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position 2; the outer layer of rubber is then worked down over the inner layer by means of the fingers until the opening for the crucible is just a little less than that of the funnel itself.

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MANNER OF FITTING RUBBER TUBING TO GOOCH FUNNEL

In preparing the asbestos mat, some short fiber (about 0.5 cm.) asbestos, which has been washed in acid and ignited, is suspended in water and let stand for a short time to allow the asbestos to settle. The top portion of the water is discarded, after which enough of the asbestos pulp is allowed to flow upon the crucible, which is under slight suction in the set-up just described, until the mat will have a thickness of 1-2 mm. when spread out evenly. The suction is now broken by slipping off the rubber connection from the side-neck Erlenmeyer flask, the crucible filled with distilled water and the asbestos stirred up by a stirring rod; suction is then applied and the mat sucked down tight; this procedure is repeated three or four times in order to get rid of very short fibers and any soluble matter. The crucible is now taken by means of a pair of tongs (the crucible should never be handled by the fingers from the time it is placed in the funnel until after the final weighing has been made) and placed over a very small Bunsen flame for a few minutes to drive off any water; the flame is then gradually increased and the crucible strongly ignited for five to ten minutes. After being allowed to cool in the desiccator, it is placed in the filter funnel and washed with about 200-300

c.c. distilled water. It is ignited a second time, allowed to cool in the desiccator and weighed. It is next washed with the same quantity of water, ignited, allowed to cool, and weighed a second time. The two weighings should agree within 0.1 mg. for platinum Gooches and 0.2 mg. for porcelain Gooches. If they do not, successive washings, ignitions and weighings are made until this concordance is attained. The great advantage of igniting the asbestos mat is not only that it brings about a much finer intermeshing of the fibers, thus allowing the filtration of the finergrained precipitates, but it also reduces the number of treatments necessary to bring the crucible to constant weight.

An asbestos mat made from a good grade of asbestos fiber, does not seem to be affected when treated by acid solutions up to one molar or by alkaline solutions which are not too strongly alkaline, such as ammonium hydroxide or dilute alkaline carbonate solutions. The practice of using a Gooch crucible for more than one determination is not to be recommended; a fresh Gooch should be prepared for each determination.

47. Munroe Crucible.11- This is really a modified Gooch crucible in which the filter medium, instead of being made of asbestos fibers, is an extremely thin layer of porous platinum deposited on the bottom of the Gooch crucible. Munroe crucibles are always of platinum, whereas Gooch crucibles may be either platinum or porcelain. The advantages of the Munroe crucible are its rapidity of filtration, retention of the finest precipitates, such as calcium oxalate, barium sulphate, etc., its (almost) insolubility in fairly concentrated acids and alkalies, and its requirement of almost no previous treatment to get it ready for a filtration.

Preparation of Munroe Crucible. According to Snelling,12 who made an extended investigation of the method for preparing a Munroe crucible, a concentrated solution of chlorplatinic acid containing the equivalent of 0.450 g. platinum is precipitated by a solution of ammonium chloride and the resulting ammonium chlorplatinate washed several times by decantation with water and then with alcohol. A perforated platinum crucible is now placed upon several layers of filter paper and held firmly in this position while the alcoholmoist precipitate of ammonium chlorplatinate is poured into it so as to form a uniformly distributed layer of from 0.25 to 0.50 cm. thickness. As soon as

11 C. E. Munroe, J. Anal. Chem. 2, 241 (1888); also Chem. News, 58, 101 (1888). 12 W. O. Snelling, J. A. C. S. 31, 456 (1909).

the alcohol has been absorbed by the filter paper, the crucible is dried in a water or electric oven at about 100° until the alcohol has been entirely driven off, after which the crucible is gently heated over a Bunsen burner until the decomposition of the ammonium chlorplatinate has been effected, the heating then being continued and increased until the crucible reaches dull redness. The salt should be shaped during ignition, while after ignition the layer of coherent platinum sponge which has been formed should be gently burnished with a glass rod flattened somewhat at the end. If the mat retains cold, freshly-precipitated calcium oxalate, it may be considered free from cracks and sufficiently fine to hold the finest precipitates.

In case cracks or imperfections have developed in the mat or subsequently develop from any cause,13 the crucible may readily be restored to prime condition by saturating it with concentrated chlorplatinic acid solution and then allowing a moderately strong aqueous solution of ammonium chloride to diffuse upwards throughout the mat by lowering the crucible slowly into a beaker containing the ammonium chloride solution, washing with alcohol, drying and igniting.

Cleaning Munroe Crucible.14 In many instances, a part, at least, of the precipitate may be shaken or gently blown out, but the mat can be completely cleansed only by the aid of solvents, and this method of procedure is an essential feature of the use of the Munroe crucible. It is to be noted that hot concentrated sulphuric acid and hot hydrochloric acid, if used alone, cause a decided loss of the mat, the former by rendering the finely divided platinum colloidal, in which condition it is readily carried through by the wash water, the latter by taking up the platinum as chloride. However, both these acids may be used freely in the crucible without causing the slightest weighable loss of platinum provided that ammonium salts, alkali or ammonium nitrates or nitric acid are added to the sulphuric acid, while ammonium salts other than the nitrate are added to the hydrochloric acid. Besides the solvents just mentioned, Swett describes about forty others, the complete list of which must be sought in the original article. The following partial list will take in such precipitates as are likely to be ordinarily encountered; the reference numbers show the proper solvent to be used with a given precipitate:

List of Solvents

(Salts are applied in aqueous solution unless otherwise specified)

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13 This paragraph drawn almost verbatim from the article of O. D. Swett, J. A. C. S. 31.

932 (1909).

14 O. D. Swett, loc. cit.

Choice of Solvent for a Given Precipitate

(Numbers indicate correspondingly numbered solvents in the preceding list. Hyphens indicate successive treatments, commas indicate alternative treatments

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10h

10, 13

1h, 12h
14h

36 dry-ignition, 10-1
10dh

4, 6, 10

48. Ignition. In igniting precipitates it was formerly the practice to dry the filter paper together with the precipitate in an oven (usually at 100°-105°), then detach as much of the precipitate as possible from the filter paper and ignite the latter separately after which the main portion of the precipitate was added to the crucible and the ignition completed. Instead of following this procedure it is now customary, except when carbon from charred filter paper will reduce the precipitate (see §§ 44, 173), to transfer the moist filter paper and precipitate directly to the crucible and conduct the ignition by first gently heating the crucible (with its cover on to prevent access of air) until all the water is driven off and the filter paper gradually and completely charred. During this stage of the ignition the crucible is maintained in a vertical position. After the carbonization has been effected, the cover is removed so as to allow free access of air. The crucible is next placed in an inclined position on the triangle and the full flame of the burner employed, while every ten minutes or so the crucible is turned slightly by means of the tongs so as to expose fresh portions of the precipitate. Usually it takes about twenty minutes to char the paper and about thirty to forty minutes more to complete the ignition. The crucible

and ignited precipitate are placed in a desiccator, allowed to cool, and weighed. The precipitate is then ignited a second time for the same period as in the first ignition, then cooled and weighed. The two weighings should agree within 0.2 mg., if a porcelain crucible has been employed; if a platinum crucible has been used, then the concordance of the weighings should be within 0.1 mg., provided that temperatures above 1000° have not been used. If temperatures above 1000° have been used, allowance must be made for the change in weight of the crucible due to the volatility of platinum above 1000° (see § 81).

The practice of using covered crucibles is to be recommended. When a hot crucible is placed in a desiccator, part of the air is driven out of the desiccator, so that upon cooling, a partial vacuum is created. When the cover of the desiccator is removed in order to take out the crucible, a sudden in-rush of air results and this will frequently blow some of the precipitate out of the crucible if the latter is not covered.

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49. Drying. As already mentioned in Chapter I, it is customary in the case of many substances, such as minerals, soils, clays, salts, etc., to report analyses to what is known as the "moisture-free basis.” For this purpose it is necessary to make a determination of the moisture in the sample. Instead, however, of actually isolating the water and weighing it, it is a more general practice to dry the sample at some arbitrary temperature between 100° and 110°, until constant weight has been attained and then regard the loss in weight as representing the moisture originally present. This determination of loss in weight should always be made on a separate portion of the sample, as other changes in the composition of the sample may occur during the process, particularly if the sample contains a substance which is easily oxidizable. The procedure is to take a platinum "milk dish," or, lacking this, a platinum or porcelain crucible, heat same in the flame of the Bunsen burner for 5-10 minutes, allow to cool in the desiccator and weigh. Two grams of sample are then placed in the dish or the crucible, after which the dish or the crucible, along with its contents, is placed in a constant-temperature oven or air bath,16 and allowed to remain there for several hours, when it is 16 An electrically heated oven with thermostatic control is most admirable.

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