ANALYTICAL CHEMISTRY
1516
Table 11. Recoveries of Known Amounts of Calcium, Iron, and A l u m i n u m from Magnesite Sample 1
2 3 4 5 6
7 8
9
Per C e n t Present 1.70 2.31 2.92 1.01 1.62 2.23 0.78 1.39 2.02
Fez03 Found 1.70 2.28 2.80 1.00 1.75 2.45 0.84 1.68 2.22
Per Cent CaO Present Found 1.50 1.45 2.65 2.88 3.79 3.82 2.70 2.83 3.85 4.07 4.97 ... 3.10 3.08 1.25 4.45 5.39 5.32
Per Cent Present 0.17 0.51 0.84 0.14 0.48 0.81 0.14 0.48 0.81
A1208
Found 0.15 0.43 0.81 0.12 0.41 0.81 0.12 0.42 0 75 -
Table 111. Replicate -4nalyses of a Magnesite Ore Sample 1 2
3 4 5 6
(70 CaO
76 FezOa
7c All08
3.60 3.40 3.30 3.30 3.20 3.25
0.80 0.90 0.81 0.78 0.81 0.80
0.12 0.11 0.11 0.11 0.11 0.11
by the wet or chemical analysis. In Table I1 are the spectrographic results of recoveries of calcium. aluminum, and iron. To the carefully analyzed samples of magnesite, known additions of calcium, iron, and aluminum were made. h i d e from the spectrographic results for iron oxide in sample 8, the recovciies are well within the range for adequate raw material or chemical control. Spectrographic reproducibility, or precision, is shown in Table 111, which records the determination of iron, aluminum. and calcium over a period of 6 days. Each percentage value i* thc nveiage of three separate exposures. I t is obvious that minor constituents of many acid-roluhle ores may be determined by this procedure. The author.; have employed the method for brucite and dolomite and contemplate the determination of manganese, aluminum, phosphoruq. titanium, etc., in iron ore. LITERATURE CITED
(1) Bovle. A. J.. etal.. J . Lab. Clin Med.., 34. , 625 11949). (2) Bride, W. 'R., "Chemical Spectroscopy," 2nd ed., Kcw T O I . ~ , John Wiley & Sons, 1945. (3) Caley, E. R.,and Elving, P. J., ISD. ENG.CHEY., A x . ~ L ED., . IO, 264-9 (1938). (4) Feldman, C.,J . 0pt.lcaZSoc. Ani., 38,1100(1948). (5) Smith, C. W., ANAL.CHEM., 20,1085 (1948). (6) Twyman, F.,and Hitchen, C. S., Proc. R o g . SOC.London, A133, 77 (1931). (7) Whitehead, T., and JJ7illiams, E. V., ISD. ENG.CHEsf.. A Y ~ L . ED.,17,490(1945). 1
magnesium. Calcium a-as also determined spectrochemically with a Beckman flame spectrophotometer. The greatest deviations between spectrographic and wet procedures to be found in Table I are those recorded for aluminum oxide. Because iron is in such excellent agreement for these samples, it w m felt that this discrepancy is not due to variabilities in sample solution procedure. The aluminon colorimetric method involves an organic extraction step which, in the authors' hands, may not have effected a complete separation from iron. This would account for the higher values obtained for aluminum oxide
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RECEIVEDJanuary 2, 1951. Presented before the Division of Anall-tiral SOCIETY, L Chemistry a t the 119th Meeting of the . ~ M E R I C A S C ~ E X I C A Boston, Mass.
Formal Potential of Cerium(V1)-Cerium(II1)
Couple in Perchloric Acid
FREDERICK R. DUKE ANI) ROBERT F. BREMER Iowa State College, Ames, Iowa and Goetz found that the potential of the cerium5voltsMITH (1V)-cerium(II1) couple rises sharply from 1.70 to 1.8i with acid concentration in from 1 M to 8 hf perchloric acid (2)
solutions. These observations cannot reasonably be explained on the basis of complex formation with perchlorate, becauscl cerium(II1) would necessarily form very stable perchlorate conipleses if this explanation were valid. A more reasonable explanation is bawd upon difference in degree of hydrogen of the two ions, the cerium(1V) effectively complexing more water than the cerium( 111)ion. Figure 1 is a plot of E as measured by Smith and ~
I
I
.I
[ ~ e + + +a ] plot of E us. log
$- should be a straight line of 820
slope 0.059 n and intercept EO,where EO is the formal potential a t aa20 = 1. The points lie very close to a straight line, having a slope of 0.63 and an intercept of 1.69 volts. It appears that cerium(1V) effectively inactivates 10 more molecules of water than does cerium(II1). The assumption in the above treatment is that the activity ratio of the metal ions remains constant. Although compensating effects could conceivably account for it,, the fact that, the points on the graph lie close to a straight line indicates that the activity ratio does not deviate largely from constancy as the perchloric acid concentration is changed. Thus it appears that cerium(1V) is the tetrapositive hydrat,ed cation in moderate concentrations of perchloric acid. Variations in the cerium(1V)-cerium(II1) potentials in nitric, sulfuric, and hydrochloric acids ( 2 ) presumably arise from a
.2
.3
.4
log k g "e0
Figure 1 combination of the above effect and the formation of c,oniplrscs cont,aining cerium(1V) and the anion of the acid. The large potential drops in these three acids from that in perchloric acid are sufficient to indicate that the effect of coniplexing far outweighs the effect of the vater activity in these cases. LITERATURE CITED
(1) Pearce, J. Tu'., and Kelson, -4.F., J . Am. Chem. Soc., 55, 30i6
(1933). (2)Smith, G . F.,a n d G o e t z , C . -%., IND. ESG.CHEX, AS.
