Work in this laboratory has shown that good can be Obtained with this procedure as it is applied to waste analysis. However, in mixtures containing amine substituted starches we have observed interference with the ureaformaldehyde determination. The results always tend to be high.
(2) Morris,
ACKNOWLEDGMENT
The author acknowledges permission of Weyerhaeuser c0,for publication, LITERATURE CITED
(1) Helbert, J. R., Brown, K. D., ANAL.
CHEM.29,1464 (1957).
D. L., Science 107, 254 (1948). ( 3 ) Samsel, E. P., Aldrich, J. C., ANAL. CHEM.29,574 (1957). (4) Viles, F. J., Silverman, L., Ibid., 21,950 (1949). RAYG. WESTENHOUSE
Weyerhaeuser Co. Pulp and Paperboard Division Springfield, Ore.
Extraction of Iron(ll1) from Concentrated Phosphoric Acid SIR: The presence of iron(II1) as an impurity in phosphoric acid made by the wet process has led to a study of the removal of this cation from phosphoric acid solutions 5.1, 7 . 7 , 8.7, and 12.1M in phosphoric acid. The presence of hydrochloric acid significantly affects the extraction of iron from phosphoric acid solution. Previous work ( 1 , 5 ) indicated an increase in the per cent of iron extracted with increasing iron concentration whereas the results of this research show a decrease in the per cent iron extracted with increasing iron concentration. Dodson, Forney, and Swift (1) reported that isopropyl ether is a good extractant for the removal of iron from aqueous solutions of hydrochloric acid. Thus in the present work concentrated hydrochloric acid has been added to solutions of iron(II1) in phosphoric acid and the tetrachloroferrate(II1) produced has been extracted into isopropyl ether.
f 0.2' C. for 45 minutes with occasional mixing to ensure that equilibrium was reached. I n all cases, phase separation was established quickly. Analyses for iron(III), phosphate, and chloride were performed only on the aqueous phase and the concentrations of species in the organic phase were determined by difference. The concentration of iron was learned by reducing Fe(II1) to Fe(I1) with stannous chloride and titrating this solution with 0.05N potassium dichromate. The amount of P20sin the aqueous phase was found spectrophotometrically using the molybdovanadate method ( 2 ) . The chloride ion concentration was determined by the Volhard method. RESULTS A N D DISCUSSION
As in previous studies of the separation of iron(II1) from hydrothe conchloric acid solution (1, 3, 4, centration of HC1 was found to play a most important part in the extraction process. I n the present investigation,
EXPERIMENTAL
lOOr
All chemicals were reagent grade and no attempts were made at further purification. The solutions of iron in phosphoric acid were prepared by dissolving ferric phosphate in concentrated (85%) phosphoric acid and diluting with water to the desired concentration. The procedure for extraction involved shaking the ferric phosphate solutions with 25 ml. of isopropyl ether and varying amounts of 1 2 . 3 s hydrochloric acid. The flasks were shaken and immersed in a constant temperature bath a t 25"
Table I.
HCI = 4 X ) M
\HsPO,
Molar concn. of HC1 necessary to initiate extraction 3.5 2.5 1.0 0
HCI
301
Effect of Acid Concentration on Extraction of Iron
[H,PO,] Moles/liter 5.1 7.7 8.7 12.1
1876
'q P 0 , = 7.7 M
5
5.1M
= 4.OM
20
Molar
concn. of HC1 at
maximum extraction -5.7 5.3 5.2 4.7
ANALYTICAL CHEMISTRY
'Ot -
0 0
0.1
0.2
0.3 0.4 0.5 Iron Taken, m q l m l .
0.6
Figure 1 . Effect of iron concentration on extraction of iron from phosphoric acid
there is a limiting hydrochloric acid concentration below which there is no extraction of iron into the ether phase (Table I ) . Depending upon the HCl concentration, there is a maximum per cent iron extracted which shifts to higher concentrations of HCl with decreasing concentration in the system (Table I). The per cent phosphate extracted from phosphoric acid solutions 5.1, 7 . 7 , and 8.7M increased with increasing hydrochloric acid concentration. For these three solutions, there was a lower limit below which no phosphate was extracted into the ether phase (Table 11). For 12.1M H3P04,however, the per cent of phosphate extracted increased with decreasing HCl concentration, 45y0 of the phosphate being extracted a t 0 molar HCl. At constant phosphoric acid and hydrochloric acid concentrations, there is a decrease in the proportion of iron transferred to the organic phase with increasing ferric concentration. This is in contrast to previous investigations (1, 5 ) which showed that during the extraction of ferric ion from concentrated HC1 solutions, the per cent of iron extracted increases with increasing iron concentration. This deviation from the Sernst distribution law was attributed to polymerization of the FeC14- species and enhanced extraction of the polymer. However, it was later found (3,6)that the deviation from the distribution law is caused by dissociation of HFeC14 in the organic phase. This dissociation results in an increase in the per cent iron extracted by decreasing the activity of the undissociated HFeC14 in the organic phase. Thus, in the present study, the extraction and subsequent dissociation of H B P Oin ~ the organic phase causes a repression of the ionization of HFeC14 and the system more closely obeys the Kernst law. I t is interesting to note t h a t the decrease in the per cent iron extracted with increasing iron concentration is evident in the more concentrated phosphoric acid solutions while the concentration of iron present appears to
be without effect in the 5 . 1 X phosphoric acid system (Figure 1). The concentration of phosphoric acid in the system has EL very pronounced effect on the separation of iron(II1) from phosphoric arid. For example, at 5M HCl, with 12.1M &Pod, 99% of the iron and 2170 of the phosphate are extracted while with 5 . l M H3P04, 89% of the iron and less than 3% of the phosphate are removed. Therefore, the most efficient separations can be made from the most dilute phosphoric acid solutions and quantitative separations of ferric ion from phosphoric acid
LITERATURE CITED
Table 11. Molar Concentration of HCI Necessary to Initiate Extraction of Phosphate
[HsP041 Moles/liter
(1) Ihdson, R. W., Forney, G. J., Swift, E. H., J . Am. ('hem. SOC. 5 8 , 2573 (1936). (2) Gee, A . , Deitz, V . R., A s . ~ I , .CHEM. 25, 1320 (1953).
[HC!I Moles/liter
5.1
3.5 3.0
12.1
1.1 0
7.7 8.7
( 3 ) Laurene, A. H., Campbell, I>. E., Wiberley, S. E., Clark, H . M., J . Phys. Chem. 6 0 , 901 (1956).
(4)Nachtreib, S . H., Conway, J. G., J . Am. Chem. SOC.70, 3547 (1948). ( 5 ) Sachtreib, N. H., Fryxell, R. E., Ibid., p . 3552. (6) Saldick, J., J . Phys. Chem. 6 0 , 500 (1956).
XoRroN H.~BERMAN
can be achieved with only a few extractions when the more dilute phosphoric acid solutions are used.
W. R. Grace & Co., Washington ~~~~~~~hCenter Clarkesville, Md.
Compositional Analysis of n Component Systems by X-Ray Absorption Method
SIR: Previous workers (1, 3) have used various absorption techniques such as calibration curves of some material immersed in a coniplex or absorption edge spectrometry with certain assumptions about the complex which contains t,he unknown. These methods are perfectly valid pro.vided that one unknown constituent and one complex are used. When large numbers of different elements in different complexes are studied, many calibration curves or certain constants must be obtained. The method proposed in this paper relies only on the linear absorption at n different wavelengtlis for an n component system. The wavelengths can be chosen completely a t random; with knowledge of the elements present, however, more accurate results can be obtained by a judicious choice of the n wavelengths. .4 simple method, into which matrix effects do not enter, for the analysis of foils and liquid materials has been t,he goal of laboratories wishing to obviate the need for calibrat.:ion curves generally used in fluorescence analysis. Since the mechanism of absorption involves either the absorption or the nonabsorption of a photon, freedom from the effect of other materials present in the unknown is achieved. Consider a material which consists of constituent .4, 30%; constituent jg, 20%; and const,ituent C, 50%, i:n which the three materials, A , B, and C, are thoroughly mixed in the unknown. The method of fluorescence analysis would necessarily derive calibrat,ion curves for each of the constituents immerscd in the other two, while on the other hand, in an absorption experiment' an x-ray photon would be either absorbed or not absorbed by
one of the three constituents. If it is absorbed by the unknown, then this would decrease the obseived count by one. Statistically speaking, if a large number of photons were to impinge on plzP1
+ PZZPZ+
p32P3
pllP1
+
+
p21P2
+...+
p31P3
pnlPn-
1 Io In - = 0 at X1 d 11 -
and similarly at x2 to A,,
+ . . . + pnzP,- d-1 In Io1 2 -
=
0 a t Xz
(3)
the sample and only 50YG of the beam was detected in the counter, then the 50% that was absorbed would divide in the ratio ~ ~ A : ~ , u B : ~ Mwhere c , PA, , p and pc are the linear absorption coefficients of A , B. and C, respectively. The absorption equation for a simple system of n components is
Pl+PZ+P3+ ~
Therefore, the algebra is
. . . +P n =1
solution
by
matrix
PI = Ai("+ I ) Pz = Azc" + 1)
(4)
Il
=
I o e-WmPn)d
(1)
where
ll lo
x-ray intensity a t detector; intensity of unabsorbed x-ray beam; p,, = linear absorption coefficients of nth component; P , = fractional parts of nth component where 2: P,, = 1 ; d = thickness of material. Rewriting Equation 1 for two components we would have pJ'1
= =
+
pz1P2
- -1 In 1-0 d
= 0
11
at
(2)
Extending t,his equation to n component.s gives
or, more generally,
P,
=
A m ( n + l )( V I = 1 , 2 , 3 .
. . n)
where the,l,, are the elements of the last row of the inverse matrix of the coefficients. If the quantity 1 ' d is kno\\-n,then the number of measurements is reduced by one and Equation 3 takes a >lightly different form; the nth equation is deleted from the set VOL. 36, NO. 9, AUGUST 1964
1877
