Chapter VI (1)

METHODS OF CHEMICAL ANALYSIS FOR THE TANNERY.

It is assumed that the reader has an elementary knowledge of chemistry, and of the common manipulations of the laboratory; but at the risk of giving information which to many is already familiar, the principles that underlie those methods of testing which are most applicable to technical purposes must be briefly explained.

_Standard Solutions._--If 40 _grm._ of pure caustic soda (NaHO) be dissolved in water, and a little tincture of litmus added, it will be coloured a bright blue. If hydrochloric acid be now added, drop by drop, the litmus will at last become purple, and a single drop more would turn it a bright red. At this point the liquid is neither acid nor alkaline, and if it be evaporated to dryness, nothing will be left but 58·5 _grm._ of common salt (NaCl), while 18 _grm._ of water will be formed and have escaped. We have therefore used exactly 36·5 _grm._ of pure HCl, and if we dissolve 40 _grm._ of caustic soda in 1 _litre_ of water, and 36·5 _grm._ of pure HCl in another, equal parts of these liquids will always exactly neutralise each other, forming nothing but common salt and water. It will be obvious that if we have a soda solution of the strength named, we can find the amount of hydrochloric acid in any solution of unknown strength, by seeing how much of it is required to neutralise, say, 10 _c.c._ (= 0·4 _grm._ soda) of the known solution. Instead of 40 _grm._ of caustic soda, we may take 56 _grm._ of potash to the _litre_, and it will exactly neutralise an equal volume of the hydrochloric solution containing 36·5 _grm._ If, again, we make a solution containing 49 _grm._ of pure sulphuric acid (SO_{4}H_{2}) per _litre_, it will neutralise an exactly equal volume of either the soda or the potash solution, thus being precisely equivalent to the HCl solution. Such solutions are called normal, and any normal acid solution will neutralise an equal volume of any normal alkali, and _vice versâ_. For many purposes normal solutions are too strong, and solutions containing 1/10 of the quantities required for normal solution are preferable; such solutions are called decinormal. All solutions containing known quantities of chemicals, and intended for use in volumetric analysis, are called Standard solutions.

_Indicators._--The tincture of litmus used to show when the solution is exactly neutral is called an indicator, and many materials are used in a similar way in different analytical processes. Thus the indigo solution in Löwenthal's process is an indicator. A more useful indicator than litmus for tannery purposes is Dr. Lunge's "methyl orange," which is indifferent to carbonic acid, and may therefore be used in the cold with solutions of alkaline carbonates; which are much more easily made and preserved than those of the caustic alkalies necessary with litmus. It is very sensitive to mineral acids, but not equally so to organic. It may be obtained of Messrs. Mawson and Swan, of Newcastle; and as a minute quantity only must be used for each test, it is really cheaper than litmus, and a few _grm._ will last a lifetime. It must be dissolved in water, and not more than 2 or 3 drops taken for each titration. (Titration signifies an estimation by means of a standard solution.) Other indicators will be named in connection with the analytical methods in which they are used.

_Instruments._--To practically carry out analysis by standard solutions, measuring glasses are required. One or more flasks marked in the neck to hold exact quantities (Fig. 9), one at least, holding 1 _litre_, are indispensable. One or two graduated cylinders (Fig. 10), holding 100 _c.c._, and divided into tenths of _c.c._, are very useful, and it is well also to have one holding a _litre_, and provided with a stopper (Fig. 11). This is called a "test mixer," but is not absolutely essential.

Pipettes (Fig. 12) are tubes with a mark on the stem by which exact quantities of liquid can be taken. Several holding 5, 10, 20, and 25 _c.c._ are necessary, and one holding 10 _c.c._ and divided into tenths is advisable. Most important of all is the burette (Fig. 13). If only one is to be had, it must be a Mohr's burette with a glass tap, but as alkaline solutions are apt to set glass taps fast, it is well to have one with a tap, and another with a pinchcock (Fig. 14). They should hold 50 or 25 _c.c._ and be divided into tenths. The burette in use is fixed in a stand (Fig. 15) and filled up to the top of the graduation, and the quantity of solution delivered is then shown by the scale. It is usual to read by the under side of the hollow of the liquid, keeping the eye carefully level with it.

A chemical balance suitable for the preparation of standard solutions and general analytical use, is shown in Fig. 16. The beam is provided with steel or rock-crystal knife-edges at the centre, which are supported on agate planes, and similar edges _a_ support the pans. Except at the moment of weighing, the beam, and in good balances the pans also (at _b_), are steadied by supports raised by turning the milled head _c_. The long pointer _d_ moving over a scale, shows when the beam is horizontal; but the weighing is performed, not by waiting till the balance comes to rest, but by noting when the oscillations are equal on each side of the zero point. The _weights_, which should run from 50 _grm._ downwards, are usually of brass (preferably gilded) down to 1 _grm._, while the fractions to 0·01 _grm._ are of platinum foil. Milligrammes and fractions are weighed by a "rider" of wire weighing 0·01 _grm._, and moved along the beam (which is graduated for the purpose like a steelyard) by the arms _e_. A fair balance should turn distinctly with 0·001 _grm._, and a good one with 0·0001 _grm._ If equal weights are placed on each pan, they should of course balance, and if changed side for side the balance should be maintained. If not, the arms of the beam are unequal. Weights always have trifling errors, but if by a really good maker, these are generally so small that they may be disregarded except in very delicate researches. The weights should always be placed on the scale in regular order, beginning with the heaviest, and it is well to accustom oneself to reading the weight by the vacant places in the box as well as by the weights on the scale.

While of course it is most important, and for accurate work essential, to have as good a balance as possible, much may be done in technical work, even with a good pair of druggists' scales; and most standard solutions may be bought ready made; while from two or three accurately adjusted solutions many others may be made volumetrically.

_Preparation of Standard Acid and Alkaline Solutions._--In practice it is very difficult to obtain perfectly pure caustic soda, free from water and carbonic acid, both of which are greedily absorbed by it from the air, so that a standard solution cannot practically be made by directly weighing out the substance as suggested in the introductory paragraph. In sodic carbonate, however, we have a substance which is easily obtained pure and dry, and which may be used for almost all the purposes to which a caustic solution could be applied. A decinormal solution is strong enough for most of the work in a tannery, though it is a convenience to have both normal and decinormal, and a stock of the stronger solution will last a longer time and is readily diluted to decinormal strength by adding 1 part to 9 parts of distilled water. To make a normal solution, about 60 _grm._ of the purest sodic carbonate are placed in a porcelain basin or platinum crucible and heated over a Bunsen gas-burner or spirit-lamp, nearly to redness, and allowed to cool closely covered up. Of the salt thus dried 53 _grm._ are accurately weighed into a beaker and dissolved in distilled water. The solution is then poured into a gauged _litre_ flask, and carefully filled up with water at a temperature of 59° F. (15° C.) to the mark on the neck. The whole is then poured into a good-sized stoppered bottle (40 oz.) and vigorously shaken for 5-10 minutes. This thorough shaking is important with all standard solutions, and without experience no one would believe how much shaking is required uniformly to mix a solution. Probably more difficulty to beginners in analysis arises from neglect of this matter than from any other cause. To make a decinormal solution, proceed in precisely the same way, using 5·3 _grm._ instead of 53; or dilute as above.

_Standard Acid Solution._--For this purpose any one of several acids may be used, each of which has its special advantages.

Oxalic acid is the easiest to make of any. A sufficient quantity of pure crystallised oxalic acid is powdered and pressed between filter paper, so as to absorb the moisture which occasionally is retained in cavities of the crystals. 6·3 _grm._ is then weighed out and dissolved in water, exactly as was done with sodic carbonate, forming a decinormal solution. It is used in Löwenthal's tannin estimation process and may also be employed to determine alkalies, but forms insoluble calcium oxalate with lime salts, and does not give a sharp reaction with methyl orange indicator. Hence litmus must be used, or a few drops of a neutral solution of calcium chloride added to the methyl orange, when hydrochloric acid will be liberated as soon as there is excess of the acid, and the indicator will be promptly reddened. Sulphuric acid is the most permanent of any acid solution, and may be generally employed. It forms insoluble sulphates with lime, baryta, and strontia. To make a normal solution, 35 _c.c._ of the pure concentrated acid are poured into at least 3 or 4 times as much distilled water, and allowed to cool, and are then made up to about 1 _litre_ and well shaken. The burette is filled with the mixture, 10 _c.c._ of the standard sodic carbonate are measured into a beaker, 2 or 3 drops of methyl orange solution are added, and the acid is run in with constant stirring till the indicator is just beginning to redden. This must be repeated, and the two titrations should exactly agree. Suppose that 9.5 _c.c._ are required, then 950 _c.c._ of the trial acid are equal to 1 _litre_ of the soda. If therefore 950 _c.c._ be measured into a test mixer, and made up to 1 _litre_, the solution should be accurately decinormal. Of course great care must be used in the whole process. If a gauged flask only is at hand it will be easier to measure into it the water required to make up the _litre_, and then fill to the mark with the trial acid. Normal hydrochloric acid may be made exactly as described for sulphuric acid, but using about 100 _c.c._ of the strongest acid. Decinormal solutions of both these acids may be made by the same methods; using 1 tenth the quantities, or by dilution of the normal solution.

Beside comparison with sodic carbonate solution, hydrochloric acid may also be checked by determining the amount of chlorine present, with silver nitrate (see p. 98) 10 _c.c._ of decinormal acid should of course be equal to 10 _c.c._ of decinormal silver nitrate.

Table giving the Quantity of the Following Substances contained in or equivalent to 1 _litre_ of Normal or 10 _litres_ of Decinormal Standard Solution.

Sulphuric acid 49 _grm._ SO_{4}H_{2} = 40 _grm._ SO_{3}
Hydrochloric acid 36·5 " ClH = 35·5 _grm._ Cl.
[G]Oxalic acid 63·0 " C_{2}O_{4}H_{2} + 2 Aq.
Acetic " 60·0 " C_{2}H_{3}O_{2}H.
Soda 40·0 " NaHO.
Sodic carbonate 53·0 " Na_{2}CO_{3}.
[G]Lime 28·0 " CaO = 37·0 _grm._ CaH_{2}O_{2}.
[G]Calcic carbonate 50·0 " CaCO_{3}.
Ammonia 17·0 " NH_{3}.
[G]Barium hydrate 76·5 " BaO = 85·5 _grm._ BaH_{2}O_{2}.
Barium chloride 104·0 " BaCl_{2}.
Zinc chloride or sulphate 32·6 " Zn = 16·0 _grm._ S. as sulphide.
Silver nitrate 170·0 " AgNO_{3} = 35·5 _grm._ Cl.
Potassic permanganate 31·6 " K_{2}MnO_{4}.

[Footnote G: Insufficiently soluble in water to form a normal solution.]

EXAMINATION OF WATER.

_Hardness_ (Hehner's process). (_a_) Temporary Hardness.--As has been stated (p. 84), this consists of lime and magnesia carbonates. As methyl orange is not affected by carbonic acid, bicarbonates of alkaline earths have an alkaline reaction, and may be estimated in solution by standard acid like the alkalies themselves. 100 _c.c._, or in soft waters 200 _c.c._, of the water is measured into a beaker, a drop or two of solution of methyl orange added, and decinormal hydrochloric or sulphuric acid run in from the burette with constant stirring till the colour just changes to pink. This is repeated, and the average taken. The two determinations should not at the most differ more than 1/10 _c.c._ Each _c.c._ represents 5 parts per 100,000 of CaCO_{3} or 2·8 parts of CaO; or corresponding quantities of magnesia (4·2 parts of MgCO_{3} or 2 parts MgO), when 100 _c.c._ of water are used.

(_b_) Permanent Hardness.--200 _c.c._ are measured into a beaker and boiled for 15 minutes with 40 _c.c._ decinormal sodic carbonate. The mixture is then allowed to cool and made up to 250 _c.c._; or the flask and its contents may be weighed before boiling and made up again to the same weight. It is then filtered, and 60 _c.c._ representing 50 _c.c._ of the original water, is twice titrated with decinormal acid and the result added. If the water were pure, exactly 10 _c.c._ should be required to neutralise the 10 _c.c._ of sodic carbonate, but if there be permanent hardness a part of the sodic carbonate will be already neutralised with the acids of the lime and magnesia salts, which have been precipitated as carbonates together with the carbonates of these bases originally present in the water. The hardness will therefore be represented by the loss, i. e. the number of _c.c._ of acid used for 100 _c.c._ of the original water must be subtracted from 20 and the remainder calculated as before, or if calculated as sulphates, each _c.c._ represents 6·8 parts of CaSO_{4} or 6 parts of MgSO_{4} per 100,000. If, as is sometimes the case, more acid is required than is needed for the sodic carbonate used, the excess corresponds to sodic carbonate originally present in the water. In this case there can be no permanent hardness.

_Chlorine in Water._--If silver nitrate be added to a solution of any chloride, the silver is precipitated as white curdy insoluble silver chloride. As indicator, a few drops of neutral potassic chromate are used. So long as any chloride is present the red silver chromate which forms is at once decomposed, and the silver converted into white chloride. But as soon as all the chloride is exhausted, the red chromate becomes permanent. To prepare a standard decinormal solution of silver, 17 _grm._ of pure recrystallised silver nitrate are dissolved in 1 _litre_ of distilled water. To perform the estimation 50 _c.c._ of water are measured into a beaker, 2 or 3 drops of strong solution of pure yellow potassic chromate are added, and then silver nitrate from the burette till a permanent red is formed. This is repeated, and the results are added together, representing 100 _c.c._ of water. Each _c.c._ of silver nitrate used represents 3·55 parts of chlorine, or 5·85 parts of sodic chloride per 100,000. If more than 10 _c.c._ of silver solution are required to 50 _c.c._, it is advisable to use a smaller quantity of water. If the process be applied to other liquids than natural water, it must be borne in mind that the solution must not contain free acids or alkalies except carbonic acid. If this is not the case the liquid may be rendered faintly alkaline, with lime-water free from chlorides, and the excess of lime removed by passing carbonic acid through it; or it may be slightly acidified with sulphuric acid, and shaken with a little pure precipitated calcic or baric carbonate.

_Detection of other Impurities._--Sulphuric acid (as sulphates) is seldom wholly absent, but its presence may be proved, by adding excess of barium chloride to the water slightly acidified with hydrochloric acid (2-3 _c.c._ of saturated solution of BaCl_{2} are sufficient for any ordinary water); if the mixture be allowed to stand overnight in a 100 _c.c._ cylinder beside a solution containing a known, and not very different quantity of decinormal sulphuric acid, the quantity present may be roughly compared by measuring the bulk of the precipitates.

Lime may be similarly detected and roughly measured by precipitation with excess of ammonic oxalate in presence of ammonium chloride, to hinder precipitation of magnesia. Lime-water, which may be used as a standard, contains about 128 parts of lime per 100,000.

Magnesia is detected by adding ammonium phosphate to the filtrate from the precipitated oxalate of lime. If the mixture be allowed to stand in a warm place for 24 hours all the magnesia will be precipitated as ammonio-magnesic phosphate.

Silica, &c.--100 _c.c._ of the water is acidified with a little HCl evaporated to dryness, moistened with HCl, and treated with a little hot water. The silica or silicic acid is left undissolved. The solution from which the silicic acid has been filtered off is evaporated to small bulk and ammonia added, when iron will be precipitated as brown ferric oxide, which is coloured black by tannin or tanning liquor. If copper be present it will give a blue solution with the ammonia. Iron may also be recognised by evaporating the water to small bulk with a trace of HCl, and adding a little sodium acetate, when if iron be present it will be coloured black by tannin, red by ammonium sulphocyanide, and blue by potassium ferrocyanide (prussiate of potash). Its quantity may be estimated (Thomson, Chem. Soc. Abstracts, May 1885) by measuring 100 _c.c._ of the water to be tested and 100 _c.c._ distilled water into two similar cylinders, adding to each 5 _c.c._ of dilute hydrochloric acid (1:5) and 15 _c.c._ of a solution of potassium sulphocyanide (40 _grm._ per _litre_), and then adding to the distilled water cylinder a very dilute standard solution of ferric salt, till its colour matches the other. If the iron contained in the water is in the ferrous condition, it must be oxidised with potassic permanganate before testing.

A suitable ferric standard solution may be made by dissolving 0·1 _grm._ of clean, bright, soft iron wire in a little hydrochloric acid in a long-necked flask, adding nitric acid so long as red fumes are produced, evaporating nearly to dryness, and making up to 1 _litre_ (more accurately 996 _c.c._). Each _c.c._ will then equal 0·0001 _grm._ Fe.

Lead (and copper) may be detected by passing sulphuretted hydrogen through the water acidified with HCl, or by adding a drop of fresh ammonium or sodium sulphide to the slightly acidified water, when a brownish coloration clearly visible in a deep beaker set on a sheet of white paper will be produced. Iron also gives a black with sulphides in alkaline solution. Copper may be distinguished from lead by the blue given with ammonia, and by a reddish-brown precipitate with potassium ferrocyanide.

For accurate quantitative estimation of these impurities, the regular works on the subject, such as Thorpe's 'Quantitative Analysis,' Sutton's 'Volumetric Analysis,' or Fresenius' 'Quantitative Analysis,' must be consulted.

EXAMINATION OF COMMERCIAL ACIDS.

_Sulphuric acid_ 10 _grm._ may be made up to 100 _c.c._ and well mixed, and of this 10 _c.c._ may be tested with normal sodic carbonate in presence of methyl orange. Each 1 _c.c._ of soda solution used corresponds to 0·049 _grm._ or 4·9 per cent. of H_{2}SO_{4}. For most purposes, the strength may be ascertained from the specific gravity, as measured by a hydrometer or weighed in a specific gravity bottle. The following table gives the strength at 59° F. (15° C.):--

─────────┬───────────────┬────────────
Specific │ Degrees │ Per cent.
Gravity. │ Twaddell.[H] │ H_{2}SO_{4}
─────────┼───────────────┼────────────
│ ° │
1·8426 │ 168·5 │ 100
1·8376 │ 167·5 │ 95
1·822 │ 164 │ 90
1·786 │ 157 │ 85
1·734 │ 147 │ 80
1·675 │ 135 │ 75
1·615 │ 123 │ 70
1·557 │ 111 │ 65
1·501 │ 100 │ 60
1·448 │ 90 │ 55
1·398 │ 80 │ 50
1·351 │ 70 │ 45
1·306 │ 61 │ 40
1·264 │ 53 │ 35
1·223 │ 45 │ 30
1·182 │ 36 │ 25
1·144 │ 29 │ 20
1·106 │ 21 │ 15
1·068 │ 14 │ 10
1·032 │ 6 │ 5
─────────┴───────────────┴────────────

[Footnote H: Degrees of Twaddell's hydrometer may be reduced to specific gravity by multiplying by ·005 and adding 1·, thus 10° Tw. = 1·050 sp. gr.]

The impurities of sulphuric acid most common and injurious for tanning purposes are iron and nitrous acid. Iron is detected on neutralising with soda or ammonia, when it falls as a yellowish precipitate, which may be recognised by the ordinary tests (p. 100). Nitric and nitrous acids are detected by pouring a strong solution of ferrous sulphate cautiously on to the top of the strong cold acid, when a dark ring is formed at the junction of the two liquids.

_Hydrochloric acid_ may be tested with soda solution like sulphuric. 1 _c.c._ of normal soda = 0·0365 _grm._ or 3·65 per cent. HCl. It may also be calculated from specific gravity.

──────────────────┬───────────────
Specific Gravity, │ Per cent. HCl.
15° C. │
──────────────────┼───────────────
1·200 │ 40
1·177 │ 35
1·151 │ 30
1·126 │ 25
1·100 │ 20
1·075 │ 15
1·050 │ 10
1·025 │ 5
──────────────────┴───────────────

The presence of iron is indicated by a yellow colour, and may be confirmed by the usual tests as in sulphuric acid.

_Oxalic acid_ should be pure white and soluble in distilled or rain-water. 6·3 _grm._ may be weighed out, and made up to 200 _c.c._ If 20 _c.c._ of the solution for a test be used, each _c.c._ of normal soda solution equals 10 per cent. of pure crystallised acid, C_{2}O_{4}H_{2} + 2 Aq. The end-reaction with methyl orange is rendered sharper by the addition of a few drops of neutral calcic chloride towards the end of the titration.

_Acetic acid_ may be similarly determined, each _c.c._ of normal alkali being equivalent to 0·06 _grm._ of C_{2}H_{4}O_{2}. Caustic soda, or lime-water and litmus, give sharper results than sodic carbonate and methyl orange. Brown pyroligneous acid is difficult to test from the dark compounds formed with soda, but may be indirectly determined by the quantity of marble, baric carbonate, or magnesia which it will dissolve (compare p. 100), or very possibly by lime-water like tan-liquors with a little tannin as indicator.

EXAMINATION OF LIME AND LIME-LIQUORS.

The quantity of caustic lime in either quicklime or lime-bottoms may be determined by weighing a quantity of the finely powdered material containing not more than 1 _grm._ of caustic lime, and shaking it thoroughly with 1 _litre_ of distilled water and filtering. 100 _c.c._ should be taken, and decinormal acid, sulphuric or hydrochloric (or if oxalic, with addition of neutral calcic chloride, or with litmus instead of methyl orange as indicator). Each _c.c._ of decinormal acid corresponds to 0·0028 _grm._ of CaO. If the filter and residue be treated with sufficient normal acid to dissolve the whole of the carbonates, and then titrated back with normal sodic carbonate and methyl orange, the loss (less soda solution required than acid was originally employed) is equal to the carbonate of lime and carbonate and hydrate of magnesia present. 1 _c.c._ of normal acid = 0·05 _grm._ of CaCO_{3}.

Lime-water and lime-liquors may be titrated as above, with sulphuric or hydrochloric acid and methyl orange; but in the latter case ammonia (and if soda ash or "Inoffensive" is used, soda and potash also), and the lime salts of weak organic acids will be estimated with it. It is difficult to get a sharp end-reaction in old liquors from the organic acids (caproic, amidocaproic, &c.) present. To determine the ammonia, 50-100 _c.c._ of the liquor may be distilled in a small retort or flask, and the escaping NH_{3} collected in a =U=-tube or "nitrogen bulb" (Fig. 17), containing 20-50 _c.c._ of normal acid, which is afterwards titrated back with sodic carbonate and methyl orange. Kathreiner employs the arrangement shown in Fig. 18. 30 _c.c._ of the liquor to be examined is placed in a shallow vessel on a piece of ground-glass, and 10 _c.c._ of normal acid in a second cup, which is supported over the other by a glass or wire triangle. The whole is covered with a small bell-glass, of which the edges are smeared with, vaseline. At the end of 24 hours, all the ammonia will have been absorbed by the acid, which is titrated back. The lime-liquor sample should be drawn after well plunging the lime, and rapidly filtered into a flask from a funnel covered with a clock-glass.

_Determination of Gelatin and Coriin in Lime-liquors._--This cannot be done directly, though considerable quantities of dissolved hide-substance are precipitated on acidification of the liquor with hydrochloric acid and saturation with common salt. If the liquor be neutralised with hydrochloric acid, and evaporated to dryness on the water-bath, nitrogen may be determined in the residue by combustion, and the hide-substance calculated from it (compare p. 108). This method is serviceable in determining the amount of hide dissolved by different solutions, or under different conditions.

The total solids of lime-liquors are estimated by evaporating 20-30 _c.c._ in a porcelain crucible at 212° F. (100° C.). The organic matter is then found by igniting and determining loss (using ammonia nitrate if necessary to complete the combustion of the carbon). The ash is mostly lime carbonate. Soda, potash, and other bases may be determined in it by the usual methods, if required.

ESTIMATION OF SULPHUR AS SULPHIDE IN SODIUM SULPHIDE, &c.

32·6 _grm._ of chemically pure zinc is dissolved in dilute sulphuric or hydrochloric acid. This is readily accomplished in a flask, if a piece of platinum foil, or a few drops of platinic chloride are added to form a galvanic couple with the zinc. After solution, sufficient ammonia is added to redissolve the precipitate at first formed,[I] and the whole is made up to 1 _litre_. Each _c.c._ = 0·016 _grm._ sulphur or 0·242 _grm._ of sodic sulphide. This solution is added drop by drop from a burette to the solution of sulphide, and forms a white precipitate of zincic sulphide. The end of the reaction is known by placing a drop (with a glass rod) side by side on a piece of white filter paper, with a drop of solution of lead acetate. So long as sulphide remains in solution, it will form a black margin of lead sulphide where the drops touch. The drops must not be placed too close, as the solid zinc sulphide is _always_ darkened if it comes in contact with lead acetate. It must be noted that tank-waste liquors, and many other sulphur solutions, contain polysulphides which are estimated by zinc, but which do not unhair, at any rate in an unaltered state.

[Footnote I: If any brown residue remains, the zinc is contaminated with iron.]

CHEMICAL EXAMINATION OF LEATHER.

_Estimation of Grease._--To determine oil and grease, a weighed quantity (5-10 _grm._) of the leather in fine shavings or raspings is exhausted with petroleum-ether (gasoline) in a fat-extraction apparatus, of which a convenient form is represented in Fig. 19. The leather is placed in the upper vessel, of which the lower opening is loosely plugged with cotton-wool, and the petroleum-ether in the flask, which is gently heated in a water-bath. The petroleum-ether boils and condenses in the inclined condenser through the casing of which a stream of cold water is passed, whence it drops back into the flask through the material to be exhausted. When the exhaustion is complete (when a drop of petroleum-ether from the leather leaves no grease when allowed to evaporate on a clean glass), the upper part of the apparatus is removed, and the ether is distilled off. If the flask has been previously weighed, it is maintained in an air-bath at 212°-248° F. (100°-120° C.) for some hours, allowed to cool, and weighed, when the gain of weight is the grease and oil. Paraffin would also be extracted and reckoned, and probably traces of resin if present. Ordinary ethylic ether cannot be used, since tannins and many of their products are soluble in it. Probably carbon disulphide might be substituted. Care must be taken to avoid explosion, as the vapours of petroleum are very combustible. The residue left in the percolator may be examined for matters soluble in water, by extracting again with hot distilled water, or for resins (and phlobaphenes) by extraction with alcohol.

_Estimation of matters soluble in water._--This is important both to detect weighting, and to draw conclusions as to the materials used in tanning. Fine raspings or shavings may be exhausted with warm water in a percolator, or roughly a weighed piece (20 _grm._) of leather, air-dry, may be well kneaded and worked in 100 _c.c._ of warm water in a basin. 50 _c.c._ of this may be evaporated to dryness in a light basin over the water-bath (or under a paper hood on a steam boiler), and the gain of weight will give the amount dissolved from 10 _grm._ This is more accurate and quicker than redrying the leather and weighing loss. The residue will contain tannins and their products, often in considerable quantities, and may be examined by the table of reactions, p. 112, though these are as yet very imperfect. It will also contain glucose, dextrin, and soluble salts, if these have been used to give weight and firmness. The absolute proof of weighting with glucose or dextrin is difficult, since tanning materials naturally contain these and analogous principles. The residue may be powdered and exhausted with cold water, and the tannins and colouring matter removed by shaking with magnesia (p. 108) or lead carbonate. Fehling's solution[J] is then added and the mixture is heated nearly to boiling. A rapidly formed and considerable precipitate of red cuprous oxide indicates weighting with glucose or dextrin. Leather extracts, however, invariably reduce Fehling's solution more or less, and a conclusion can only be drawn after some experience and comparative tests. Gallotannic acid and pyrogallol reduce it when heated, but not cane sugar or gum arabic. If a solution of cane sugar be heated to 68° C. for 1/4 hour with 10 per cent. of fuming hydrochloric acid, it is "inverted," and then after neutralising the acid with potash or soda, will reduce Fehling's solution when heated.

[Footnote J: 4 _grm._ cryst. cupric sulphate are dissolved in 20 _c.c._ of water; and 16 _grm._ of neutral potassic tartrate and 13 _grm._ of fused sodic hydrate are dissolved in 60 _c.c._ The two are mixed, made up to 100 _c.c._, and boiled for some minutes. It should always be tested before use by boiling a portion, which should remain perfectly clear.]

The soluble mineral salts are detected by igniting the residue left after evaporation of a separate portion in a porcelain crucible.[K] From unweighted leather, the quantity is very small. The ash is exhausted with a few _c.c._ of distilled water, which will dissolve most sulphates and chlorides, which may be detected in small portions of the solution by baric chloride and silver nitrate respectively. Baric chloride and lead acetate are precipitated by a drop of sulphuric acid, and the latter is blackened with ammonic or sodic sulphide. Lime is precipitated by addition of ammonic chloride, ammonia, and ammonic oxalate; magnesia by the subsequent addition of sodic phosphate (see p. 109). The carbonates in the insoluble part (mostly derived from salts of organic acids) may be taken up by dilute hydrochloric acid and tested separately, or the acid may be used at first. Any residue undissolved by the acid is probably lead chloride, and will be dissolved by hot water.

[Footnote K: A platinum crucible must not be used for fear of its destruction by lead, unless this metal has been proved absent.]

_Estimation of ash._--The leather in small pieces (either after or before extraction with water) is incinerated in a porcelain crucible. The ash is extracted with hydrochloric acid. The insoluble portion may contain barium sulphate (barytes), lead sulphate, sand, clay, &c. For further examination, ordinary chemical text-books must be consulted. Any large amount of ash indicates weighting. Müntz found only about 0·5 per cent. of ash from bark-tanned leather.

_Determination of hide substance._--It is sometimes of interest to determine the proportion of dry hide-substance in a sample of leather, but there is no known means of doing this directly. If, however, the leather be dried, finely powdered by rasping, and the nitrogen determined by combustion, either with soda-lime (Will and Varrentrapp's method), or with copper oxide (Dumas), the hide-substance may be calculated, since tannin contains no nitrogen. Müntz found unhaired skin dried at 230° F. (110° C.) to contain 51·43 per cent. of nitrogen (compare also p. 20).

DETERMINATION OF FREE ACIDS IN TAN-LIQUOR.

The lime-water method mentioned on p. 172 is, from its simplicity, well suited for daily use in the tannery as a control method for ordinary working; but where it is necessary to make very exact estimations, or to determine the various acids separately, it is not so satisfactory as one recently published by Kohnstein and Simand (Dingl. Polyt. Jour., 1885, cclvi. 38).

The acids usually present in liquor consist of several members of the fatty or acetic group, which distil over with boiling water, of other non-volatile organic acids, and sometimes sulphuric acid, which is added to assist the swelling of the leather.

To determine the acids of the acetic group, Kohnstein and Simand proceed as follows:--100 _c.c._ of the liquor are distilled, in a flask or retort with a good condenser, to about 30 _c.c._, allowed to cool a little, made up again to 100 _c.c._, and again distilled; and this is repeated till about 300 _c.c._ have passed over. The distillate is then made up to 300 _c.c._, well mixed by shaking, and the acid is determined with standard soda. Methyl orange and sodic carbonate is not so suitable for this titration, as caustic soda and litmus, since methyl orange is not very sensitive to vegetable acids. If it be desired to ascertain what quantity of acids of the acetic group exist in combination with lime and other bases in the liquor, small excess of sulphuric acid may be added to the residue in the retort, and the distillation repeated, when the organic salts will be decomposed and the volatile acids come over.

To determine the total free organic acids, Kohnstein and Simand shake about 80 _c.c._ of the liquor with 3-4 _grm._ of freshly ignited magnesia, quite free from carbonate and from lime, and allow to stand for some hours with frequent vigorous shaking, till the liquor, which at first is brown or dirty green, becomes almost colourless and gives no reaction of either acid or tannin. The mixture is then filtered, and the tannin and colouring matter are retained on the filter in combination with magnesia, while the organic salts of magnesia, which are mostly soluble, pass through with the filtrate. 10-30 _c.c._ of the filtrate, according to the amount of acid present, is evaporated to dryness, and gently ignited so as not to decompose any magnesic sulphate present. The residue is moistened with water saturated with carbonic acid, to convert any magnesic oxide into carbonate, and then dried, in order to make the mass powdery, and easier to wash, It is next taken up with hot distilled water, filtered and well washed. Any sulphate which is present passes into the filtrate, while the carbonate, which corresponds to the organic salts present before ignition, remains on the filter, and after solution in hydrochloric acid, is estimated as magnesic pyrophosphate. To the hydrochloric solution is added excess of ammonia and sufficient ammonic chloride to redissolve the precipitate formed, and prevent the precipitation of the magnesia; the solution is heated and then ammonic oxalate solution, first dilute, and then concentrated, is added to precipitate any lime which may be derived from lime salts present in the liquor. After filtering out and washing the precipitate, 10-15 _c.c._ of 10 per cent. sodic phosphate solution is added, and the liquid is stirred with a glass rod without touching the sides of the beaker, and allowed to stand 12 hours. The crystalline precipitate is then rinsed on to a filter, and washed with a mixture of 1 of ammonia and 3 of water, till the washings no longer give any milkiness with silver nitrate. The filter is then dried and the precipitate is placed in a platinum crucible and first gently, and then strongly ignited with the cover on; the filter paper, freed as much as possible from the precipitate, is burnt in the usual way on the crucible lid, the ashes are added to the precipitate in the crucible, and the whole is again ignited and allowed to cool in the desiccator, and finally weighed. 111 parts of magnesia pyrophosphate correspond to 120 parts of acetic, or 180 parts of lactic acid. Kohnstein and Simand calculate the pyrophosphate corresponding to the acetic acid already found by distillation, and after deducting it reckon out the remainder as lactic acid. Of course the volatile acids are really a mixture consisting of acetic, propionic, butyric and other members of the fatty group; but it would be difficult if not impossible to separate them. Similarly other fixed acids exist in mixture beside the lactic acid, but as their action is similar and lactic acid is always the most abundant, these acids are to be reckoned as lactic.

It has been mentioned that when sulphuric acid is present in the liquor it is found in the filtrate from the magnesia carbonate as sulphate. After removal of the lime as oxalate, as previously described, the magnesia may be similarly determined as pyrophosphate, and reckoned out as sulphuric acid (111 parts of pyrophosphate being equal to 98 parts sulphuric acid, H_{2}SO_{4}). It may also be estimated with barium chloride, but in this case regard must be had to the sulphates originally present in the liquor.

Since waters invariably contain both lime and magnesia salts, a portion (50 or 100 _c.c._) must be evaporated, ignited, and after precipitation of the lime, the magnesia must be estimated as already described, and deducted from the amount found in a similar amount of liquor after saturating with magnesia. If, together with the organic acids, the liquor contains sulphuric acid, the correction may be divided equally between the two.

The method is not applicable in presence of phosphoric, tartaric, or oxalic acids. To overcome this difficulty, Messrs. Kohnstein and Simand are at present investigating a method dependent on decolorisation of the liquor with bone charcoal, completely free from mineral salts, and subsequent titration with soda.

It may be interesting to add the determinations of a complete set of handlers in a Continental upper-leather tannery, in which larch bark is used. 100 _c.c._ of liquor contained as follows, in _grm._:--

─────────┬────────────┬───────────────┬──────────────────────
No. of │Total Acids │ Volatile Acids│ Fixed Organic Acids
Handler. │ reckoned │ reckoned │ reckoned as Lactic.
│ as Acetic. │ as Acetic. │
─────────┼────────────┼───────────────┼──────────────────────
1 │ 0·205 │ 0·050 │ 0·232
2 │ 0·628 │ 0·237 │ 0·586
3 │ .. │ 0·372 │ ..
4 │ 0·688 │ 0·426 │ 0·393
5 │ 0·569 │ 0·432 │ 0·206
6 │ 0·509 │ 0·453 │ 0·084
7 │ 0·487 │ 0·456 │ 0·047
─────────┴────────────┴───────────────┴──────────────────────

QUALITATIVE DETECTION OF TANNINS.

It is often desirable to determine from what tanning materials an extract or liquor is made, or with what a sample of leather is tanned. The following table gives reactions of the principal tanning materials, which will enable any one of them to be recognised with certainty, and in many cases will determine the constituents in a mixture of several, though this is naturally far more difficult. In such cases, colour reactions are apt to mislead, that of one tannin being modified by another, and it is safest to rely on the categorical test of precipitate or no precipitate, coloration or no coloration, without regard to the tint. The infusions must be very weak, not exceeding 1-2° Bktr., or precipitates will be formed where mere coloration or clouding is noted. In some cases only negative peculiarities can be given, and the material cannot be positively determined in mixture with materials where these peculiarities are present. Thus myrobalans could not be distinguished from divi with certainty, where any other material, such as gambier, was present, which gave a deep coloration with concentrated sulphuric acid. The writer will feel greatly obliged by the communication of more distinctive reactions.

CHEMICAL ANALYSIS FOR THE TANNERY

────────────┬────────────┬────────────┬─────────┬───────────┬─────────────┐
Reagent. │Myrabolanes.│ Divi-divi. │ Valonia.│ Oak Bark. │Chestnut wood│
────────────┼────────────┼────────────┼─────────┼───────────┼─────────────┤
Boiled with │Pale deposit│Pale deposit│ Slight │Slight pale│ Slight red │
equal volume│ (eliagic │ (eliagic │ pale │ deposit or│ deposit on │
of sulphuric│ acid) on │ acid) on │ deposit.│ turbidity │ cooling. │
acid (1 vol.│ cooling. │ cooling. │ │on cooling.│ │
to 9 vol. │ │ │ │ │ │
water). │ │ │ │ │ │
────────────┼────────────┼────────────┼─────────┼───────────┼─────────────┤
Bromine │ No pp. │ No pp. │ No pp. │ Pale pp. │ No pp. │
water. │ │ │ │ │ │
────────────┼────────────┼────────────┼─────────┼───────────┼─────────────┤
Dilute │Blue-black │Dark blue │Blue- │Bluish │Blue- │
ferric │pp. │ pp. │black pp.│ black pp. │ black pp. │
chloride. │ │ │ │ │ │
Add │Brown pp. │Dark red pp.│Red brown│ Red brown │Dull red pp. │
ammonia. │ │ │ pp. │ pp. │ │
────────────┼────────────┼────────────┼─────────┼───────────┼─────────────┤
Sol. │ No pp. │Faint │ No pp. │ No pp. │Slight │
tartar │ │ clouding. │ │ │ clouding. │
emetic. │ │ │ │ │ │
Add ammonic │Light pp. │Dense pp. │Pale pp. │Whitish │ Pale pp. │
chloride. │ │ │ │ pp. │ │
────────────┼────────────┼────────────┼─────────┼───────────┼─────────────┤
Copper │Faint │Slight │ No pp. │Slight pp. │ No pp. │
sulphate. │ clouding. │ green pp. │ │ │ │
Add │Dense dark │Dense dark │Dark │Brown pp. │Dark │
ammonia. │ pp. │ pp. │ reddish│ │ brown pp. │
│ │ │ pp. │ │ │
────────────┼────────────┼────────────┼─────────┼───────────┼─────────────┤
Lime-water. │Yellow pp. │Yellow pp. │Yellow │Brown pp. │Purplish │
│ turning │ turning │ pp. │ │ brown pp. │
│ greenish. │ purple. │ turning│ │ │
│ │ │ red- │ │ │
│ │ │ purple.│ │ │
────────────┼────────────┼────────────┼─────────┼───────────┼─────────────┤
Ammon. │Dirty │Dark │Dark │Greenish │Dirty │
molybdate │ yellow │ greenish │ greenish│ pp. │ green pp. │
in nitric │ pp. │ pp. │ pp. │ │ │
acid. │ │ │ │ │ │
────────────┼────────────┼────────────┼─────────┼───────────┼─────────────┤
With sodic │Yellow │ Yellow │Turns │Turns │Reddish pp. │
sulphide │ colour. │ colour. │purpulish│ red. │ │
exposed to│ │ │ red. │ │ │
air on a │ │ │ │ │ │
tile. │ │ │ │ │ │
────────────┼────────────┼────────────┼─────────┼───────────┼─────────────┤
Add │Yellow │Intense │Deep │Deep red │Dark brown. │
concentrated│ colour. │ crimson. │ yellow.│ pp. on │ │
sulphuric │ │ │ │ dilution │ │
acid to │ │ │ │ │ │
1 drop │ │ │ │ │ │
infusion. │ │ │ │ │ │
────────────┼────────────┼────────────┼─────────┼───────────┼─────────────┤
Lead │Light │Dark │Pale pp. │Brown pp. │ Brown pp. │
Nitrate │ yellow pp.│ yellow pp.│ │ │ │
────────────┼────────────┼────────────┼─────────┼───────────┼─────────────┤
Cobalt │Buff pp. │Buff │Dirty │ Ditto. │Dirty │
Acetate │ │ pink pp. │ pink pp.│ │ yellow pp. │
────────────┼────────────┼────────────┼─────────┼───────────┼─────────────┤
Manganese │ Yellow pp. │ Yellow pp. │ Dirty │ Ditto. │ Grey pp. │
acetate. │ │ │ yellow │ │ │
│ │ │ pp. │ │ │
────────────┼────────────┼────────────┼─────────┼───────────┼─────────────┤
Uranium │Dark red │Dark red │Dark red │Dark brown │ Dark red │
acetate. │ colour. │ colour. │ colour.│ pp. │ colour. │
────────────┼────────────┼────────────┼─────────┼───────────┼─────────────┤
Ammoniacal │ No pp. │ No pp. │Brown pp.│ No pp. │ No pp. │
picric │ │ │ │ │ │
acid sol. │ │ │ │ │ │
────────────┼────────────┼────────────┼─────────┼───────────┼─────────────┤
Potassic │ Brown pp. │ Brown pp. │Brown pp.│ Brown pp. │ Brown pp. │
dichromate.│ │ │ │ │ │
────────────┴────────────┴────────────┴─────────┴───────────┴─────────────┘

CHEMICAL ANALYSIS FOR THE TANNERY -- Continued.

┬───────────┬───────────┬───────────┬────────────┬────────────┬───────────┐
│Hungarian │ Hemlock │Mimosa │Cutch │ Gambier │Gallotannic│
│Larch │ (Extract).│ bark. │ (Pegu). │ (Cuba). │ Acid, 1 │
│(Extract). │ │ │ │ │ per cent. │
┼───────────┼───────────┼───────────┼────────────┼────────────┼──────────-│
│Yellow │Abundant │Heavy red │Light red │ Reddish │ Usually │
│Flocculent │ red │ deposit │ deposit │ deposit │ some pale │
│deposit │ flocculent│ on │ on cooling.│ on cooling.│ deposit. │
│separates │ deposit. │ cooling. │ │ │ │
│quickly. │ │ │ │ │ │
┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤
│Yellow pp. │Yellow pp. │Yellow pp. │ Yellow pp. │ Yellow pp. │ No pp. │
┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤
│Dull brown │Dirty green│Full brown │Green-black │Intense │ Blue │
│ pp. │ pp. │ pp. │ pp. │ green │ -black │
│ │ │ │ │ colour. │ pp. │
┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤
│Dull red │Reddened │Purple │ Dark red │ Reddened │ Reddened │
│ pp. │ pp. │ colour. │ pp. │ pp. │ pp. │
┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤
│No pp. │No pp. │White pp. │ No pp. │ No pp. │ No pp. │
┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤
│Pale pp. │Slight pale│Dense white│ Pale pp. │ Faint │ White pp. │
│ │ pp. │ pp. │ │ clouding. │ │
┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤
│Slight │Pale pp. │Slight pp. │ Dense pp. │ No pp. │ No pp. │
│ cloud. │ │ │ │ │ │
┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤
│Deep blue │Dark green │Deep red │Deep violet │Dark green │ Brown pp. │
│coloration.│coloration.│ pp. │ coloration.│ coloration.│ │
┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤
│Dirty │ Brown pp. │ Slight │Slight cloud│ No pp. │ Pale pp. │
│brown pp. │ │reddish pp.│ soluble in │ │turns blue.│
│ │ │ │ excess. │ │ │
┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤
│Slight │ Slight pp.│ Brown pp. │ Ditto. │ Ditto. │ Yellow │
│ clouding. │ │ │ │ │ colour. │
┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤
│No change. │ No change.│ Turns red.│ Slight │ No change. │ No change.│
│ │ │ │ reddening. │ │ │
┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤
│Dark brown │ Intense │ Intense │ Deep red no│ Dark brown │ Yellow. │
│or crimson.│ crimson. │purple-red.│ pp. on │ or crimson.│ │
│ │ │ │dilution. │ │ │
┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤
│Pale pp. │ Pale pp. │ Clouding. │ No pp. │ Faint │ White pp. │
│ │ │ │ │ clouding. │ │
┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤
│ Purplish │ Purple pp.│ Brown pp. │ Brown pp. │ No pp. │ Purple pp.│
│ pp. │ │ │ │ │ │
┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤
│Slight │ Slight pp.│ No pp. │ No pp. │ Ditto. │ White pp. │
│ clouding. │ │ │ │ │ │
┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤
│Slight │Light brown│ Dark red │ Dark red │ Dark red │ Crimson │
│ darkening.│ pp. │ colour. │ colour. │ colour. │ colour. │
│ │ │ │ │ │ Brown pp. │
┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤
│ No pp. │ Clouding. │ No pp. │ No pp. │ No pp. │ No pp. │
│ │ │ │ │ │ │
┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤
│ Ditto. │ Brown pp. │ Brown pp. │Brown │ Brown pp. │ Brown pp. │
│ │ slowly │ │ colour. │ slowly │ │
│ │ formed. │ │ │ formed. │ │
┴───────────┴───────────┴───────────┴────────────┴────────────┴───────────┘

QUANTITATIVE DETERMINATION.

Many processes have been proposed for the quantitative estimation of tannins, but it cannot be said that any method yet known is wholly satisfactory. The oldest, that of Sir H. Davy, recently improved by Stoddart and others, consists in precipitating with gelatin, and drying and weighing the precipitate. This is almost impossible to filter off as directed by Davy; but by the use of a little alum, and by pouring hot water on the precipitate, it becomes curdled into a mass which may be washed by decantation. As the precipitate contains varying quantities of tannin, according to the strength of solution employed; as it is soluble in excess of gelatin solution, and as it is almost if not quite impossible to wash it free from gelatin and alum, the method can hardly lay claim to much accuracy. A somewhat better one consists in the employment of a standard solution of gelatin with a little alum, determining the end of the reaction by filtering off a portion and ascertaining if another drop of the reagent produces a further precipitate. This method is very tedious, the end reaction is difficult to hit, the standard solution is very unstable, it is inapplicable to gambier and cutch because the mixture will not filter clear, and its results are irregular, probably from the power of tannin to combine with various proportions of gelatin. A plan, which has a seductive appearance of simplicity, is that of Hammer; he takes the sp. gr. of the infusion, then absorbs the tannin with slightly moistened hide-raspings, again takes the sp. gr., and from the difference calculates the percentage of tannin, a difference of 5 per cent. of tannin corresponding to one of 1·020 sp. gr. (20° barkometer). Unfortunately the hide is more or less soluble in the liquor, and absorbs acids other than tannic with considerable energy; the moistening of the raspings introduces an error, and the smallness of the quantity to be measured makes a slight error completely vitiate the results. With extreme care, due corrections for temperature, for the water introduced with the raspings, and for their solubility, and by substituting evaporation of the infusions to dryness for mere calculation from their sp. gr., the method is useful as giving almost the only information obtainable as to the actual weight of tannin in any material capable of being absorbed by hide. It is, however, only suitable for use as a check on easier and more rapid methods, such as Löwenthal's, which give accurate relative results, but no information as to absolute weight of unknown tannins. A modification of Hammer's method has been introduced by Müntz and Ramspacher, in which the liquor whence the tannin is to be removed is forced through a piece of raw hide by pressure. This method, except that it is more rapid, has all the evils of Hammer's in an intensified form, and gives such variable results as to be quite useless in practice. A set of very careful determinations of one sample of sumach gave results ranging from 18 to 28 per cent., and similar variations occurred when the experiment was repeated with valonia. Wagner's method by precipitation with a standard solution of cinchonine and magenta has proved wholly unreliable.

Gerland's method with a volumetric solution of tartar emetic, used in presence of ammonic chloride, gives constant results with sumachs, 2/3 of those given by permanganate and Neubauer's equivalent. Tartar emetic does not precipitate the tannins of cutch and gambier. Fleck's, by precipitation with copper acetate, and subsequent washing with ammonic carbonate and gravimetric estimation, either of the tannate dried at 212° F. (100° C.), or of the copper oxide left on ignition; and Carpene's, by precipitation with ammoniacal zinc acetate, and subsequent estimation with permanganate and indigo, though giving fairly accurate results on some tannins, are only of limited application. They may therefore be passed over, as well as Jean's method with a volumetric solution of iodine in presence of sodic carbonate, and Allen's method with lead acetate, which are tedious and difficult, and present no advantage over Löwenthal's improved process. This last is easy of execution, constant in results, and universally applicable. Before proceeding to describe it in detail, it may be well to give some hints as to the best modes of sampling and preparing tanning materials for analysis, since this is often more difficult and tedious than the actual analysis.