Comparing the weights of lanthanum recovered with the weights of lanthanum present, the above separation may be considered satisfactory. The amount of cerium in the lanthanum precipitates as observed from the yellow color was minute, and its presence may be attributed to reduction of the ceric sulfate stock solution (5). Cerous sulfate was previously found not to precipitate until pH 7.07.
2 Chemistin the Experiment Station, Emeritus. 3 Grateful.acknowledgment is hereby made for helpful comments from the staff of the Division of Plant Nutrition and to the Works Progress Administration for much clerical assistance on O.P. 465-03-3-587. 4 Italic numbers in parentheses refer to "Literature Cited" at the end of this paper.
We have previously published the results of experiments on Nitella cells, which emphasized the primary importance of illumination and of temperature in determining the accumulation' of halogens in the cell sap and at the same time a number of preliminary observations were reported with regard to certain inter-ionic effects in relation to the process of accumulation (5). This latter question had also been studied earlier in connection with experiments on the absorption of ions by barley plants (4). While it appears that the accumulation of solutes by a plant cell is del)endent on the growth or metabolic activities of the cell, it is also clear that the magnitude of the actual accumulation under any given conditions of light and temperature is influenced by the concentration and composition of the culture solution. It is this latter aspect of the question that we now wish to discuss.The general methods of experimentation heretofore described were used in the present work. Nitella cells were immersed in the solutions under investigation and at the end of the experiments the cells were thoroughly rinsed with distilled water and the larger cells (from 1-3 inches in length) were broken individually, the sap being collected in amounts varying between 1 and 25 cc. Since a single cell yielded only one or a few hundredths of a cubic centimeter of sap, each composite sample of sap collected represented a very large number of cells.2 The method of HIBBARD (3), specially developed for this purpose, was employed in analyzing the sap for its halogen content, which procedure made it possible to obtain results of sufficient accuracy even when dealing with very small volumes of sap. (Determinations were made in duplicate or triplicate.)
THEpurpose of this paper is to describe methods for quantitatively estimating the small amounts of zinc which may loe found in plant materials and other substances. These methods require ashing to remove organic material, then separation of the zinc from most other associated metallic ions. The ash is extracted with hydrochloric acid and the zinc separated by means of hydrogen sulfide or oxalic acid and ferrocyanide. The isolated zinc may be estimated by nephelometry, by iodometric titration, by a micromodification of the long-known ferrocyanide titration, or by the polarigraph (5). The amount of zinc to be measured may be 0.5 to 0.1 mg., quantities sometimes found in 5 grams of plant material. The polarigraph is capable of measuring very much smaller amounts.
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