Exercise 9. Ferrous Chloride

This salt is much more difficult to obtain in dry, pure form than is the corresponding sulfate. It is very soluble in water, but from concentrated solution it is deposited in crystals which in color are almost identical with those of the sulfate. Upon exposure to air, even during filtration with the pump, the crystals turn yellow through oxidation. Probably this is not due to greater ease of oxidation on the part of the solid salt but to the physical conditions that prevail. The saturated, cold solution is very viscous and sticky, and absorbent paper absorbs this liquid very slowly. Owing to the great solubility of the salt, the solution has a low vapor pressure and does not tend to evaporate in the air. Consequently, the crystals remain coated with a film of concentrated mother liquor which oxidizes very fast in the air. 

The logical way to meet this situation is to wash the crystals with some liquid that will dissolve the film and leave them clean and bright. Evidently, ferrous chloride will have to be soluble in this liquid, but its solubility should be rather small, so that the solution left upon the crystals will not be viscous and will dry rapidly. An alternative plan is to dilute the original solution with a solvent that will diminish the solubility of ferrous chloride and thus avoid some of the difficulties. Alcohol will serve this latter purpose, but it increases the solubility of oxygen in the solution, and this promotes oxidation. A better liquid is concentrated hydrochloric acid. This suggests the following procedure  

When satisfactory crystals have been obtained, bring them quickly upon a Witte plate, washing them on with the mother liquor. Wash the crystals with successive small portions of concentrated hydrochloric acid, being careful that none escapes the washing. Pump off the liquid each time, then spread the crystals upon absorbent paper, press them lightly with a wad of absorbent paper, and transfer them to a desiccator over sulfuric acid. As soon as they are dry, put them in a perfectly dry bottle and seal with paraffin, since they are deliquescent. They should be bright green in color and not at all yellow. It is very hard to get a perfect specimen, but it can be done, and, when successful, the specimen will keep indefinitely. 

Additional Exercises Chromous acetate, Cr(C2H302)2 chromous sulfate, CrS04-7H20 chromous chloride, CrCl2-4H20 Anderson and Riffe, J. Ind. Eng. Chem., 8, 24 (1916) Reeoura, Ann. Chim. Phys., (6) 10, 6 (1887). 

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First prepare a pure solution of calcium chloride by following the directions for the preparation of calcium nitrate in Exercise 7 but substituting hydrochloric acid for nitric acid and using 100 g. of marble. Do not add the excess of acid at the end of the operation but leave the solution strongly alkaline. It is also well to add about 10 cc. of bromine water to the acid solution while boiling it and before the addition of the slaked lime. This is to oxidize any ferrous salts that may be present. 

Additional Exercises Lead chromate, PbC rO, calcium phosphate, Ca3(PC>4)2 ferrous oxalate, Fe(C2C>4) sodium chloride, NaCl, precipitated from a saturated solution by HC1 gas, Richards and Wells, Revision of the atomic weights of sodium and chlorine, J. Am. Chem. Soc., 27, 459 (1905). 

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