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color. The following weights of precipitate were obtained on successive washings : Initial weight, gram After 6 washings with acetone, gram After 6 more washings, gram -4fter 6 more washings, gram
0.5627 0.5625 0.5625 0.5624
0.3675 0.3672 0.3671 0.3671
The greater difference between the first and second weights may in part be due to the condensation of moisture on the crucible while the weights were being adjusted; in subsequent weighing the weights were placed on the pan of the balance before removing the crucible from the desiccator. But even disregarding this possibility, it is evident that the compound is much less soluble in acetone than in 80, per cent alcohol. The use of acetone has several advantages over the series of wash liquids recommended by Spacu and Dick. It removes the adhering ammonium dichromate as readily as the alcohol, and since acetone is almost as volatile as ether, the use of absolute alcohol and ether is unnecessary when the gravimetric method is used. The procedure adopted was to wash six to eight times with ordinary acetone after the precipitate had been transferred and washed with the ammonium dichromatepyridine solution. Air was then drawn through the crucible for 5 minutes to evaporate the acetone, after which the crucible was wiped with a clean cloth and placed in a vacuum desiccator for 10 to 15 minutes. Gravimetric and iodometric results when acetone was used as a final wash liquid are shown in Table 11. These results are not as superior to those obtained with the alcohol wash as had been expected. It may be that in the presence of the dichromate which adheres to the precipitate little or none of the precipitate dissolves and that the six to eight washings recommended are just sufficient to remove this excess completely without dissolving any ap-
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preciable quantity of the precipitate. Such an assumption would account for the excellent results reported by Spacu and Dick. It appears, however, that acetone is the more advantageous wash liquid, if for no other reason than that by its use three washing mediums may be replaced by one. TABLE11. RESULTS WITH ACETONE HgCIz Taken Gram 0.1443 0.1792 0.1619 0.1579 0.2654 0.1733
HgClz Volume of HgClz Weight of Found 0.0975 N Found [HgPyz]CrzO7 (from 001. 2) NatSzOs (from 001. 4) Gram Gram M1. Gram 0.3057 0.1444 32.76 0.1445 0,3785 0.1788 40.53 0.1788 0.3421 36.69 0.1616 0.1019 0.3338 0.1577 35.69 0.1575 0.6625 0.2657 60.07 0.2650 0.3672 0.1735 39.25 0.1732
Summary Mercury may be determined indirectly by precipitating [HgPy2]Cr2O? and titrating the dichromate in the precipitate. The titration may be effected iodometrically or with ferrous sulfate, either potentiometrically or with diphenylamine as indicator. The importance of the use of pure pyridine is emphasized. The substitution of acetone for the wash liquids previously proposed simplifies and improves the washing technic.
Literature Cited (1) Briggs, 2. anorg. allgem. Chem., 56, 254 (1908). (2) Caley and Burford, IND.ENQ.CREM.,Anal. Ed., 8, 43 (1936). (3) Spacu and Dick, 2.anal. Chem., 76, 273 (1929). RECEIVED July 20, 1936. Constructed from part of a dissertation submitted by Harold M. State in partial fulfillment of the requirements for the degree of doctor of philosophy, Princeton University, 1936.
A Constant-Volume Dialyzer CHESTER B. KREMER College of the City of New York, New York, N. Y.
C
OKSTANT-volume dialyzers previously described are of two main types. The first is the “pressure dialyzer” ( I ) , while the second involves the “evaporation method” technic which depends upon evaporation to maintain (or attain) the original volume. A weakness of the first lies in the fact that the final v o l u m e is usually f o u n d to have increased, as it is difficult to fill the dialyzing sac i n i t i a l l y t o the distended condition it attains a t the end of the dialysis. The second method may be criticized because deposited material on the sides of the container may become irreversible; and because of the e f f e c t of a t m o s pheric gases, and of temperature if heat is used as an aid in evaporating. T h e
latter point is frequently overlooked, although it is well known that many metal oxide hydrosols undergo extensive changes upon heating (2-6). To overcome these difficulties, a simple and practical constant-volume dialyzer has been developed in these laboratories. Details are shown in the figure. A bottle of the type illustrated is fitted with a two-hole rubber stopper. A membrane, wide enough t o fit tightly over the stopper chosen and long enough just t o touch the bottom of the bottle when the stopper is tightly inserted, is used. Through the rubber stopper pass an inlet tube extending to the bottom of the dialyzing sac, and an outlet tube. The liquid to be dialyzed is placed in the bottle, the distended membrane, filled with distilled water supplied from a source 3 feet above the table, is inserted, and the bottle is tightly stoppered. Care should be taken t o exclude air bubbles. Distilled water is then passed through the dialyzing tube at any desired rate.
Literature Cited (1) Holmes, H. N., “Laboratory Manual of Colloid Chemistry,” p. 19, New York, John Wiley & Sons, 1928. (2) Thomas, A. W., and Kremer, C. B., J. Am. Chem. Soc., 57, 1821 (1 R.Z.5). \ - - - - I -
(3) Thomas, A. W., and Owens, H. S., I b i d . , 57, 1825 (1935). (4) Thomas, A. W., and Tai, A. P., Ibid.,54, 841 (1932). (5) Thomas, A. W., and von Wicklen, F. C., I b i d . , 56, 794 (1934).
RECEIVED February 8, 1936.
