Determination of Mercury An Indirect Volumetric Method Based upon a Critical Study and Improvement of the Bichromate-Pyridine Method of‘ Spacu and Dick N. HOWELL FURMAN
AND HAROLD M. STATE Frick Chemical Laboratory, Princeton, N. J.
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RIGGS (1) prepared and described pyridine-bichromate complexes of the ions of Cu, Ni, Co, Zn, Mn, Ag, Hg, and U. All these complexes are more or less insoluble. Spacu and Dick (3) have recommended the precipitation and weighing of mercury as [HgPy2]Cr207. I n their hands this method gave excellent results. It seemed feasible to titrate the dichromate in the precipitate either with a standard reducing agent or iodometrically. Such a titration would have a very favorable factor, since the equivalent of mercury in any titration of this sort is Hg/6. The washing technic proposed by Spacu and Dick has been found to be successful through a compensation of errors, and a less empirical method has been devised for this part of the procedure.
Preparation of Materials Reagent grade mercuric chloride was used as the material for analysis. Analysis of this salt after drying at 110” C. gave by the method of Spacu and Dick 73.91 and 73.80 per cent of mercury and by the method of Caley and Burford (I)73.83 and 73.99 per cent. Theory requires 73.88 per cent. The material was accordingly used without further purification. Merck’s “reagent” pyridine and ammonium dichromate of the same quality were used throughout the investigation. It was found more convenient to add 10 ml. of a solution containing 20 grams of ammonium dichromate per 100 m!. instead of 2 grams of the solid salt as recommended by the original investigators. [It is desirable t o use pure pyridine of the correct boiling point (115.2’ C.). Very erratic results may be obtained if the pyridine contains other or anic bases.] Approximately 0.1 solutions of ferrous ammonium sulfate and of sodium thiosulfate were prepared from reagent grade salts and standardized against triply recrystallized potassium dichromate according to established procedures. In the case of the former solution, the standardization was done just before use, and the method of determining the end point was the same as that to be used for the titration of the mercury salt. The potassium iodide used in the iodometric titrations, although slightly yellow in color, gave no color with starch either in neutral or in acid solution. Two wash solutions were required. The first, used to transfer the precipitate to the filtering crucible, contained 0.5 gram of ammonium dichromate and 0.5 ml. of pyridine per liter; the second, for washing out the above liquid, was made by adding 12 ml. of water to 88 ml. of 95 per cent alcohol. If the gravimetric method is to be used, a third wash mixture containing 1 drop of pyridine per 10 ml. of absolute alcohol is used. The residue of this wash solution is removed from the precipitate prior t o weighing by washing with ether. The indicator solutions were freshly prepared 1 per cent “soluble” starch and 1 per cent diphenylamine in concentrated sulfuric acid.
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PRECIPITATION OF MERCURY AS [HgPyzl Crz07.
The precipitation of mercury as [HgPyJ C1-207 was done exactly as described by Spacu and Dick. The procedure is in brief as follows:
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Individual samples of mercuric chloride were weighed into 250-ml. beakers and dissolved in 140 ml. of water. When dissolved, 10 ml. of ammonium dichromate solution were added. The addition of 1 ml. of pyridine t o the solution, while stirring it vigorously, precipitated the bright yellow corn ound which after standing 10 minutes was filtered through a gyms crucible. The recipitate was transferred completely to the crucible with the $chromate pyridine solution, and then washed six to eight times with 2- t o 3-ml. portions of 80 per cent alcohol to remove the excess dichromate. It is important that the walls of the crucible 467
as well as the precipitate be washed with this liquid. The crucible was then placed in a drying oven for 15 to 20 minutes to remove the alcohol, Failure to do this always resulted in low values, due to reduction of the dichromate by the alcohol when the precipitate was dissolved in acid. The procedure to this point is common to each of the three volumetric methods.
Conventional volumetric methods for the titration of bichromate were applied after the precipitate had been dissolved. For the potentiometric method with ferrous sulfate and for the iodometric method, 50 ml. of 1.2 N hydrochloric acid were used in small portions to dissolve the precipitate; the acid was 2.4 N in the other case. The solutions which were titrated with ferrous sulfate contained 20 ml. of 25 per cent phosphoric acid, and the initial volume was approximately 100 ml. Typical results by three methods are presented in Table I. TABLEI. DETERMINATION OF MERCURY BY TITRATION OF SOLUTION OF [HgPyz]Crz07
THE
Potentiometric Titration with Ferrous Sulfate HgClz present, gram 0.1236 0.2755 0.0871 0.1301 0.1322 0.1962 HeCLfound. 0.1235 0.2752 0.0868 0.1300 0.1321 0.1946 . gram Titration with Ferrous Sulfate; 1% Diphenylamine Indicator HgClz resent, gram 0.1398 0.1741 0.0875 0.1487 0.1275 HgClzfound, gram 0.1397 0.1732 0.0872 0.1488 0.1272 Liberation of Iodine and Back-Titration with Thiosulfate HgClz present, gram 0.0908 0.1138 0.1026 0.1874 0.2263 HgClzfound, gram 0.0906 0.1137 0.1027 0.1869 0.2261
Diphenylamine sulfonic acid was also tried as indicator instead of diphenylamine, but the color change was erratic and the end point could not be determined with precision. USE OF ACETONEFOR WASHINGAND DRYING [HgPy2]Cr207. On the average, results by all methods are somewhat too low. A check of the various steps in the procedure revealed that, even after twenty washings with 80 per cent alcohol, this wash liquid always had a faint yellow tinge, indicating the removal of dichromate from the precipitate. I n no case, however, could mercury be detected in these washings with either hydrogen sulfide or stannous chloride. I n spite of Spacu and Dick’s insistence that the complex compound is insoluble in all wash liquids recommended, a steady loss of weight was observed each time the precipitate was washed with 80 per cent alcohol: First weight of precipitate, gram After 7 more washings, gram After another 7 washings, gram
0.2750 0.2741 0.2733
0.3072 0.3062 0.3055
The density of the alcohol agreed with that given in the original paper; the solution was only slightly acid, showing a p H of approximately 6 with nitraeine paper. To determine whether the slight acidity might be responsible for the solubility of the compound, some alcohol was left standing over calcium oxide for 20 hours and then distilled from the oxide. This was diluted t o a specific gravity of 0.849 with boiled water. The washings with this liquid were still yellow, however, even after fourteen washings. The addition of a drop of pyridine to 10 ml. of the alcohol did not decrease the yellow tint of the washings. The results with 95 per cent alcohol were no better. Acetone, on the other hand, apparently dissolved the compound only to a slight extent, for the washings showed no
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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. E d . , 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.
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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., Ibid., 57, 1825 (1935). (4) Thomas, A. W., and Tai, A. P., Ibid.,54, 841 (1932). (5) Thomas, A. W., and von Wicklen, F. C . , Ibid., 56, 794 (1934). RECEIVED February 8, 1936.
