APPENDICES.
APPENDIX A.
METHOD OF THE INTERNATIONAL ASSOCIATION OF LEATHER TRADES CHEMISTS FOR THE ANALYSIS OF TANNING MATERIALS.
Including Alterations adopted at the Leeds Conference in 1902.
Section I.—Sampling from Bulk.[189]
[189] See London Report, pp. 22-29 and 124.
1. Liquid Extracts.—In drawing samples, at least 5 per cent. of the casks must be taken, the numbers being selected as far apart as possible. The heads must be removed, and the contents mixed thoroughly by means of a suitable plunger, care being taken that any sediment adhering to sides or bottom shall be thoroughly stirred in. All samples must be drawn in the presence of a responsible person.
2. Gambier and Pasty Extracts.—Gambier and pasty extracts should be sampled from not less than 5 per cent. of blocks, by a tubular sampling tool, which shall be passed completely through the block in seven places. Solid extracts shall be broken, and a sufficient number of portions drawn both from the inner and outer parts of the blocks to fairly represent the bulk. In both cases samples shall be rapidly mixed and enclosed at once in an air-tight bottle or box, sealed and labelled.
3. Valonia, Algarobilla, Divi-divi, and General Tanning Materials. Valonia, algarobilla, and all other tanning materials containing dust or fibre, shall be sampled, if possible, by spreading at least 5 per cent. of the bags in layers one above another on a smooth floor, and taking several samples vertically down to the floor. Where this cannot be done, the samples must be drawn from the centre of a sufficient number of bags. While valonia and most materials may be sent to the chemist ground, it is preferable that divi-divi, algarobilla, and other fibrous materials shall be unground. Bark in long rind, and other materials in bundles, shall be sampled by cutting a small section from the middle of 3 per cent. of the bundles with a saw.
4. Samples for more than one Chemist.—Samples to be submitted to more than one chemist must be drawn as one sample, and well mixed; then divided into the requisite number of portions, not less than three, and at once enclosed in suitable packages, sealed and labelled.
Section II.—Preparation for Analysis.[190]
[190] See London Report, p. 40 et seq.
1. Liquid Extracts.—Liquid extracts shall be thoroughly stirred and mixed immediately before weighing, which shall be rapidly done to avoid loss of moisture. Thick extracts, which cannot be otherwise mixed, may be heated to 50° C., then stirred and rapidly cooled before weighing, but the fact that this has been done must be noted in the Report.
2. Solid Extracts.—Solid extracts shall be coarsely powdered and well mixed. Pasty extracts shall be rapidly mixed in a mortar, and the requisite quantity weighed out with as little exposure as possible, to avoid loss of moisture. Where extracts are partly dry and partly pasty, so that neither of these methods is applicable, the entire sample shall be weighed and allowed to dry at the ordinary temperature sufficiently to be pulverised, and shall then be weighed, and the loss of weight taken into calculation as moisture.
In such cases as gambier, in which it is not possible to grind, or by other mechanical means to thoroughly mix the constituents of the sample, it is permissible to dissolve the whole, or a large portion of the sample, in a small quantity of hot water, and immediately after thorough mixing to weigh out a portion of the strong solution for analysis.
3. Barks, and other Solid Tanning Materials.—The whole sample, or not less than 250 grms., shall be ground in a mill until it will pass through a sieve of 5 wires per centimetre. Where materials such as barks and divi-divi contain fibrous materials which cannot be ground to powder, the ground sample shall be sieved, and the respective parts which do and do not pass through the sieve shall be weighed separately, and the sample for analysis shall be weighed so as to contain like proportions.
Section III.—Preparation of Infusion.
1. Strength of Solution.—The tannin solution employed shall contain from 0·35 to 0·45 grms. per 100 c.c. of tanning matters absorbed by hide. (Paris 1900.)
2. Solution of Liquid Extracts.—A sufficient quantity shall be weighed into a covered basin or beaker, from which it shall be washed into a liter flask with boiling water and well shaken, and the flask shall be filled to the mark with boiling water. The neck being covered with a small beaker, the flask shall be placed under a cold water tap or otherwise rapidly cooled to a temperature between 15° and 20° C., and made up accurately to the mark, after which it shall be thoroughly mixed, and the filtration at once proceeded with.
Note.—Tannin infusions may be kept from fermenting by the addition of 3 to 5 drops of essential oil of mustard per liter. (F. Kathreiner.)
3. Filtration.—The filtration of the solution for analysis may take place through any paper which may be considered most suitable for the particular case, and with or without the use of kaolin, absorption of tanning matter, if any, being corrected for by an amount determined by a similar filtration of a clear solution. Perfectly clear solutions need not be filtered.
To determine the correction, about 500 c.c. of the tanning solution of the strength prescribed for analysis is obtained perfectly clear, preferably by the method of filtration which is to be corrected for. After thorough mixing, 50 c.c. is evaporated to determine “total soluble No. 1.” A portion of the remainder is then filtered in the manner for which correction is to be made, and 50 c.c. of the filtrate is evaporated for “total soluble No. 2.” Deducting No. 2 from No. 1 the difference is the correction required, which must be added to the total soluble found by analysis. It is generally advisable, both in analysis and in the second filtration for correction, to filter first 150 c.c. (which in analysis may be used for the determination of non-tannins), and then to employ the next 50 c.c. for evaporation, keeping the filter full during the operation; but whatever procedure is adopted must be rigidly adhered to in all analyses to which the correction is applied. Where kaolin is employed, a constant weighed quantity (1 or 2 grm.) must be used, which is first washed with 75 c.c. of the liquor by decantation, and then washed on to the filter with a further quantity of liquor, of which 200 c.c. is filtered as above.[191]
[191] It is obvious that in the first instance it will be necessary to determine the correction for each particular material employed, but it will soon be found that the correction is practically constant for large groups of tanning materials, so long as the same method of filtration is rigidly adhered to.
4. Solid Extracts.—Solid extracts shall be dissolved by stirring in a beaker with boiling water, the undissolved portions being allowed to settle, and treated with further quantities of boiling water, and the solution poured into a liter flask. After the whole of the soluble matter is dissolved, the solution is treated similarly to that of a liquid extract.
5. Extraction of Solid Materials.—Such quantities shall be weighed as will give an infusion of the strength already prescribed. (Preparation of Infusion, Resolution 1.) Not less than 500 c.c. of the infusion shall be extracted at a temperature not exceeding 50° C., after which the temperature shall be gradually raised to 100° C.,[192] and the extraction continued till the percolate is free from tannin and the whole made up to one liter, the weaker portions of the solution being first concentrated if necessary by evaporation in a flask, in the neck of which a funnel is placed.
[192] In substances which, like canaigre, contain a large quantity of starch, the extraction should be completed at a temperature of 50° C.—H. R. P.
Section IV.—Determination of Tanning Matters and Non-Tannins; etc.
1. Total Soluble Matter.—100 c.c.[193] of the clear filtered tanning solution, or a smaller quantity if the balance employed is of sufficient delicacy, shall be evaporated in an open weighed basin of platinum, hard glass, porcelain, or nickel, on the water-bath, and the basin shall afterwards be dried till constant in an air-oven, at a temperature of 100° to 105° C., or at a temperature not exceeding 100° C. in vacuo till constant, care being taken that no loss occurs by splintering of the residue. The use of the vacuum-oven for drying the residues is recommended when possible.
[193] 50 c.c. is sufficient, and is the quantity now generally employed.
2. Determination of Non-Tannins.—That the filter method shall remain the official method until the next Conference, but that members be permitted to employ the chromed hide-powder method of the American Association of Official Agricultural Chemists of 1901 (Appendix C) where it is desired, the fact being clearly stated on the report that the A.O.A.C. method has been employed, and not that of the I.A.L.T.C. (Leeds, 1902, see [note], [p. 480].)
That the “bell form”[194] of filter, as described by Professor Procter, shall be employed; not less than 5 grms. of hide-powder be used; the hide-powder should be so packed in the tube that the detannised liquor shall come over at a rate of about one drop in two seconds; and the filtrate be rejected so long as it gives a turbidity with a clear tanning solution. The filtrate may be used for the determination of non-tannin so long as it gives no reaction with salted gelatine solution.[195] The first 30 to 35 c.c. should be thrown away, and the next 50 c.c. of detannised solution, or an aliquot part of it, evaporated in a weighed basin on a water bath, and then dried till constant in an air-oven at a temperature of 100° to 105° C., or, in vacuo, not exceeding 100° C.
[194] It is obvious that the exact form and dimensions of the filter must be adapted to the character of the hide-powder available, as considerable differences exist in the absorptive power of different samples.
[195] 8 to 9 grams of good gelatine are dissolved in 500 c.c. of hot water, 100 grams of salt added, and the whole cooled and filtered.
3. Hide-Powder.—That the hide-powder must be sufficiently absorbent for use in the filter, and that in a blank experiment conducted with distilled water in the same way as an analysis, the residue from the evaporation of 50 c.c. should not exceed 5 milligrams.
The Freiberg Hide-Powder, made by Mehner and Stransky, containing between 10 and 20 per cent. of cellulose (as suggested by Cerych), is recommended by the Conference (Liège, 1901) and is very suitable for the filter method; but the powder, when analysed by the Kjeldahl method and calculated to 18 per cent. of moisture, must not contain less than 11·5 per cent. of nitrogen (Leeds, 1902).
4. Determination of Moisture and “Total Dry Matter.”—That the moisture in the sample be determined by drying a small portion at the temperature adopted in the determination of the “total soluble.” In extracts yielding turbid solutions which can be thoroughly mixed, it is generally preferable after mixing the solution and before filtration, to measure off and evaporate 50 c.c. for the determination of total dry matter (and moisture) in the same manner as the “total soluble.”
5. Statement of Results.—It is recommended, when full analysis is given, that the Statement should be made in the following manner:—
(1) Tanning Matters Absorbed by Hide.—Obtained by deducting the “soluble non-tanning matters” found by evaporating the hide-powder filtrate from the “total soluble.”
(2) Soluble Non-Tanning Matters.—Found by evaporation of filtrate from hide-powder filter.
(3) Insoluble.—By deducting “total soluble matter” from the “total dry matter.”
(4) Moisture.—Determined by drying a portion at the temperature adopted in the determination of “total soluble.”
If other determinations are given they shall form a separate additional statement.
Density.—The statement of densities of extracts, etc., should be given as specific gravity in preference to arbitrary degrees, such as Baumé, Twaddell, etc.
Section V.—Colour Measurement.
Colour Measurement.—It is recommended that the method used by English chemists, namely, measuring with Lovibond’s Tintometer (as described by Professor Procter and Dr. Parker, Journ. Soc. Chem. Ind., 1895, 125), shall be used, and the results stated in units of red, yellow and black. The measurement may be made on the solution used for analysis, but must be calculated to one containing 0·5 per cent. of tanning matter, in a centimetre cell.
Analysis of Used Liquors.
It was decided at Liège, 1901, and Leeds, 1902,[196] that the “Shake Method” with chromed hide-powder, of the American Association of Official Agricultural Chemists, 1901 (A.O.A.C.), should be employed in the detannisation of used tanning liquors, as with these the filter method is apt to give too high results owing to the amount of non-volatile acids which they contain. The method of the A.O.A.C. is given in [Appendix C].
[196] Procter and Blockey quoted experiments at the Leeds Conference, proving that gallic acid and some other non-tanning substances were largely absorbed by the hide-powder filter, though probably not permanently retained by leather; while the error, though still considerable, was much less when the chromed hide-power shake method was employed. Where only gallotannic and gallic acids are present, as in the case of sumach and commercial gallotannic acids, the most accurate quantitative estimation is probably that by the Löwenthal method carried out as described L.I.L.B., p. 123, but considerable skill is required in its execution.
Analysis of Spent Tans.
It was decided at Leeds, 1902, that spent tans must be analysed like fresh tanning materials; but where the prescribed strength of solution cannot otherwise be obtained it is permissible to concentrate the entire solution by evaporation. It is advisable, where suitable apparatus is available, to concentrate in vacuo; but failing this, an ordinary flask may be used, in the neck of which a funnel is placed.