ON T H E CONDITIONS O F PRECIPITATIO;?;, BY ELECTROLYTES, O F SELENIUM HYDROSOL AND OTHER HYDROSOLS BY JAMES J. DOOLAN
While there is difference of opinion as to the manner in which colloid particles receive an electric charge, the statement of Jevonsl, and of Hardy2, that such a charge exists, and is the chief factor in preventing coagulation is now generally accepted. Schulze3, and somewhat later, Linder and Picton4, working with arsenious sulphide hydrosol, shewed that trivalent cations exert a greater precipitating effect than divalent ones, and that monovalent cations were least effective . Whethams, pointed out that the figures of I h d e r and Pictori for the ratios of the precipitating powers of equimolecular solutions of monovalent, divalent and trivalent cations may be represented by the formula I : x : : x2,x being a constant of 30 to 40. Many exceptions to this Schulze-Picton-Linder “Law” have since been observed. Some of the most striking deviations from the “Law” are found in the results obtained by O d W in his investigation of the precipitating effect of electrolytes on sulphur hydrosols. An attempt is here made to determine whether similar discrepancies occur with hydrosols of the allied elements, selenium and tellurium. O n Selenium Hydrosol. The existing literature contains various methods for preparing this sol. The method used by Schulze7, using sulphurous acid, was used throughout this work. Preparation and Purification of Selenium Dioxide. It was found impossible to purify selenium dioxide by sublimation when prepared by the methods ordinarily described; some decomposition to the element caused a pink discolouration. Accordingly recourse was made to sublimation in a current of air. Even now a faint rose pink persisted. Hereupon, at the suggestion of Professor Partington the idea was employed of passing over this sublimed product a stream of ozonised oxygen at steam heat. This proved effective in bleaching the sublimate. Although selenium trioxide was then unknown, tests were made with barium chloride in case any of the higher oxide had appeared. KOprecipitate was formed, although the solubility of barium selenate is only 82. j gms. per litres. Trans. Manchester Phil. Soc. 1870, p. 78. J. Physiol. 24, 288 (1899). 3 J. prakt. Chem. 25, 431 (1882). J. Chem. Soc. 67,63-74 (1895). Phil. Mag. (5) 48, 474 (1899). 6 Z . physik. Chem. 78, 682-707 (1912). 7 J. prakt. Chem. (2) 32, 390 (1885). 8 Meyers and Friedrich: Z. physik. Chem. 102, 369. 1
PRECIPITATION O F SELENIUM EIYDROSOL
I79
Further, selenium trioxide has since been isolated by the action of ozone on selenium, but its discoverers’ found it necessary to dissolve the element in selrnium oxychloride. The pure colourless selenium dioxide eventually obtained as above was sublimed without any trace of decomposition, but was so hygroscopic that it was necessary to keep it in a desiccator over phosphoius pentoxide to prevent its passing to selenious acid. Preparation of Selenium 801. A uniform concentration of nominally, 0.5 gm. of selenium dioxide to the litre, was employed. The best procedure was found to be that of dissolving the compound in a small quantity of water at about ZOOCand then passing gaseous sulphur dioxide until 0.58 gm. had been added. (Gutbier’s optimum for precipitation.) The whole was then made up to a litre with conductivity water, when the red precipitate first formed redissolved completely to a ruby sol. Dialysis of Selenium Sol. References in the literature of the subject are mainly to the undialysed sol, or to sols which had been stablised, say by sodium lysalbinate or sodium protalbate. In this work however it did not appear justifiable to assume that a stabilsed Selenium hydro sol possesses the characteristics of one in which selenium and water were alone concerned. Zsigmondy2 states, for example, that these three phase complexes take on many of the characteristics of the emulsoid. Further, results obtained by Billiter on the effect of electrolyte on the mobility of the particles of protected gold sol were not confirmed by Thitney and Blake3, using the same sol unprotected. They attribute the discrepancy t o the gelatine used as a stabiliser. It was therefore decided to attempt purification by dialysis, of unprotected sols. As might be expected, considerable difficulty was here encountcred. Parchment Faper “sausage skin dialysers” were first employed. Although these were well washed to remove any sulphates, so easily did the sol precipitate in them that they were abandoned, as being too destructive on material, in favour of collodionised Soxhlet thimbles. Green’s 30 n1.m. x I O O m.m. thimbles were selectrd as being of suitable capacity. Even now precipitation occurred at first, leaving a very dilute sol. On the second attempt with the same thimble, however, no precipitation occurred. Hence by using a battery of these “srasoned” thimbles dialysis proceded successfully. On starting new thimbles at any stage in subsequent working the phenomenon was again encountered. In the absence of any coagulating electrolyte it may have been due to membrane adsorption. The sign of the particles was determined by cataphoresis with the ordinary Lodge apparatus and found to he negative. During the research, all colloid ‘solutions’ had to be kept in the darkroom, and dialysis was also carried out there. This was due to the fact that light causes the rapid precipitation of the sol, apart from electrolyte action. R.R. Le G. Worsley and H. B. Baker: J. Chem. SOC. 123, 2870 (1923).
* “Kolloidchemie”
Eng. Trans. p. 86, (1917). J. Am. Chem. Sac. 26, 1339 (1904).
I 80
JAMES J. DOOLAN
The hydrosols were not dialysed entirely free from electrolyte. Attempts to do so always resulted in the deposition of the selenium as a more or less coherent layer on the walls of the thimble. G. liosail foui!d the same occurs with sulphur hydrosol containing sulphur compounds. Consequently a method was employed in which dialysis was carried on until a content of electrolyte was reached which was the minimum consistent with stability of the sol. The attainment of this concentration was indicated by the use of ,z dipping electrode, which was thoroughly cleaned and placed in the sol at intervals; the resistance being determined each time by means of Kohlrausch’s appaiatus, until a certain known value was obtained. Dialysis was usually complete in about a weck, with changes of conductivity water twice in the twenty four hours. The known value to which the resistance was taken was 9000 ohmq, as against 60,000 ohms with conductivity water in the same apparatus and about 1000with ordinary water. This represents a sol of specific conductivity 2.4 x IO-5 mho. In this manner the sol could be reproduced exactly whenever a fresh batch was required. The dialysed sol, when placed in containers of resistance glass which had been steam blown after the usual chemical cleaning, were quite stable for a month or six weeks in the dark room. One batch which had been left for a year was found to have thrown down a slight sediment, but the supernatant sol was clear.
Additaon of Electi.olyte to the Eznlysetl H!ld~osol. T’arious standard solutions were made up from materials pure for quantitative analysis, and small quantities of the hydrosol were titrated by them until perceptible coagulation occurred. This method does not take account of the time factor, and that the concentration of the sol varies with different amounts of the coagulating solution. For exact determinations the method selected was one due to Hatschek*; the titration expts. being, however, useful in affording a rough idea of the concentrations required, and thus indicating the strengths of precipitating solutions to be employed. A set of resistance-glass coagulation tubes were obtained, each tube fitted with a glass stopper and graduated at 18 C.C. and at 2 0 C.C. Eighteen cubic centimetres of the dialysed sol were placed in a tube and two cubic centimetres of the coagulating solution added. Thus the Coagulating solution was diluted to one tenth of its original concentration and the concentration of the hydrosol was always the same. The sol and solution were mixed in each case by a uniform procedure and allowed to stand for two hours before examining. Kolloid-Z. 30, 228-230 (1922). “Laboratory Manual of Colloid Chemistry,” p. 91 .”
PRECIPITATIOS O F SELENITM HYDROSOL
I81
llillimols per litre
Reagent
NaCl AmCl MgSOr ZnS04 Ba C1, Alz(S04)a
1000 1000
30 40 15 2
More dilute solutions of known strength were prepared by mixing in testtubes, 8 c.c. of solution with 2 C . C . of water; 6 C.C.of solution with 4 C.C.of water; 4 C.C. of solution with 6 c.c. of water; and 2 c.c. of solution with 8 c.c. of water; these being labelled 0.8; 0.6, 0.4, and 0.2; their respective concentrations referred t o the stock solution. I n cases, where, say, solution 0.6 failed to precipitate the sol after two hours, and that marked 0.8 completely precipitated it; a third solution of strength intermediate to these was tried. and in this way the exact precipitating concentration was obtained. The limiting concentrations so found are set out in the following table:Reagent
NaCl WHdCl IZnSO4 MgSO, BaCL .412(S04)z
Precipitating Sol. *4
.4
.6 Stock ( I .o) .9
.5
Limit Concentration 40.00 40. do 2.40
millimols per litre. ” lf
3 .oo
’,
1~35
lJ
0.10
f l
>l
Jf
Jf
1,
11
)J
)l
>J
11
I t can at once be seen that the order of importance is trivalent, divalent and monovalent for the ions, no monovalent ion being as powerful a precipii,ant as a divalent one, and aluminium being the most powerful. The lines of demarcation are clearly marked, also, as between monovalent ions and divalent ions, and between the latter and trivalent ions. This shows the behaviour of selenium hydrosol to be materially different from that of OdBn’s sulphur, for which barium acts with
