Chapter VI (2)

Sampling.--Samples should always be drawn from at least 10 sacks or separate parts of the bulk, and, in the case of valonia, special care should be taken to have a fair average quantity of "beard." No attention is usually paid to this point by merchants, and the proportion varies greatly in different parts of the same cargo. If several sacks are spread in layers on a level floor, and then portions going quite to the ground are taken from several parts of the floor, this will be accomplished. Where samples must be dealt with which have not been specially drawn, it might be safest to weigh out from each the same proportion of beard and whole cups, bearing in mind that the beard is always the richest part of the valonia. In sampling myrobalans, it should be remembered that the poor and light nuts will rise to the top, and hence the hand should be plunged well into the sack. Grinding of valonia and myrobalans when practicable is probably best done in a small disintegrator, fitted with gratings. The material, of which some pounds must be used, is screened over a sieve of say 15 wires per in., and all coarser parts are returned to the mill till they will pass. The mill must grind into a close box, that no dust may be lost. Bark may be reduced to fine saw-dust by cutting a portion of each piece in the sample with a circular saw or rasp driven by a lathe. The advantage of these methods is that samples can be ground without previous drying, and thus in many cases time may be saved and separate determination of moisture avoided. When this is not practicable, the sample of some lbs. at least is ground in an ordinary bark-mill, well mixed, spread out flat on a floor or table, and several portions are taken as already described, say 50-100 _grm._ in all, and dried in a water- or air-oven at 212° F. (100° C.). The moisture is best determined, to save time, in a small separate portion of 10 _grm._, which must be dried till it ceases to lose weight, and the loss taken as moisture. It must be weighed in a covered capsule, as it is very hygroscopic. When the larger portion of the sample has been dried some hours, it is passed twice through a good coffee-mill, and then returned to the oven till thoroughly dried, for which, 12-24 hours is generally sufficient. Another method sometimes convenient is to take each acorn, or each piece of bark of the sample to be tested, and snip a piece from it with a pair of tinners' shears, taking care that in the case of valonia the section runs right to the centre of the cup; and in bark, that fair shares of the outer and inner layers are taken. The reason for drying before grinding is, that unless hard dried, tanning materials cannot be passed through a small mill. Bark and valonia usually contain 12-16 per cent. of moisture.

Exhaustion.--10 _grm._ of valonia, 20-30 _grm._ of bark, or corresponding quantities of other material, are boiled briskly for half an hour with 1 _litre_ of distilled water, a funnel being placed in the neck of the flask, and great care being taken at first to avoid frothing and boiling over. The flasks used should have a capacity of at least 1-1/2 _litre_. The whole contents are finally rinsed into a gauged flask, allowed to cool to 59° F. (15° C.), and made up to 1 _litre_. In the case of sumach, a little more boiling even than this is desirable. This method has been found by the writer to give better results than boiling with successive portions of water. Another method is to boil for 1/2 hour with 250 _c.c._ of water, then pour the whole on a filter, wash with boiling water so long as a drop of the filtrate blackens paper moistened with a dilute solution of ferric acetate, and finally make up to 1 _litre_. Many materials, however, clog the filter to such an extent that washing is almost impossible. Kathreiner has used 15 _litres_ of water, and corresponding quantities of material, in a large steam-jacketed copper pan, for each exhaustion, making the weight up finally to 15 _kilos._, with very uniform and excellent results. (See also p. 130.) With all materials which deposit ellagic acid or other insoluble derivatives, on cooling and standing, considerably higher results will be obtained if the titration be made as soon as the liquor is cold, than if it be allowed to stand 24 hours; in this respect, a uniform practice should be adhered to. Addition of 1/2 _c.c._ of glacial acetic acid renders the infusions less liable to change.

Analysis.--Of all the methods which have been proposed for the estimation of tannins, the only one which has met with any general acceptance is that of Löwenthal, and indeed it is the only one which in rapidity of execution and constancy of results is fitted for general use. The method, as originally proposed, depends on the oxidation of the astringent solution by permanganate in presence of indigo, which not only serves as an indicator, but controls the oxidation, limiting it to those bodies which are more oxidisable than indigo. As, however, these include gallic acid and other substances which are useless to either tanner or dyer, it is necessary to remove the tannin, and by a second titration to obtain its value by difference. This Löwenthal (Zeitschrift f. Anal. Chemie, 1877, p. 33) accomplished by a solution of gelatin and common salt, to which, after mixture with the tannin infusion, a small quantity of sulphuric or hydrochloric acid was added. It was necessary to let this stand at least some hours before a clear filtrate could be obtained, and the gelatin remaining in solution had a slight though generally negligible effect on the permanganate. In some cases, even after long standing, perfect filtration was extremely difficult and tedious, and it was also clearly proved by Simand (Ding. Polyt. Jour., ccxliv. 400) that a certain proportion of the tanno-gelatin precipitate, varying with the acid present, and with the species of tannin, remained in solution, and thus gave too low a result. He therefore proposed to revert to the old method of separating tannin with hide raspings, or, as an improved substitute, with the gelatinous tissue of bones, and this is probably the most accurate method, but has the disadvantage of requiring considerable time for its execution. (See also p. 130.) The writer has therefore tried, and he thinks successfully, so to modify Löwenthal's original method as to increase its accuracy, and at the same time to make it more rapid and easy of execution. It was found that by saturating the clear filtrate with salt, a further precipitate containing tannin was formed, but unfortunately, it was so finely divided that no amount of standing, or even of warming, and repeated passing through the paper, would obtain a clear filtrate. Finally, he hit on the device of mixing with the liquid, before filtration, a portion of the pure kaolin used by photographers. The effect was instantaneous and complete. A perfectly clear filtrate was obtained without any of the tedious waiting which before was necessary, and it was not only free from tannin, but also nearly so from gelatin, so that it only gave the faintest cloudiness with tannin solution. Gelatin gives a more considerable precipitate, but this is simply due to its insolubility in the saturated salt solution, and it is redissolved on dilution with water.[L]

[Footnote L: Hunt (Jour. Soc. Chem. Industry, April 1885) states, that saturation with salt causes partial precipitation of gallic acid when present, and that results agreeing more closely with those obtained by absorption with hide are obtained by employing a mixture of 50 _c.c._ liquor, 25 _c.c._ 2 per cent. gelatin solution, and 25 _c.c._ saturated solution of salt containing 50 _c.c._ of concentrated sulphuric acid per _litre_ and a teaspoonful of kaolin. This approaches very nearly to Löwenthal's original method, but with the addition of the kaolin, and as in it, it is to be feared that a portion of the tannate of gelatin will remain in solution. For accurate work, therefore, absorption by hide-raspings is preferable, though even that has been shown by the writer to remove gallic acid and other matters beside tannin. Hunt states that raw hide also absorbs catechin.]

A slight error is introduced by the presence of a trace of oxidisable matter in the gelatin, and when very great accuracy is required, it is well to make a blank estimation of "not-tannin" without tannin infusion, and deduct 1/2 of the permanganate consumed as a correction from the not-tannin; but this may usually be disregarded. Each titration should be made twice, and successive tests should not differ by more than 0·1 _c.c._ of permanganate.

Reagents.--Solutions are required of (1) Pure potash permanganate, 1 _grm._ per _litre_. (2) Pure soda or potash sulphindigotate, 5 _grm._, and concentrated sulphuric acid, 50 _grm._ per _litre_. (3) Pure oxalic acid, 6·3 _grm._ per _litre_ (decinormal). The sulphindigotate (indigo carmine), must be filtered, and when oxidised by permanganate, should give a pure clear yellow, free from any trace of brown or orange. Any contamination with indigo-purple, which gives brown oxidation-products, is quite fatal to the accuracy of the analysis. The permanganate solution is standardised by measuring 10 _c.c._ of the (decinormal) oxalic acid solution, adding a little pure sulphuric acid and distilled water, warming to 136° F. (58° C.), and running in the permanganate till a faint permanent pink is produced, for which about 32-33 _c.c._ should be required. The indigo-carmine solution should be of such strength that 14-16 _c.c._ of permanganate are required to bleach the quantity employed, which may be 20-25 _c.c._, as convenient. (4) Gelatin solution: 2 _grm._ of Nelson's or other good gelatin are allowed to swell in distilled water for two hours, melted by setting the glass in a pan of boiling water, and made up to 100 _c.c._ This will not keep. (5) Dilute sulphuric acid: 10 _c.c._ of pure concentrated acid are added to 90 _c.c._ of distilled water. (6) Good table salt. (7) Purified kaolin.

The analysis is performed in the following manner:--20 _c.c._ of indigo solution, and 5 _c.c._ of the infusion of tanning material is added, in a white basin as recommended by Kathreiner, to about 3/4 _litre_ good water, which it is best to measure approximately, so that if it contains any impurity which affects the permanganate it should be constant, and thus be eliminated with the indigo. Permanganate solution is then allowed to drop in, with constant stirring till the pure yellow liquid shows a faint pinkish rim, most clearly seen on the shaded side. This end-reaction, which is of extraordinary delicacy, is due to Kathreiner, and is quite different to the pink caused by excess of permanganate, being an effect common to all pure yellow liquids. It is not needful to make the titration so slowly as has been advised--the permanganate may be dropped in steadily with vigorous stirring, so long as there is large excess of indigo, but as soon as the bottom of the basin can be seen through the solution, it must be added very cautiously, one or two drops at a time, and with occasional pauses, to allow time for its complete mixture through so large a mass of fluid. The titration is repeated twice, and the results added together and denoted by _a_. Then take 50 _c.c._ of the infusion, and add 28·6 _c.c._ of the gelatin solution of Nelson's gelatin of 2 _grm._ to 100 _c.c._ After shaking, the mixture is saturated with salt, which brings the volume up to 90 _c.c._, and 10 _c.c._ of the dilute sulphuric acid (containing 1 vol. of concentrated acid in 10) and a teaspoonful of pure kaolin are added. It is best to do this in a flask in which it can be well shaken, after which, filtration may be at once proceeded with, although it is safer to let it stand an hour or two: (the flask may be cleansed with caustic soda solution). 10 _c.c._ of this filtrate (= 5 _c.c._ of the original infusion) are employed for a second pair of titrations, which are added as before, and the result denoted _b_. If, further, _c_ be the quantity of permanganate required to oxidise 10 _c.c._ of decinormal oxalic acid, and 10 _grm._ of the tanning material have been employed to make 1 _litre_ of infusion, _c_ : (_a_ - _b_) :: 6·3 : _x_, where _x_ is the percentage of tannin expressed in terms of crystallised oxalic acid. If it be desired to calculate the gallic acid and non-tannin substances contained in the infusion, the value in permanganate of the indigo alone must be determined. Calling this _d_, as _c_ is to (_b_ - _d_), so is 6·3 to the percentage of non-tannins in terms of oxalic acid, and for the present it is best invariably to calculate results in this way, since we do not actually know the relation of any single tannin to permanganate, even Neubauer's number for gallotannic acid being probably too high, according to the recent investigations of Councler and Schroeder,[M] and Oser's for quercitannic being at most only approximate. It happens, moreover, that this last equivalent (62·36 _grm._ of quercitannic acid = 63 _grm._ of crystallised oxalic acid) does not differ from that of oxalic acid more than the ordinary limits of error of such estimation, and the substitution is therefore of no commercial importance, while it is surely better to employ a standard which is easily and exactly verified than one which is certain to be modified by further research, and so to run the risk either of having our results made useless for future comparison, or of establishing a false or arbitrary equivalent. What is wanted for practical purposes is not the absolute weight of tannins in the various materials, but only a means for the relative comparison of two samples of the same material; cross comparisons of different tannins being simply delusive. If, however, it is necessary at any time to give actual percentages of gallotannic acid, it is probably best to stick to Neubauer's number for the present, as it is in general use. Neubauer states that 63 _grm._ of oxalic acid consume as much permanganate as 41·37 _grm._ of gallotannic acid. Tshekawa found 41·688 as the equivalent for tannin from Japanese gall nuts (Chem. News, xlii. 274). Councler and Schroeder on the other hand give only 34·3 _grm._ Simand gives 61·1 _grm._ as the equivalent of quercitannic acid. Commercial "pure tannin" always gives results higher than the truth, as the gallic acid which it contains consumes more permanganate than an equal weight of tannin, or even than the tannin which would yield it if boiled with acid. When this is done the equivalent used should be definitely stated, or it will certainly lead to confusion. Neubauer's equivalent is only properly applicable to gall nuts, and possibly to sumach and myrabolans. For oak bark Oser's number or that of oxalic acid is most likely nearly correct; and this may also be approximately true of oak wood and valonia, but as respects all other materials we have no information whatever, and the oxalic equivalent is as likely to be right as any other. (Compare note, p. 128.)

[Footnote M: From researches by von Schroeder, published since the above was penned, it seems that the permanganate consumed by tannin is largely influenced by the way in which the titration is conducted, see p. 128.]

A few results are given below, not as showing the relative values of the materials, which, of course, cannot be directly compared by any analytical process, but for comparison with those obtained by other methods and modes of calculation:--

─────────────────────────────────┬──────────────────┬──────────────────┐
│ │ Other Bodies │
│ Tannin │ Oxidised │
│ (as Oxalic Acid).│ (as Oxalic Acid).│
─────────────────────────────────┼──────────────────┼──────────────────│
Spent Liquor │ O·12 │ 11·0 │
Valonia (good Smyrna). Sample 1 │ 29·1 │ 2·3 │
" " Sample 2 │ 30·7 │ 2·1 │
" " Sample 3 │ 30·5 │ 1·9 │
Hungarian Larch Extract. Sample 1│ 14·78 │ 1·95 │
" " Sample 2│ 18·08 │ 2·33 │
Chestnut-wood Extract, 25° B. │ 25·53 │ 3·68 │
Pegu Cutch │ 63·59 │ 2·45 │
─────────────────────────────────┴──────────────────┴──────────────────┘

It is proved by experiment that kaolin removes nothing which is oxidised by permanganate, but simply facilitates the precipitation and filtration; and it is often found useful to clarify the original infusions and liquors before the first titration. On the other hand, there is no doubt that the salt and acid of Löwenthal's method precipitate of themselves a large proportion of certain tannins. In the case of cutch this amounted, in the analysis given, to 67 per cent. of the whole. There is, however, good reason to believe that this would also have been absorbed, or at least removed from solution by hide in the process of tanning. This is shown by the analysis of the spent liquor above given, which originally contained the tannins of oak bark, valonia, myrabolans, gambier, hemlock, and oak wood extracts, &c., to the extent of 10 to 15 per cent., but which was reduced by contact with hide to 0·12 per cent. That a portion had not been absorbed but decomposed is proved by the large accumulation of oxidisable impurities (equal to 11 per cent. of oxalic acid); at the same time this example shows that the method is capable of estimating a very small portion of tannin in presence of much gallic acid and other analogous substances. It is worth remark that such spent liquors become very pale in colour, and also that the filtrates, freed from tannin by precipitation, are nearly colourless, thus proving that the colouring matters present in tanning materials are of the nature of tannins, at least as regards their precipitability by hide and gelatin.

Simand (Dingl. Polyt. Jour., ccxlvi. 133) has recommended instead of precipitation with gelatin, the use of the gelatinous tissue of bones to remove the tannin. For this purpose porous bones, such as horn piths, are coarsely powdered, and after treatment with dilute soda solution to remove the fat, are steeped in weak hydrochloric acid till all the calcareous matter is dissolved. They are then thoroughly washed, ground wet through a steel mill, washed again and dried at a low temperature; the tannin is removed more quickly than by raw hide, and the amount of gelatinous matter dissolved by cold water is a very trifling one. This method, or that with purified hide-powder, is to be recommended for scientific research, since no element capable of precipitating substances other than those absorbed by the hide is introduced, while it is not certain in all cases that saturation with salt and acidification may not remove other constituents of the liquor besides tannins. It has, however, for technical purposes the great disadvantage of requiring a much longer time for absorption of the tannin than is the case with gelatin solution, and of the process being much more difficult of execution. If hide-powder be employed, it must be moistened with a small quantity of water before adding to the infusion, and this water must be taken into account in the quantity of the filtrate employed for the titration of the "non-tannin." The digestion with the hide- or bone-powder must be continued till the filtered liquid does not give the faintest clouding with a drop of clear gelatin solution, and it is always very difficult to be sure that the tannin is so completely removed as with gelatin and salt. Hide- or bone-powder may be employed to determine the actual weight of any unknown tannin absorbable by hide, by evaporating equal quantities of the original infusion and of that freed from tannin by digestion with the powder; the difference giving the tannin absorbed. The evaporation must be conducted as far as possible in absence of air, for instance in _vacuo_, or in a current of carbonic dioxide, and the residues both dried at 212° F. (100° C.) so long as they lose weight. The amount of matter dissolved from an equal quantity of the hide- or bone-powder by water must also be ascertained and taken into the calculation.

Ammoniacal solution of cupric acetate or sulphate has been employed by several chemists to remove tannin from solutions. N. H. Darton of New York, who has a large practice in tannin analysis, employs cuprammonic sulphate in the following manner.

The infusion, for which 20 _grm._ of hemlock bark or a corresponding quantity of other material must be used, is made by exhausting with 2 or 3 quantities of water successively, first cold, and then with heat (by placing the flask in a pan of boiling water), each portion of water being poured off into a _litre_ flask. The last should be almost colourless. The liquor is thus made up to nearly 1 _litre_, 25 _c.c._ of dilute sulphuric acid (about 1 vol. concentrated in 10) is added, and the liquor is filtered through a small filter, which is finally rinsed with a small quantity of water. Liquid ammonia is now added till the liquor slightly smells of it, and, if any precipitate is formed, it is filtered off as before; 25 _c.c._ of dilute sulphuric acid is again added (which should give the liquid an acid reaction), and it is made up to 1 _litre_. The titration is done as described under Löwenthal's method, but instead of precipitating with gelatin, 100 _c.c._ is mixed with 100 _c.c._ of a solution of copper sulphate to which sufficient ammonia has been added to redissolve the precipitate first formed, and containing 1-1/4 per cent. of copper sulphate. This is well shaken and filtered, and the "not-tannin" is determined in the filtrate just as with gelatin; a little dilute sulphuric acid being added in the basin to neutralise the ammonia. The writer has examined this method with regard to a few tanning materials. With valonia (and therefore probably with oak bark) the preliminary treatment is unnecessary, and copper precipitation gives results practically identical with the improved gelatin, while it is less troublesome. On the other hand, a sample of Miller's Hungarian Larch Extract which gave tannin equal to 18·08 per cent. (by the gelatin method) gave no precipitate with cuprammonic sulphate, and hence a result in tannin of _nil_ by Darton's method. It is worth remark that by the copper method it is therefore possible to estimate the valonia tannin alone in a mixture of larch and valonia tannin. Probably this mode of analysis may also be utilised to separate other tannins. With chestnut extract the results seem satisfactory, as regards the precipitation of the tannin by copper, the figures agreeing very closely with those by gelatin, but the preliminary treatment with sulphuric acid and ammonia precipitates about 75 per cent. of what is usually reckoned as tannin, leaving 7·53 per cent. of tannin only instead of 25·53 per cent. as reckoned by the gelatin method; which, judging by practical results in tanning, can hardly be accepted as correct. The results of the gelatin method are found to agree fairly with those of direct absorption by hide-powder, which is strong confirmation that what is estimated as tannin is what is absorbed by the hide. It is well known that sulphuric acid precipitates many tannins, and in an experiment with cutch it was found by the writer that saturation with salt and the addition of dilute sulphuric acid as for Löwenthal's process, but without the gelatin, precipitated 67 per cent. of the total tannin as usually reckoned.

It is obvious that it is impossible by analysis to compare the relative value of different tannins, such as those of myrobalans and gambier, or hemlock and valonia. All that analysis can reasonably be expected to do is to give the relative values of different samples of the same substance, or at the most, of materials of the same class. All other comparisons are misleading; and would be so, even if the exact percentage of each tannin could be calculated; since the commercial and practical value of different materials does not depend on the quantity of tannin only, but on the character of the leather it produces, hard or soft, dark- or light-coloured and heavy- or light-weighing.

A Commission of German technical chemists, under the presidency of Dr. Councler of Eberswalde, and including Messrs. Eberz, Kathreiner, Schaun, von Schroeder, and Simand, have recently reported on methods of tannin estimation ('Bericht über die Verhandlungen der Commission zur Feststellung einer einheitlichen Methode der Gerbstoffbestimmung,' Cassel, 1885). After reviewing earlier methods, they recommend the following modifications of the Löwenthal method, for general adoption.

Chemicals employed.

(1) _Permanganate solution._ 10 _grm._ of the purest potash permanganate are dissolved in 6 _litres_ of distilled water.

(2) _Indigo solution._ 30 _grm._ dry sulphindigotate of soda (Carminum cærul. opt., "pure Indigotin I" of Gehe & Co., Dresden), air-dry, are dissolved in 3 _litres_ of dilute sulphuric acid (1 vol. H_{2}SO_{4} to 3 vols, water), 3 _litres_ of distilled water are added, the whole is shaken till dissolved, and filtered. In each titration, 20 _c.c._ are used in 3/4 _litre_ of water, and reduce about 10·7 _c.c._ of permanganate.

(3) _Hide-powder_ must be white and in a fine woolly state of division, and should yield to cold water no substance capable of reducing permanganate. Such a powder is prepared by Dr. Both of Berlin,[N] and by the Vienna Research Station.

[Footnote N: Messrs. Mawson and Swan, of Newcastle, have kindly undertaken to keep these, and the other reagents mentioned in this book, in stock for the convenience of English tanners and chemists.]

Mode of Titration.

Instead of adding the permanganate solution drop by drop, to the mixture of indigo, water, and liquor (as described, p. 121), it is recommended to add it 1 _c.c._ at a time,[O] vigorously stirring 5-10 seconds after each addition. When the liquid has become bright green, 2-3 drops at a time are cautiously added with stirring, till the liquid is pure yellow. Either a beaker on a white tile or a white basin may be used (compare p. 121). It is advantageous in strong sunlight to shade the window with white tissue-paper.

[Footnote O: It has been noted by several chemists, and especially by Kathreiner, and later by Prof. von Schroeder, that the quantity of permanganate reduced by a given amount of tannin varies within rather wide limits, according to the rate at which the permanganate is added; and the "1 _c.c._ method" was suggested by Prof. von Schroeder, to secure uniformity in this particular. It has, however, been found by the writer, in the course of experiments not yet completed, that the quantity of permanganate required, was a function not simply of time, but of the rapidity of diffusion through so large a bulk of liquid; and by the alternate use of a simple glass rod, and of a specially constructed perforated stirrer, he was able, while adhering strictly to Prof. von Schroeder's directions, to obtain results even more divergent by the "1 _c.c._ method" than could be obtained by the drop method previously recommended, when properly carried out. Employed in conjunction with the use of tannin for standardising, as recommended by the Commission, either method gives perfectly dependable results.

The explanation of the variation is a simple one. The oxidation in the Löwenthal process should be limited to indigo, and bodies more oxidisable than indigo, but there exist both ready formed in liquor, and among these oxidation products many substances which in the absence of indigo will readily reduce permanganate. When the latter is added rapidly, and with insufficient stirring, it destroys the indigo and tannin in contact with it, and proceeds also to oxidise the other matters present, although in other parts of the beaker indigo and tannin still exist. Thus more permanganate is reduced than corresponds to the indigo and tannin, and this is especially so towards the end of the process, when very little of either remains. The more slowly the permanganate is added, and the more vigorously it is stirred, the more closely it will approximate to the theoretical quantity required merely to oxidise the indigo and tannin. It seems to the writer more scientific to approach this as nearly as possible, than to attempt to establish a purely arbitrary standard such as the "1 _c.c._ method;" but he would rather refrain from committing himself to a definite opinion till his experiments are complete.]

_E. & F. N. Spon, London & New York._

"INK-PHOTO." SPRAGUE & CO. LONDON.

SOFTENING THE SKINS.]

Standardisation of the permanganate.

To avoid the uncertainty involved in comparing tannin (which reduces different quantities of permanganate according to the method of titration) with so dissimilar a reducing agent as oxalic acid, it is recommended to employ tannin, titrated under precisely the same conditions as the tanning material, so that whatever method be employed, the differences will be common to both, and will so be eliminated. Prof. von Schroeder has shown (Report, p. 45) by careful experiment, that with the purest samples of tannin the permanganate value estimated on the total dry substance of the tannin varied by very little from that of the part of the tannin absorbed by hide as determined by Hammer's process, but on the average bore the proportion of 1 : 1·05. The percentage of water in an air-dried tannin must be estimated by drying a portion at 201°-212° F. (94°-100° C.) and determining the loss, and a quantity equivalent to 2 _grm._ must be dissolved in 1 _litre_ of water and 10 _c.c._ titrated with indigo in the usual way. If the permanganate value thus obtained be multiplied by 1·05, it will be equivalent to that of 2 _grm._ of chemically pure tannin. It is only necessary to determine the moisture occasionally, if the tannin be kept in a well-closed box or bottle.

To ascertain if a tannin is pure enough for this, use a solution made as above described (it is not necessary to determine the moisture) and 10 _c.c._ are titrated with permanganate in the usual way. 50 _c.c._ are then digested in the cold with 3 _grm._ hide-powder (previously moistened with distilled water and well squeezed in linen) for 18-20 hours, with frequent shaking, filtered, and 10 _c.c._ again titrated. If the second titration ("not-tannin") does not exceed 5 per cent. of the total, it is _good_, but it may be _used_ so long as the "not-tannin" does not exceed 10 per cent.[P] The purest tannin examined by Prof. von Schroeder was Schering's Phar. Ger., which may be obtained of Messrs. Mawson and Swan.

[Footnote P: Gallic acid suggests itself to the writer as being a good standard, since it behaves with permanganate like tannin, and being crystalline is easily purified and of definite composition.]

The course of analysis is as follows:--

_Preparation of the infusions._--Extracts are dissolved in hot water, and if necessary, filtered. Barks and other solid materials are treated in Prof. von Schroeder's extraction-apparatus (Fig. 20) (which seems very well adapted for its purpose). This consists of a perfectly cylindrical vessel of cast-tin, about 12·5 _c.m._ deep and 7 _c.m._ diameter. A strainer covered with fine muslin fits it like a piston.[Q] The powdered material is placed in the cylinder, and stirred up with 200 _c.c._ of cold water. At the end of an hour, the piston is inserted and pressed down gently, the clear liquor is poured off, and the process is 4 times repeated with hot water, at intervals of 1/2 hour, placing the cylinder in a water-bath. The liquid is made up to 1 _litre_, and, if necessary, filtered (Report, p. 66). The quantity of material used should be such as to give an infusion of which 10 _c.c._ do not reduce more than 8 _c.c._ permanganate. If it is desired to determine separately the "easily soluble tannin" (viz. that extracted by cold water), Real's Press (Fig. 21) is employed, which consists of a cylinder _a_, through which water may be forced by the pressure of a column of liquid. The small sieve _d_, covered with a disc of linen, is placed in _a_, next the tanning material previously thoroughly moistened with water, and the tap is closed. The press is then filled with water and left 15 hours under a pressure of 1-1/2 _metres_. The tap is then opened and 1 _litre_ is allowed to run through in the course of about 2 hours, and mixed by shaking. The material is finally exhausted like a new material in von Schroeder's apparatus to extract the difficultly soluble tannin.

[Footnote Q: Both this apparatus, and the Real's press, may be obtained from C. Focke, Zinngiesser, Grosse Kirchgasse 3, Dresden.]

The titration is carried out as before described; in each infusion separately to determine the "not-tannin" 50 _c.c._ are treated with 3 _grm._ hide-powder, and 10 _c.c._ are titrated.

It may be well in conclusion for the writer to state for the information of the non-chemical reader, that though for purposes of comparison of the results of different chemists, it is most desirable to have a standard method of the highest possible perfection; any of the accepted modifications of the Löwenthal method will give excellent practical results in careful hands.