  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, 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. Fig. 9. 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 Fig. 10. Fig. 11. Fig. 12. Fig. 13. Fig. 14. 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 Fig. 15. 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 Fig. 16. While of course it is most important, and for accurate work essential, to have as good a balance as possible, 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 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.
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. (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 CaSO4 or 6 parts of MgSO4 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 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 BaCl2 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 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 H2SO4. 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.):—
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
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, C2O4H2 + 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 C2H4O2. 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 Fig. 17. Fig. 18. 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 NH3 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 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, CHEMICAL EXAMINATION OF LEATHER. Fig. 19. 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 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. The soluble mineral salts are detected by igniting the residue left after evaporation of a separate portion in a porcelain crucible. Estimation of ash.—The leather in small pieces (either after or before extraction with water) is incinerated in a porcelain 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. 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 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, H2SO4). 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, 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.:—
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
CHEMICAL ANALYSIS FOR THE TANNERY — Continued.
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 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 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 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 11/2 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 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 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 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 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:—
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 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 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 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 (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, 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, 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. Pl. V. 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. 74) 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 Fig. 20. Fig. 21. 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. 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. |
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