Determination of Potassium in Solids ANTOINE M. GAUDIN AND JAMES 13. PANNELL Massachusetts Institute of Technology, Cambridge, illoas. A previously described method for the determination of potassium by u s e of a Geiger counter has been extended and modified. The procedure obviates errors due to self-ahsorption and is applicable to concentrations of potassium above 1%.
T
lecays, with a half-life of about 4 X 108 years, to either Ca4@ 134%) or AIQ(66%). I n the former case beta-radiation of maxiInum energy 1.45 m.e.v. is emitted. The range of hctsiparticles is given approximately by the Feather (6) relationship:
HE natural radioactivity of potassium has been proposed by
Barnes md Salley ( I ) as the basis of an analytical method for determining potassium in aqueous solution. The naturally radioactive elements are a small group consisting of certain isotopes of uranium, thorium, lutecium, samarium, rubidium, and pota&um, and the disintegration pi oduets of the first two. The rare earth elements, lutecium and samarium, are curiosities;
0
0.2
0.4 0.6 0.8 1.0 THICKNES!.. GRAMS PER SO. CM.
1.9
R
=
0.513E,,,
- 0.160
R is the range expressed in grams of absorbent per square :entimeter, and Emax.is tho maximum energy in million electron rolts. Application of this formula to the radiation emitted by K'O Cives, for the maximum range, 0.6 gram per hq. cm. Some ;ammaradiation is also emitted by this isotope in decaying to L", hut i s of very low intensity and not generally detectable. rho above figure ropresents a depth of sample which is just infilite far this particular beta-energy and it has the obvious prop-
Tvhere
14
Figure 1. Saturation Curve for K'o Radiation in Potassium Chloride rubidium is rare, and uranium and thorium, with their offspring elements, emit alpha-rays as well as beta-rays. The specific beteaotivity of uranium, in equilibrium with its decay products, is roughly 2000 times that of potassium. Thus, B mineral sample containing 0.0005% UaO, exhibits about the same amount of measurable bet&aotivity as one containing 1 % potassium oxide The correction needed in a radiometric analysis of a n average granite, containing 5% potassium ($2) and 0.0005% UaOs (4), iS therefore 20%. The method proposed by Barnes and Salley is particularly attractive. However, it scem desirable to show that the method can be extended to solid samples which in many casos are more conveniently handled. than solutions. Where sufficient sample material is available, employment of a depth which is virtually infinite for the K40 activity has advantages. By retaining the fundamental cylindrical geometry of Barnes and Salley, applying i t to powdered mineral samples, and making the sample depth sufficient to be infinite for practical purposes (1 em.), the method becomes even more sensitive and useful. As the absorption of beta-radiation by elements of low atomic number is primarily a mass effect, the thickness penetrable by the radiation is measured in units of weight per unit area. Thus 0.5 cm. of a. material having a density of 1is equivalent to 0.25 em. of another material having a density of 2. The maximum energy of the beta-radiation from the isotope K40 has been recenHy determined (7) and its decay scheme verified (6). The nucleus 1154
Figure 2. Counting Apparatus
Figure 3. Components of Sample Holder
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V O L U M E 20, N O , 12, D E C E M B E R 1 9 4 8
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I155 Figure 2 shows the apparatus used. The thinwalled cylindrical Geiger-Miiller tube is mounted vertically a t the axis of a lead shield and is supported rigidly by ita lower end alone. The shield, or chamber, is lead 3.25 cm. (1.25 inches) thick with a thin brass lining. Employment of a lead
in redicing thetime necessaryfor obtainingstatistical accuraoy. The sample holders employed for this work were designed with the following requirements in mind: They must fit the counter tube as closelv ss possible in order to obtain optimum effickney- in counting, they must be easily filled and emptied of very finely nowdered matmid8 which are not alwavs freeflowing, and they should cause the mini6um .ab sorption of radiation directed toward the counter. ~~
Figure 4.
Geiger Tube Mount
erty that iurther increments of thickness, no matter how great, cause no increase in the, intenvity of emitted radiation. It is desirable, in counting beta-particles, tbat all samples whose weight may not be constant be infinitely thick. I n this way, corrections for differences in density may be avoided. I n view of the distribution of energies (5) of the beta-particles emitted by one isotope, very few of the partic.es have the maximum energy. Experimental d a t a o n self-ahsorption of the K40 r o d i a tion has been obtained in the authors’ laboborittory and plotted in the form of the “saturation curve” described by Libby (8)and shown in Figure 1. The maximum range has been determinod by the authors to be 0.35 gram per sq. em. in potsssium chloride and
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length, which is about 7.5 c&. (3 incses). They are 5 em. (2 inches) in over-all diameter and about 2.5 em. (1 inch) in inside diameter, and the volume of the annular space is about 100 ml.
.
1156
ANALYTICAL CHEMISTRY
Table I. Times Required to Obtain 2% Probable Error %K 0.1 1.0 10.0 50.0
Net Counts per Minute 2 20 200 1000
Time, Min. 6250 113 8 1
Table 11. Comparison of Chemical and Radioactive Assays Sample No. 1
2 3 4 5 6 7 8 9 10 11 12 13 14
Kz0, Chemical, %
Kz0, Counter, %
26.70 36.34 27.81 29.23 16.12 25.10 20.92 30.12 16.00 30.00 35.55 45.70 60.05 63.00
26.9 36.5 28.6 29.8 15.9 25.2 20.8 31.0 15.0 31.0 40.3 48.5 60.3 61.8
Difference, % -0.2 -0.6 -0.8 -0.6 0.2 -0.1 0.1 -0.9 1.0 -1.0 -0.7 1.2 -0.2 1.2
analyzed, but the apparatus is not recommended for such a use. The times necessary to count samples to 2y0 probable error, with a background of exactly 20 counts per minute, are given in Table I.
The figures in Table I1 show the accord in assays obtained by
’
chemical analysis and by the apparatus described above in the analysis of sylvite ores and concentrates. In order to show the proportionality existing between counting rate and per cent potassium in a low range, Figure 6 was determined experimentally for synthetic mixtures of sand and potassium dichromate. The sand, although radioactively inert, did not decrease the background when counted alone. Hence, a constant background prevailed for all mixtures. These mixtures were prepared from washed screened silica and C.P. screened potassium dichromate. Standards prepared thus are free from errors caused by segregation of the undersize fraction. However, it is desirable to use, as a standard, material that will not change in composition or decrease in uniformity. For this purpose the authors employ a sealed sample holder tightly packed with an ore of relatively high activity. This standard is used for frequent calibration of counter tubes to detect changes in sensitivity. LITERATURE CITED (1) Barnes and Salley, IND.ENGI. CHEM.,ANAL.ED., 15, 4-7 (1943). (2) Clarke, U. S. Geol. Survey, Bull. 770, 5th ed., p. 441 (1924). (3) Evans, “Science and Engineering of Xuclear Power. Chap. I. Fundamentals of Nuclear Physics,” p. 26, Cambridge, Mass., Addison-Wesley Press, 1947. (4) Evans and Goodman, Bull. GeoZ. SOC.,52, 464 (1941). (5) Feather, Proc. Cambridge Phil. SOC.,34, 599 (1938). (6) Gleditsch and Graf, Phys. Reu., 72, 640 (1947). (7) Henderson, Ibid., 71, 323-4 (1947). (8) Libby, ISD. ENG.CHEM.,As.41.. ED.,19, 2 (1947). RBCEXYED February 21, 1948.
Photometric Determination of Magnesium in Water with Brilliant Yellow JIICHAEL T A R i S Department of Water Supply, Detroit, Mich.
Magnesium hydroxide in strongly alkaline solution converts to red the normal orange color of Brilliant yellow dye. The positive effect of aluminum and calcium is compensated for by raising the concentration of these ions to a level where their influence is predictable and constant. Starch and Colloresin are permissible stabilizing agents. High concentrations of chlorine must be removed. The dye has been applied successfully to the analysis of typical natural waters, conventionally treated with filter alum or lime-softened.
B
RILLIANT yellow and several common azo indicators were first suggested by Kolthoff (5) for the colorimetric determination of magnesium, but no critical evaluation of these dyes on a quantitative basis was attempted a t the time. Accordingly, a study of Brilliant yellow was undertaken with a view to ascertaining its admissibility for the magnesium determination. The color characteristics of Brilliant yellow in the presence of magnesium hydroxide closely resemble those of Titan yellow (6). Although an instrument for measuring light absorption a t various wave lengths was not available, experimentation Tvith a series of Wratten filters disclosed that minimum transmittancy occurs in the spectral range covered by the green filter (approximately 525 mp). Despite the close parallel between the Brilliant yellom-magnesium reaction and the Titan yellow reaction, certain peculiar differences confer advantages on the Brilliant yellow system-for example, aluminum ion above 0.5 p.p.m. reduces the Titan yellow color (2) but has an intensifying effect on the Brilliant yelloL7
color. Therefore, in the presence of moderate amounts of aluminum, Brilliant yellow can be considered a more desirable reagent. A method has been devised for determining magnesium directly in a natural water sample without preliminary removal of common interfering ions like calcium and aluminum. Inasmuch 88 interference from these ions is relatively constant a t specific concentrations, compensation can be made by bringing the concentrations up to this uniform level, and a photometric curve can be plotted yith the ions in solution. APPARATUS AND REAGENTS
A Cenco-Sheard-Sanford Photelometer was used in this investigation, with green filters having wave lengths near 525 mp. Stock magnesium solution was prepared by dissolving 10.1353 grams of C.P. magnesium sulfate heptahydrate in 1 liter of dietilled water. Standard magnesium solution was prepared by diluting the stock solution 1 to 10. The solution wm equivalent to 100 p.p.m. of magnesium. The magnesium content was confirmed by gravimetric analysis.
