COLLOIDAL BORON BY ALBERT L. ELDER AND NAOMA D. GREEN
Svedberg' reports that colloidal boron has been known nearly as long as amorphous boron. Kuze12was granted patents for the preparation of colloidal boron. Ageno and Barzette3 have contributed to the study of colloidal boron. A more extended report was given by Gutbier* who prepared sols by reduction of Bz03 with Mg, Na, and K. The fusion mixture was purified by treatment with dilute and conc. HC1 followed by washing with distilled water until the material which went through the filter was in a colloidal state. His failure to obtain consistent results suggested the possibility of continuing this investigation. Sols prepared using identical procedure by Astfalk and Boss under his direction varied greatly in rate of settling, stability under dialysis, color, rate of flocculation with electrolytes. They recognize that the colloidal boron was not pure boron. Our preliminary experiments using the procedure outlined by Gutbier gave negative results or at best extremely dilute sols. The general procedure was the fusion of Mg and BzOs in approximately the ratio of I to 3. The fused mixture was cooled, pulverized, treated with conc. HCl, and washed on a suction filter with hot distilled water. An extremely dilute sol was sometimes obtained on long washing. Further washing yielded no sol. In an attempt to prepare a more concentrated sol fusions containing Mg and Bz03 in ratios varying from I to I O to I O to I were made. The most concentrated sol was with a mixture of Mg and Bz03 in the ratio I to 3 . It was discovered that refluxing from 0.5 to I g of dried purified boron with IOO to 2 5 0 cc of distilled water for from 2 to 2 . 5 hours gave on filtration much more concentrated sols. The fusion container was an open iron vessel 8.5 cm deep by 4 cm in diameter having walls 4 mm thick. The reaction was started by the use of two oxygen-gas blast lamps which were focused on the lower part of the container. T o insure rapid fusion the time of heating was less than five minutes. The reaction once started is very vigorous. It is advisable to grind the fused, cooled mass before treating with conc. HCl. The addition to the fusion mixture of certain impurities which might have been found in the raw materials of previous investigators altered the concentration of the colloidal material. For example the addition of NaZS04in concentration of 0.01and 0.001% of BzOs by weight caused definite increase in the concentration of sols obtained. NaN03 and NaCl in equivalent concentrations were not as effective as NazS04. The addition of traces of iron filings to the fusion mixture increased the concentration of the sol. 2
3
Gutbier: Kolloid-Z., 13, 137 (1914). British Pat., 25,864;French Pat., 371,799;German Pat., 197,379; C.A., 1, 1930(1906). Atti. Accad. Lincei, 1, 1824 (1910). Kolloid-Z., 13, 137 (1914).
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ALBERT L. ELDER AND NAOMA D. GREEN
Study was made of some of the properties of these sols. The sols prepared by refluxing were found by electrophoresis measurements to be electronegative. The acid sols were coagulated very quickly when subjected to 220 volt potential while the neutral sols were not. Since the colloidal boron was prepared by refluxing the fusion residue with boiling water the sols are obviously rather insensitive to coagulation by heat. In contrast with the colloidal solutions of boron reported by Gutbier' and others* the sols prepared in these experiments were extremely resistant to coagulation by electrolytes. HCl, I N A&(S04)3, I N CaClt and I N NaCl gave no decisive coagulation in any concentrations. The addition of crystals of Al2(S0a) and of SnCla was effective only in exceedingly high concentrations. Mixing boron and iron sols caused complete coagulation of both. The concentration of iron in the iron sol used was 4.27 X IO-^ gjcc. One cc of the iron sol coagulated 3 cc of boron sol in one hour. The concentration of solid matter in the boron sol was 0.052% of the total weight. On ultrafiltration of this sol, and evaporation of the ultrafiltrate a residue of 0.017% was obtained. This indicates that it was the electrolyte present in the boron sol which coagulated the iron sol which in turn carried down with it the colloidal boron. The boron sol ultrafiltrate was effective in equivalent concentration in coagulating the Fe(OH)3 sol. All attempts a t concentration of the boron sol by pervaporation and dialysis were unsuccessful. Attempts to stabilize the sol by addition of gelatin solution to give total gelatin concentrations of 0.02% to 0.4%, as well as soap solution to a Concentration of 0.4% and of a trace of zinc ammonium alginate were unsuccessful. The addition of a trace of glycol bori-borate had some effect in slowing the rate of sedimentation. The rate of settling in refluxed sols was much slower than that in sols prepared by direct washing. LMost of the sedimentation in the first case took place in from one week to two or three months and in the second case always within four days. A brief study of the preparation of boron sols by the reduction of B$O3 with Na showed the sols to be similar to those described above but blue or bluish green in contrast with the yellowish brown sols obtained from the Mg reduction. The stability of these sols was increased from several hours to three months by refluxing although the concentration was not greatly increased. The following results are indicative of the adsorptive capacity of the boron used. Percent adsorption by weight, ethyl acetate 17.5, acetone 20.8, benzene 14.3, ethyl alcohol 8.7, water 7.6. summary
A method for the preparation of and some of the properties of colloidal boron are described. Department of Chemzstry, Syracuse Unauersety. 2
Kolloid-Z., 13, 137 (1914). Ageno and Brtrzetti: Atti. Accad. Lincei, (5) 1, 1824 (1910).
