V O L U M E 20, NO. 12, D E C E M B E R 1 9 4 8 2.35 nig. of folic acid instead of the 1.0 taken. This interference is slio~vnas a direct enhancement of the folic acid wave and is caused by the direct superimposition of the iron wave onto that of folic. acid. .4s found in this laboratory, the half-wive potential of thc ferric-ferrous reduction v a s -0.94 volt with the tetramethyl aninn~niumhj-tlroxide solution used. This was to be expected, since half-Ivave potent,ial for ferric-ferrous iron in potassiuni hydroside and mannitol has bcrn reported as -0.9 volt (J). Although it is possible that use of au oxalate medium ( 8 ) may shift the iron half-wave so that i t ~vouldno longer interfere. this of the problem was of minor importance in this laboratory and sufficient t i n e could not br devoted to its solution. Ttilizirig higher instrument sensitivit,y with correspondingly lon-er radmium ion concentratioris, it was found feasible to test for COIIrenti,atiorir oi’ folic acid a‘ low as 1 mg. per liter. This sensitivity is not LO great as with the turbidimetric microbial method of Rohcrts and Snell (6, 10 micrograms per liter). The latter nitathod, however, required a minimum of 16 hours per aqzaj- as ronip:wed to 10 to 15 minutes for the polarographic, 1~roc~diire. In thr. rcrcwt study of drgradation prodncts of rhiztq)tmin by
1201
Rickes et al. (6) polarographic studies indicated a general pterin class reduction at approximately 0.8 volt in 0.1 JP lithium borate ( p l l 9.12) buffer. Thus, in a study of folic acid of presumably high purity pterin contaminants should be considered as likelv to intei fere with polarographic assays. LITERATURE CITED
Hutchings, B. L., Stokstad, E. L. R., Boothe, J. H., Momat, J. H., Waller, C. W., Angier, R. B., Semh, J., and Subbarow, J., J . B i d Chem., 168, 705-10 (1947). Kolthoff, I. M., and Lingane, J. J., “Polarography,” p. 170, New York, Interscience Publishers, 1941. Ibid., p. 484. Lingane, J. J., IND.ENG.CHEM.,ANAL.ED.,15, 553 (1943). Rickes. E. L..Trenner, N. R., Conn, J. B.. and Keresztesy. J. C., J . Am. Chem. SOC.,69,2751 (1947). lioberts, E. C., and Snell, E. E., J . B i d . Chem., 163, 499 (1946). Stokstad, E. L. R., Fordham, D., and de Grunigen, rl., I b i d . , 167, 877 (1947). RECEI\-EI) X a y 11, 1948. Presented before the Division of Analytical and CHEMICAL S O C I E T Y . l\Iicro Cheiiiiatry a t the 113th hleeting of the AXERICAX Chicago, Ill.
CHEMISTRY OF THORIUM Quantitative Estimation of Thorium by Precipitation with Radioactive Pyrophosphate ‘I’lIEKILI) TIOELLEK
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GEORGE I
