POTASSIUM DETERMINATION BY THE COBALTINITRITE METHOD AS AFFECTED BY TEMPERATURE AND pH.

Rapid chemical analysis of plant and soil extracts for plant nutrients is beingv given widespread attentioii. Potassium is one of the most important and expensive planit nutrienits and requires, therefore, special coiisideration of methods employed for determining, its coneentrations in soils and plant tissues. The cobaltiniitrite iimetlhod for potassiumi deteriliinations in extracts of soils anid plants is becomiing universal; maniy investigators, however, have not considered the importaniee of eontrolling temperatuire and pH duLring, the precipitation reactioni. Early investigators noted the sparing solulbilitv of the double salt prepared by precipitation of cobalt salts with strong solutions of potassiumii anid sodium nitrite which led to the development of DE KONINCK'S reagent as a qualitative test for potassium. ADIE anid WOOD (1) in gravimetric and volumetric deterimiinationis of potassium in fertilizers and soils by the cobaltinitrite method founcd it necessary to have relatively high concentrations of potassium (0.5-1 per ceuit.) in the precipitating medium in order to obtain reliable results. The preeipitation of potassium salts in the sodiuim cobaltinitrite is appreciably more senisitive on additioni of silver salts but this depends on the solution being7 halogeni-free (2). Lithium, thallium, and ammonium salts must not be present as these also give crystalline precipitates with sodium cobaltinitrite. SCHUELER and THOMAS (3) report that it is necessary to keep the precipitating medium cooled to 50 to 80 C. for several hours in order to obtaiii satisfactory results. In the course of developing a satisfactorily sensitive method for the rapid determinationi of potassiuiii in extracts emplovin(g sodium cobaltinitrite, it was necessary to determine the effeet of temperature over a wide range of potassium concentrations in order to establish a satisfactory set of conditions for the precipitation reaction. Potassium standards were prepared from potassiuii clhloride. All solutions used were cooled in water baths to the respective temperatures indicated in fi(gure 1 before adding the dry sodium cobaltinitrite. In preliminary trials it was found that sodium cobaltinitrite in the dry form is more sensitive than saturated solutions in equivalent anmounts, even when the latter are accomlpanied by higher concentrations of sodium attained by addition of salts such as sodium initrate and sodium chloride. Sodinm eobaltinitrite in the dry form was used at a