George E. Knudson
Luther College Decorah, Iowa
Quantitative Determination of Potassium
by Natural Radioactivity
The quantitative determination of potassium by means of its natural radioactivity has been report,ed by numerous investigators working with a wide variety of materials and equipment. This determination is easily performed by students, using standard equipment, and illustrates several important principles involved in the quantitative measurement of radioisotopes. Naturally occurring potassium contains 0.012% of K", of which most decays by emission of 1.32 Mev B particles and the remainder (13%) by emission of 1.46 Mev y radiation. The p particles are easily detected by a thin-window geiger tube, but are absorbed to a considerable extent by the sample itself when the sample is thick enough to give good counting rates. Pure potassium chloride is a convenient standard which contains a higher percentage of potassium than other common salts. Placing the loose crystals in a planchet presents problems of spillage and irregular geometry. Covering the crystals reduces the already low activity. We have found that by grinding the crystals in a mortar and pressing the powder firmly with a metal spatula into a rimmed planchet, samples may be prepared which are uniform and not easily spilled. A self-absorption curve may easily be prepared by measuring the activity of samples varying in thickness from 0 to 0.3 g/cm2. Since the activity is very low, the counting time is long and we have made this a class project, with each student contributing one point to the curve. The activity rises rapidly to a plateau hut does not level off completely because of the more penetrating y component. From the curve, students should
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see the desirability of operating with a standard sample thickness, and may decide on the optimum size for these samples. We have chosen to use 2-g samples in 32-rnm diameter copper planchets, with a sample thickness of 0.124 g/cm2. In this region not much activity is gained by increasing the weight of the sample, and a slight error in weighing the sample has a negligible effect on the activity. Using a Nuclear-Chicago D-34 tube as detector, and with the sample mounted several millimeters below the tube, a rate of about 10,000 counts/hr was observed with a 2-g sample of KCl. Students next prepare a calibration curve, using 2-g samples in all cases, by measuring the activity of KCI-NaC1 mixtures of known composition. Again each student is asked to contribute one point to the curve. Each student also measures an unknown and calculates the percentage potassium, using the data of his calibration sample and using a linear relationship between activity and percentage potassium as shown by the calibration curve. We have used various pure lahoratory salts as unknowns and have also used commercial fertilizers. Students in their first semester of aualytical chemistry usually obtain results which are within 1-27& of the accepted values. Because of the low activity, it is necessary that the student pay more attention than usual to the statistics of counting, including the count time required for a certain degree of precision, and the optimum time required for the background count. We have found that the precision of the entire method is limited by the stability of our counting systems over the time interval required for the measurements.
