T H E WETTING O F BARIUM SULFATE BY VICTOR LENHER AND H. GORDON TAYLOR
Des Coudres’ has shown that there are three possible cases when a solid is shaken with two immiscible or partially miscible liquids. This solid may be wetted by one of the liquids to the practical exclusion of the other, in which case it goes into the first liquid. I t may be wetted by the second liquid tc the practical exclusion of che first, in which case it goes into the second liquid. It may be wetted by both liquids simultaneously, in which case it tends to go into the dineric interface, the surface separat,ing the two liquids. Cases in point with benzene and water are calcium sulfate which goes into the wat,er phase; copper powder which goes into the benzene phase; and calcium carbonate which goes into the interface. Commercial use of this is made in the ‘pulping’ of white lead when the water is displaced by linseed oil. According t o Mr. Euston of the Euston Lead Company, this displacement of the water does not take place rapidly enough for factory work if the acid number is less than four, so the effect is very possibly due more t o the free acid than t o the oil. Barium sulfate has been found to give all three types with suitable pairs of liquids. When barium sulfate with a specific gravity of 4.5 is shaken with SeOClz (spg. 2.44) and the hydrocarbon heptane (sp.g. 0.66), with which selenium oxychloride is immiscible, the barium sulfate settles completely to the bottom, leaving both liquids and the interface clear. Evidently the barium sulfate is wetted completely by the selenium oxychloride. If the barium sulfate is moistened first with heptane, the selenium oxychloride will displace the latter completely. When a little barium sulfate is shaken with 70% Hi304 (sp.g.1.6) and selenium oxychloride (these liquids are immiscible and the acid is the upper layer), the barium sulphate is found on tap of t,he interface. Bot,h liquids are left clear and there is not a trace of barium sulfate on the bottom. The reasons for believing that the barium sulfate is on and not in the interface are: firstly, no emulsion is formed; secondly, much more of the solid is supported than would constitute a layer one particle thick in the interface; thirdly, when the tube is given a slight rotary shaking, which causes an eddy current in the liquids without disturbing the level position of the meniscus, the barium sulfate is swept into the upper liquid just as dust is picked up in a whirlwind, while none goes into the lower liquid, which remains clear. The barium sulfate is evidently wetted completely by the upper liquid, 707c HzS04. When barium sulfate is shaken with heptane and water, the heptane is emulsified in the water, forming globules about as large as bird shot which collect as a deep layer on top of the water. Each globule is coated completely with barium sulphate and the emulsion shows no tendency to crack even on 1 Arch. Entwickelungsmechanik, 7,.325 (1898); Hofmann: Z. physik. Chem. 83, 385 (1913);Bancroft: “Applied Colloid Chemistry,” 83 ( 1 9 2 1 ) .
THE WETTING OF BdRIUM SULFATE
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standing for months. This is similar to the results obtained by Pickering,l Schlaefer,2 Moore,8 and S h e ~ p a r d .It ~ sometimes happens that some of the solid stays at the interface while some of it sinks. This may happen in the second case, where the solid is wet by the upper liquid which in this case is 70% H2S04, if the system contains too much BaS04, or if some of the particles are much larger than the rest. The particles which sink will, however, carry a layer of adsorhed sulphuric acid with them. Some of the solid is more apt to sink, however, in the third case where the solid is held in the interface. Here the conditions are more delicately balanced, so that the amount, size, and even shape of t,he particles may determine whether they will stay in the interface or be pulled down by gravity. Experiments were made with carbon tetrachloride, heptane, selenium oxychloride, water, sulfuric acid, and phosphoric acid; but no immiscible pair could be found which would float barium sulfate and not strontium sulfate, so a separation of these two salts by this method is impossible as yet. When the sulfates of barium, strontium, and calcium are shaken singly with heptane and water, barium sulfate floats the most satisfactorily and calcium sulfate the least well. Previous experiments by Lenher have shown that a decidedly gelatinous mass is obtained when precipitated barium sulfate or the ground mineral barytes, is treated with selenium oxychloride. Barium sulfate can be obtained in a gelatinous condition when it is precipitated in selenium oxychloride as a medium instead of in water. If a dilute solution of barium chloride in selenium oxychloride is mixed with a dilute solution of sulfuric acid in selenium oxychloride, barium sulfate is precipitated in a voluminous transparent condition, identical in appearance with a precipitate of alumina. This precipitated gel, as well as the apparently gelatinous mixture of ground barytes and selenium oxychloride, goes over immediately into the pulverulent form when the selenium oxychloride is either removed cr destroyed by such substances as water, carbon tetrachloride, sulfuric acid or other reagents. Studies have been made to determine whether there was any corrosive action of selenium oxychloride on polished surfaces of barite; but no such effect has been detected. Furthermore, careful analysis has shown that the selenium oxychloride does not penetrate the barium sulfate crystals and that selenium oxychloride contains no dissolved barium sulfate even after long standing in contact with it. The evidence therefore seems to be conclusive that the gelatinous barium sulfate obtained by means of selenium oxychloride is merely barium sulfate made plastic or gelatinous by adsorbed selenium oxychloride. This seems to be of distinct importance for the general theory of gelatinous precipitates. The general results of tliis investigation are :I . Barium sulfate goes into the selenium oxychloride layer when shaken with selenium oxychloride and heptane. 1
a
J. Chem. SOC.91, 2010 (1907);Kolloid-Z. 7, 14 (1910). J. Chem SOC. 113, 522 (1918). J. Am.Chem. SOC.41,940 (1919). J. Phys. Chem. 23,634 (1919).
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VICTOR LENHER AND H. GORDON TAYLOE
2 . Barium sulfate goes into the acid layer when shaken with selenium oxychloride and seventy percent sulfuric acid. 3. Barium sulfate goes into the dineric interface when shaken with heptane and water. 4. The gelatinous barium sulfate obtained by means of selenium oxychloride appears to be barium sulfate made plast,ic or gelatinous by adsorbed selenium oxychloride.
University of Wisconsin.
