TANNINS AND ALLIED BODIES.
588. Occurrence and Composition.—The tannins and allied bodies, which are of importance in this connection, are those which occur in food products and beverages and also those made use of in the leather industry. The term tannin is applied to a large class of astringent substances, many of which are glucosids. Tannic acid is the chief constituent of the tannins, and is found in a state of comparative purity in nutgalls. The source from which the tannic acid is derived is indicated by a prefix to the name, e. g., gallotannic, from nutgalls, and caffetannic, from coffee etc. The tannins have lately been the theme of a critical study by Trimble, and the reader is referred to his work for an exhaustive study of the subject.[606] Tannin is one of the most widely diffused compounds, occurring in hundreds of plants. Commercially, the oaks and hemlocks are the most important plants containing tannin. The sumach, mangrove, canaigre, palmetto and many others have also been utilized as commercial sources of tannin. The tannins as a class are amorphous and odorless. They are slightly acid and strongly astringent. Their colors vary from dark brown to pure white. They are soluble in water, alcohol, ether and glycerol and insoluble in chloroform, benzol, petroleum ether, carbon bisulfid and the oils. The tannins give blue or green precipitates with iron salts and most of them brown precipitates with potassium bichromate. They are all precipitated by gelatin or albumin. Tannins are not only generally of a glucosidal nature, but are found quite constantly associated with reducing sugars, or in unstable combination therewith.
The reducing sugars may be separated from the tannin by precipitating the latter with lead acetate and determining the glucose in the filtrate after the removal of the lead. A separate portion of the tannin is hydrolyzed with sulfuric or hydrochloric acid and the reducing sugars again determined. Any excess of sugars over the first determination is due to the hydrolysis of the tannin glucosid.
589. Detection and Estimation of Tannins.—The qualitive reactions above mentioned serve to detect the presence of a tannin. Of the iron salts ferric acetate or chlorid is preferred. Ferrous salts do not give any reaction with dilute tannin solutions. An ammoniacal solution of potassium ferricyanid forms with tannins a deep red color changing to brown. In quantitive work the tannins are mostly determined by precipitation with metallic salts, by treatment with gelatin or hide powder, or by oxidation with potassium permanganate. Directions for the estimation of glucosids in general are found in Dragendorff’s book.[607]
590. Precipitation with Metallic Salts.—The methods depending on precipitation of the tannins with metallic salts are but little used and only one of them will be mentioned here. A full description of the others is contained in Trimble’s book.[608] A method for the determination of caffetannic acid in coffee has been worked out by Krug and used with some satisfaction.[609]
In this method two grams of the coffee meal are digested for thirty-six hours with ten cubic centimeters of water, a little more than twice that volume of ninety-five per cent alcohol added and the digestion continued for a day. The contents of the flask are poured on a filter and the residue washed with alcohol. The filtrate contains tannin, caffetannic acid and traces of coloring matter and fat. It is heated to the boiling point and clarified with a solution of lead acetate. A caffetannate of lead containing forty-nine per cent of the metal is precipitated. As soon as the precipitate has become flocculent it is collected on a filter, washed with ninety per cent alcohol until the soluble lead salts are all removed, then with ether and dried. The composition of the precipitate is represented by the formula Pb₃(C₁₅H₁₅O₈)₂. The caffetannic acid is calculated by the equation: Weight of precipitate: weight of caffetannic acid = 1267: 652.
591. The Gelatin Method.—The precipitation of tannin with gelatin is the basis of a process for its quantitive estimation which, according to Trimble, is conducted as follows:[610] Two and a half grams of gelatin and ten grams of alum are dissolved in water and the volume of the solution made up to one liter. The solution of gelatin and also that of the tannin are heated to 70° and the tannin is precipitated by adding the gelatin reagent slowly, with constant stirring, until the precipitate coagulates, and, on settling, leaves a clear liquor in which no further precipitate is produced on adding a few drops more of the reagent. In case the clearing of the mixture do not take place readily, the process should be repeated with a more dilute tannin solution. The precipitate is collected on two counterpoised filter papers one placed inside the other, dried at 110° and weighed, the empty filter paper being placed on the pan with the weights. For pure tannin (gallotannic acid) fifty-four per cent of the weight of the precipitate are tannin. Ammonium chlorid and common salt have been used in place of the alum in preparing the reagent, but if the proportion of alum mentioned above be used, satisfactory results will be obtained in most cases.
592. The Hide Powder Method.—The principle of this method is based on the change in specific gravity, i. e., total solids, which a tannin solution will undergo when brought into contact with raw hides in a state of fine subdivision. The hide powder absorbs the tannin, and the total solid content of the solution is correspondingly diminished. The method is conducted according to the official directions as follows:[611]
Preparation of the Sample.—The bark, wood, leaves or other materials holding the tannins, are dried and ground to a fine powder and thoroughly extracted with water as mentioned below. In each case the solution or extraction is made as thorough as possible and the volume of the extract is made up to a definite amount.
Quantity of Tanning Material.—Use such an amount of the tanning material as shall give in 100 cubic centimeters of the filtered solution about one gram of dry solids. In the case of barks, woods, leaves and similar materials, transfer to a half liter flask, fill the flask with water at approximately 80° and let stand over night in a bath which is kept at 80°, cool, fill to the mark, shake well and filter. In the case of extracts and sweet liquors, wash the proper quantity into a half liter flask with water at approximately 80°, almost filling the flask, cool and fill to the mark.
Determination of Moisture.—Dry five grams of the sample in a flat bottom dish at the temperature of boiling water until the weight becomes constant.
Determination of Total Solids.—Shake the solution, which should be at a temperature of about 20°, and immediately remove 100 cubic centimeters with a pipette, evaporate in a weighed dish and dry to constant weight at the temperature of boiling water.
Determination of Soluble Solids.—Filter a portion of the solution through a folded filter, returning the filtrate to the filter twice and adding a teaspoonful of kaolin, if necessary. Evaporate 100 cubic centimeters of the filtrate and dry as above.
Determination of Tanning Substances.—Extract twenty grams of hide powder by shaking for five minutes with 250 cubic centimeters of water, filter through well washed muslin or linen, repeat the operation three times and dry as much as possible in a suitable press. Weigh the wet powder and determine the residual moisture in about one-fourth of the whole by drying to constant weight at 100°. Shake 200 cubic centimeters of the unfiltered solution of the tannin with the rest of the moist hide powder for about five minutes, add five grams of barium sulfate, shake for one minute and filter through a schleicher and schüll folded filter, No. 590, fifteen centimeters in diameter, returning the first twenty-five cubic centimeters of the filtrate. Evaporate 100 cubic centimeters of the clear filtrate and dry the residue to constant weight at a temperature of boiling water. The difference between the soluble solids obtained in the filtered tannin solution and the residue as obtained above is the amount of tanning material absorbed by the hide powder. This weight must be corrected for the water retained by the hide powder. The shaking must be conducted by means of a mechanical shaker, in order to remove all the tannin substance from the solution. The simple machine used by druggists, and known as the milkshake, is recommended.
Testing the Hide Powder.—Shake ten grams of the hide powder with 200 cubic centimeters of water for five minutes, filter through muslin or linen, squeeze out thoroughly by hand, replace the residue in the flask and repeat the operation twice with the same quantity of water. Pass the last filtrate through paper until a perfectly clear liquid is obtained. Evaporate 100 cubic centimeters of the final filtrate in a weighed dish, dry at 100° until the weight is constant. If the residue amount to more than ten milligrams the sample should be rejected. The hide powder must be kept in a dry place and tested once a month.
Prepare a solution of pure gallotannic acid by dissolving five grams in one liter of water. Determine the total solids by evaporating 100 cubic centimeters of this solution and drying to constant weight. Treat 200 cubic centimeters of the solution with hide powder exactly as described above. The hide powder must absorb at least ninety-five per cent of the total solids present. The gallotannic acid used must be completely soluble in water, alcohol, acetone and acetic ether and should contain not more than one per cent of substances not removed by digesting with excess of yellow mercuric oxid on the steam bath for two hours.
Testing the Non-Tannin Filtrate. For Tannin:—Test a small portion of the clear non-tannin filtrate with a few drops of a ten per cent solution of gelatin. A cloudiness indicates the presence of tannin, in which case the determination must be repeated, using twenty-five grams of hide powder instead of twenty grams.
For Soluble Hide:—To a small portion of the clear non-tannin filtrate, add a few drops of the original solution, previously filtered to remove reds. A cloudiness indicates the presence of soluble hide due to incomplete washing of the hide powder. In this case, repeat the determination with perfectly washed hide powder.
593. The Permanganate Gelatin Method.—This process, which is essentially the method of Löwenthal, as improved by Councler, Schroeder and Proctor and as used by Spencer for the determination of tannin in teas, is conducted as described below.[612] The principle of the process is based on the oxidation of all bodies in solution oxidizable by potassium permanganate, the subsequent precipitation of the tannin by a gelatin solution, and the final oxidation, by means of permanganate, of the remaining organic bodies. The difference between the total oxidizable matter and that left after the precipitation of the tannin represents the tannin originally in solution.
Reagents Required.—The following reagents are necessary to the proper conduct of the potassium permanganate process:
(1). Potassium permanganate solution containing about one and a third grams of the salt in a liter:
The potassium permanganate solution is set by titration against the decinormal oxalic acid solution mentioned below. The end reaction with the indicator must be of the same tint in all the titrations, i. e., either golden yellow or pink.
(2). Tenth-normal oxalic acid solution for determining the exact titer of the permanganate solution:
(3). Indigo-carmin solution to be used as an indicator and containing six grams of indigo-carmin and fifty cubic centimeters of sulfuric acid in a liter. The indigo-carmin must be very pure and quite free of indigo-blue.
(4). Gelatin solution, prepared by digesting twenty-five grams of gelatin at room temperature for one hour in a saturated solution of sodium chlorid, then heating until solution is complete, cooling and making the volume up to one liter:
(5). A salt acid solution, made by adding to 975 cubic centimeters of a saturated solution of sodium chlorid, enough strong sulfuric acid to bring the volume of the mixture to one liter:
(6). Powdered kaolin for promoting filtration.
The Process.—Five grams of the finely powdered tea (or other vegetable substance containing tannin) are boiled with distilled water in a flask of half a liter capacity for half an hour. The distilled water should be at room temperature when poured over the powdered tea. After cooling, the volume of the decoction is completed to half a liter, and the contents of the flask poured on a filter. To ten cubic centimeters of the filtered tea infusion are added two and a half times as much of the indigo-carmin solution and about three-quarters of a liter of distilled water.
The permanganate solution is run in from a burette, a little at a time, with vigorous stirring, until the color changes to a light green, and then drop by drop until the final color selected for the end of the reaction, golden yellow or faint pink, is obtained. The number of cubic centimeters of permanganate required is noted and represented by a in the formula below. The titration should be made in triplicate and the mean of the two more nearly agreeing readings taken as the correct one.
One hundred cubic centimeters of the filtered tea infusion, obtained as directed above, are mixed with half that quantity of the gelatin reagent, the first named quantity of the acid salt solution added, together with ten grams of the powdered kaolin, the mixture well shaken for several minutes and poured on a filter. Twenty-five cubic centimeters of the filtrate, corresponding to ten of the original tea solution are titrated with the permanganate reagent, under the conditions given above, and the reading of the burette made and represented by b. The quantity of permanganate solution, viz., c, required to oxidize the tannin is calculated from the formula a - b = c. The relation between the permanganate, oxalic acid and tannin is such that 0.04157 gram of gallotannic acid is equivalent to 0.063 gram of oxalic acid. The relation between the oxalic acid solution and the permanganate having been previously determined the data for calculating the quantity of tannin, estimated as gallotannic acid, are at hand.
594. The Permanganate Hide Powder Method.—Instead of throwing out the tannin with gelatin it may be absorbed by hide powder. The principle of the process, save this modification, is the same as in the method just described. As described by Trimble, the analysis is conducted according to the following directions:[613]
Reagents Required.—The reagents required for conducting the permanganate hide powder process are as follows:
1. Permanganate Solution.—Ten grams of pure potassium permanganate are dissolved in six liters of water. The solution is standardized with pure tannin. The moisture in the pure tannin is determined by drying at 100° to constant weight and then a quantity of the undried substance, representing two grams of the dried material, is dissolved in one liter of water. Ten cubic centimeters of this solution and double that quantity of the indigo solution to be described below, are mixed with three-quarters of a liter of water and the permanganate solution added from a burette with constant stirring until the liquid assumes a greenish color and then, drop by drop, until a pure yellow color with a pinkish rim is obtained. Fifty cubic centimeters of the pure tannin solution are digested, with frequent shaking, with three grams of hide powder which has been previously well moistened and dried in a press for eighteen or twenty hours, the contents of the flask thrown on a filter and ten cubic centimeters of the filtrate titrated with the permanganate solution as directed above. The difference between the amount of permanganate solution required for the first and second titrations represents the amount of pure tannin or oxidizable matter removed by the hide powder.
2. Indigo Solution.—The indicator which is used in the titrations is prepared by dissolving thirty grams of sodium sulfindigotate in three liters of dilute sulfuric acid made by adding one volume of the strong acid to three volumes of water. The solution is shaken for a few minutes, thrown upon a filter and the insoluble residue washed with sufficient water to make the volume of the filtrate six liters.
3. Hide Powder.—The hide powder used should be white, wooly in character and sufficiently well extracted with water to afford no other extract capable of oxidizing the permanganate solution.
The Process.—The reagents having been prepared and tested as above, the solution of the substance containing the tannin, prepared as described further on, is titrated first with the permanganate solution in the manner already given. Fifty cubic centimeters of the tannin solution are then shaken, from, time to time for eighteen hours, with three grams of hide powder, thrown upon a filter and ten cubic centimeters of the filtrate titrated with the potassium permanganate as above described. From the data obtained, the quantity of permanganate solution corresponding to the tannin removed by the hide powder is easily calculated. The value of the permanganate solution having been previously set with a pure tannin, renders easy of calculation the corresponding amount of pure tannin in the solution under examination.
595. Preparation of the Tannin Infusion.—A sample weighing about a kilogram should be secured, representing as nearly as possible the whole of the materials containing tannin in a given lot. The sample is reduced to a fine powder and passed through a sieve containing apertures about a millimeter in diameter. The quantity of the sample used for the extraction depends largely upon its content of tannin. Five grams of nutgalls, ten grams of sumach or twenty grams of oak bark represent about the quantities necessary for these classes of tannin-holding materials. The sample is boiled for half an hour with half a liter of water, filtered through a linen bag into a liter flask and washed and pressed with enough water to make the volume of the filtrate equal to one liter. The proper quantities of this solution are used for the analytical processes described above.