Total soluble matter.—The solution of which the preparation has been described, must now be filtered, the size and kind of paper, and exact method of filtration prescribed by the International Association being strictly adhered to. All papers and methods of filtration absorb traces of tanning matters, and but few will give a clear filtrate with such solutions as those of quebracho and hemlock extracts; and to obtain uniform results exact uniformity of method is essential.[156] Deviations from the exact method, in the case of quebracho, easily cause discrepancies of several units per cent. in the result. The object of rejecting the first portions of the filtered solution is to prevent, as far as possible, errors which would arise from the absorption of tannin by the paper, and to insure a clear filtrate. 50 c.c. of the clear filtrate is now measured by an accurate pipette, and evaporated to dryness in a weighed porcelain basin, on a steam-bath, in order to determine the “total soluble.” This and succeeding operations should be done in duplicate, even if this has not been the case in making up the original solution, which is certainly desirable.
[156] Methods of correction for absorption of filter-papers have been worked out in the Author’s laboratory, and adopted by the last conference of the International Association. Cp. Collegium, pp. 145-158, 1902, and [App. A], [p. 477].
Ordinary light porcelain basins, generally of about three inches diameter, are employed for evaporation, which takes place somewhat more rapidly if they are flat-bottomed (saucer-shaped). In place of porcelain, thin glass basins of hemispherical form may be used, and, but for the cost, platinum would be better than anything else. Aluminium and nickel basins have been tried, but are slightly attacked by some liquors, and hence are more liable to vary in weight, though they have the advantage in rapidity of evaporation. Evaporation takes place most quickly if the steam-bath can be placed in a draught of air, so as to rapidly carry away the vapour formed, but the basins must be protected from dust. Under favourable circumstances, evaporation of 50 c.c. in porcelain basins occupies one to one and a half hour. An ordinary pan fitted with a lid of thin copper perforated with holes of two and three-quarter inches in diameter, makes a useful water-bath; but where much work is done, it is desirable to have a rectangular bath of thin sheet copper, taking a single, or at most a double row of basins, and fitted with the usual appliance for keeping the water at constant level; or with a supply of steam from a boiler, and an overflow for condensed water.
As soon as the contents of the basins appear completely dry, they may be transferred to the drying oven. The most satisfactory form is one in which the basins are placed in a closed chamber, surrounded by steam at the atmospheric pressure, and at the same time subjected to a vacuum maintained by a water-jet air-pump; but as this apparatus is somewhat costly, it will probably only be provided in laboratories which make a speciality of such work. Next to the vacuum-oven, an air-oven, heated by a gas-burner, and with its temperature controlled by a mercurial regulator to 100-105° C., gives the best results, and it is also the cheapest; but considerable care and some scientific knowledge are required to work it satisfactorily. In intelligent hands good results may be got from the small “breakfast cooker” gas ovens made by Fletcher of Warrington, which are placed on an iron plate heated by a gas burner, the supply of gas to which is regulated by a thermostat, or mercurial gas-regulator, inserted, together with a thermometer, through holes drilled in the top. The basins must not be placed too near the bottom of the oven, which must be protected by a perforated metal plate supported perhaps one inch above it, to prevent radiation and to distribute the hot air. Any cold air required for ventilation should be admitted below this plate, and care should be taken to exclude the products of the burning gas. Contact of the basins with any heated part of the metal-work should be carefully avoided, and they are best supported on grid-shelves covered with wire gauze or perforated metal, so as to allow of free circulation of air. If perforated zinc is used, it must be well supported, as it is much softened at the temperature used. The least satisfactory appliance in skilled hands, but probably the most easy to work by the inexperienced, is the ordinary water- or steam-oven. It is impossible, in this apparatus, to raise the temperature of the interior fully to boiling point, and below this gambier, quebracho, and other solutions containing catechins ([p. 298]), dry very slowly. On the other hand, so long as it is kept boiling and supplied with water, the temperature is necessarily constant, and there is no danger of overheating, which easily occurs in ovens heated directly by gas. Such ovens are often fitted with openings at the top for use as a steam-bath. To get the best results, the basins must be as freely exposed as possible to the air in the interior of the oven (in no case must basins be set one inside another, except in the exsiccator for cooling), and little or no ventilation from the outside is required, as only traces of moisture remain after evaporation on the steam-bath; so that, after an hour’s drying, any ventilators may safely be closed. As a good deal of cooling takes place through the door, it is best to protect it with some non-conducting material, such as asbestos millboard, which may be attached with rivets, or even with ordinary paper-fasteners. One to one and a half hours will be required to dry to constance in the vacuum-oven; two to three in the air-oven at 105°; and probably about four hours in the water-oven, except in the case of gambiers, which may require somewhat longer. Too long heating is disadvantageous, as the residues begin to oxidise and gain in weight. As soon as it is judged that the basins will be constant in weight, they are withdrawn from the drying oven, and at once placed in an exsiccator (a glass vessel with an accurately fitted lid, which should be slightly greased, in the bottom part of which is placed either dry calcium chloride or concentrated sulphuric acid, to absorb the moisture of the air it contains). In this they are left till thoroughly cold, which if several basins are put in together, may require half an hour. They are then weighed accurately, but as rapidly as possible; returned to the drying oven for half an hour; and replaced in the exsiccator. The exact weight of each basin, as it comes in turn to be weighed, is now placed on the balance before removing the basin from the exsiccator, so that it can be seen instantly if there is loss or gain of weight, before it has time to absorb any moisture from the air. The weight should not be more than a milligram or so less than at the first weighing; if weight has been gained, it is caused by oxidation, and the first weight should be taken as correct if it is certain that the basin was then perfectly cold; a very slight amount of warmth easily reducing the apparent weight by several milligrams. If material loss has occurred, the basin must of course be returned to the oven, and re-weighed in another half hour; but with experience, this should rarely be needed.
It is necessary that the balance used should weigh accurately to milligrams; and it must carry at least 50 grm. on each pan; while it is more convenient that it should carry 100 or more, it is always possible with a little ingenuity, to manage within 50 grm.; and if a cheap balance must be used, the smaller size will probably be more accurate. Balances of this sort can now be got for two or three pounds, though it is in all respects better to obtain one of first rate quality, which should cost about ten pounds. The balances of Verbeek and Peckholdt, of Dresden, from their simplicity and rapidity of weighing, have given great satisfaction in technical work in the Yorkshire College. Whatever economy be exercised in the choice of the balance, it is essential that the set of weights should be of the greatest accuracy, and especially that all the weights of one denomination (10 grm., 1 grm., etc.), should accurately balance each other. Even after all precautions are taken, it is desirable that those weights which are in duplicate should receive distinguishing marks (e.g. with a centre-punch), and should always be placed on the scale in the same order; and, not only on account of possible inaccuracy, but to save time, it is desirable to reject basins which are so nearly of an even weight (20, 25, 30 grm.) that when weighed with the residue (0·3-0·4 grm.), a change of the larger weights may be required, since it is to be remembered that any error of the weights employed is concentrated on the small weight of the residue.
After deducting the weight of the empty basins, the weight in milligrams of the two residues of 50 c.c., which should be practically alike, are added together, and the sum divided by the weight in grams of tanning material used; which gives the percentage of “total soluble matter.”
Fig. 66.
Hide-powder Filter.
Non-tannins.—It is now necessary to determine the proportion of the “total soluble” which consists of “non-tanning matters,” that is, of substances not removed from the solution by treatment with hide-powder. The so-called “tanning matters” removed, include colouring matters and some other substances, which though absorbed by hide, are certainly not tannins in a strictly chemical sense. (See [note], [p. 480].)
According to the method of the International Association, the apparatus shown in [Fig. 66] is employed for this purpose. The glass bell is carefully and uniformly stuffed with hide-powder, care being taken that no channels are left, especially at the sides, through which the liquor can reach the syphon without traversing the hide-powder. Before filling the bell, the short leg of the syphon-tube should be loosely plugged with cotton-wool (of which a little is allowed to project from the end), in order to prevent the powder from gaining access to the tube. The powder is retained in its position in the bell by a piece of muslin held by an indiarubber band, and the bell is then placed in a beaker or tumbler as shown in the figure; and filtered liquor is gradually added, as it is absorbed by the powder, till the whole is uniformly wetted. The liquor which was first filtered through the paper, and rejected for “total solubles,” may be used for this purpose, and it is not necessary that it should be absolutely clear. The syphon is now gently sucked, and the filtrate is allowed to fall, drop by drop, into a gauged cylinder. The first 30 c.c. which collects is rejected, since it contains traces of dissolved hide-substance even from the purest hide-powder; and the next 50 c.c. should give no turbidity if a few drops are mixed either with clear tannin solution (absence of dissolved hide-substance), or with the first 30 c.c. (absence of tannin). This 50 c.c. is used for determination of non-tannins, by evaporation and drying precisely as has been described in the case of “total soluble.” Some chemists, with very accurate balances, prefer to evaporate only 25 c.c., which effects a little saving of time in evaporation; but in any case the whole of the 50 c.c. must be allowed to run through the filter before it is measured, as the filtrate varies somewhat in solid contents as the filtration proceeds. The filtration and evaporation should be done in duplicate. The weight of the residue is calculated into percentage as “soluble non-tanning matters” precisely as has been described for the “total soluble”; and when subtracted from the latter, the remainder is the percentage of “tanning matters.” If the hide-powder now employed by the English members of the International Association (manufactured by Messrs. Mehner and Stransky in Freiberg in Sachsen), be employed, no difficulty will be found in the filtration. This powder is quite neutral, and contains between 10 and 20 per cent. of cellulose to render it more absorbent. It does not swell in the filter, and hence should be stuffed into the bell almost as tightly as possible, about 10 grm. being required. If the bell is properly filled, the filtration should altogether take about one hour, but if the liquid runs too fast, it must be regulated by a pinchcock on the indiarubber tube of the syphon. If other powders are used, which often contain acid, and swell very much in the bell, the filling is much more difficult, and while the sides of the bell must be closely packed, great care is requisite to keep the powder loose in the centre, or the filter will not run. One point requires mention with regard to neutral hide-powders. If an extract which has been rendered soluble by the addition of alkalis or sulphites ([p. 388]) be analysed with a perfectly neutral powder, it has been shown by Paessler and Appelius[157] that a part of the tannin combined with the alkali will not be absorbed, while with acid powders, the whole will be estimated.