i t is evident that a significant concentration of hydrolyzed aluminum species is present. A full interpretation of the results would require the study of simpler systems. The utility of this wave length depends on the absorptivities of the aluminum nitrate and nitric acid species as well as the change in water concentration due to displacement of water by the salt and acid. The practical use of the 295-mp2.667-micron correlation depends mainly on the design of a suitable instrument.
A single-beam, narrow-band-n5dth1 filter instrument n-ith an optimum damping circuit a-ould give a calculated improvement in precision of a factor of seven. The use of quartz in the optical system and the fast response time intrinsic in the design of the Cary Model 14 spectrophotometer produces an unfavorable signal to noise ratio a t 2.667 microns. LITERATURE CITED
(1) Blaedel, 1%'. J., Panos, J. J., ANAL. CHEM.22,910-14 (1950). (2) Booman, G. I,., Elliott, hZ. C., Kim-
ball, R. B., Cartan, F. O., Rein, J. E., Ibid., 30, 284-7 (1958). (3) Falk, M., Giguere, P. .1.,Can. J . Chem. 35, 1195-204 (1957). (4) ORNL Analytical Chemistry Division, Annual Progress Rept., U. S. Atomic Energy Commission, Rept. ORNL-2453, 12 (1957). (5) Pepkowitz, L. P., Sabol, W. W., Dutina, D., ASAL. CHEM.24, 1956-9 f\19.52). ----,
(6) Schneider, H., Schwiege:, L. G., U. S.
Atomic Energy
Commission, Rept.
IDO-14419 (1957) confidential. RECEIVED for review December 21, 1959. -4ccepted March 28, 1960. The Idaho Chemical Processing Plant is operated by Phillips Petroleum Co. for the U. 9.
Atomic Energy Commission under Contract Xo. A4T(10-1)-205.
ZirconyI- Aliza rin Chelate in Spectrophotometric Determination of Trace Amounts of Fluorine R. P. ASHLEY Aluminium laboratories Ifd., Arvida, Quebec, Canada
The preliminary separation of the fluoride ion from fluoride-bearing materials by the Willard-Winter steam distillation invariably yields a distillate which is slightly contaminated with the acid used for the distillation. This free acid is an undesirable feature of absorptiometric procedures requiring precise control of pH. A simplified method for the determination of microgram quantities of the fluoride ion involving a zirconium-sodium alizarin sulfonate reagent is presented. Good precision and accuracy are obtained without the tedium of pH adjustment, and adequate sensitivity coupled with strict observance of the Beer-Lambert law in the fluoride range 0 to 1.9 y per ml. makes possible the precise measurement of concentrations of the order of 0.05 y per ml.
cision can be achieved only by careful control of pH, and where large numbers of samples are handled routinely, this is undesirable because of time expenditure. EXPERIMENTAL
Stability of the Zirconyl-Alizarin
Lake.
I n t h e search for a simpler method, unaffected b y small changes in hydrogen ion concentration, t h e zirconyl-alizarin complex which had been recommended b y various workers -for example, Sandell (6)-was investigated. I t s reported greater stability (3) at relatively high acidities in contrast to most of the other metal chelates was promising. The formation of the zirconyl-alizarin lake and its subsequent bleaching by the fluoride ion may be represented by the following equations:
A
various photometric methods are available for the determination of the fluoride ion in microgram quantities, there has long been a need for a simple procedure capable of good accuracy and precision. One very widely used method of measuring fluorine in vegetation relies on the bleaching action of F- on the thoriumalizarin chelate ( 2 ), folloFving preliminary isolation of fluorine by the steam distillation of Willard and Kinter ( 7 ) , in conjunction with some suitable ashing technique (4, 5). This absorptiometric procedure offers good sensitivity but the thorium lake is seriously affected by small changes of hydrogen ion concentration. Thus accuracy and pre-
Preliminary tests showed that an alizarin concentration of 0.01 mg. per ml. of lake solution containing zirconium and alizarin in equimolar ratio gave n reasonably deep color for spectrophotometric measurement with cuyettes of 5cm. light path. Absorbance readings within the p H range 2.86 to 0.66 confirmed that the lake could be used at fairly high hydrogen ion concentration, thereby minimizing the effect of free acid in steam distillates containing the fluoride ion. Apparatus and Reagents. Beckman Model DU (or B) spectrophotometer with 5-cm. light path cuvettes. Stock fluoride solution reagent grade sodium fluosilicate is dissolved in 500 ml. (0.8249 gram) of water. Ten milliliters of this solution are diluted t o 1 liter and stored in a
LTHOUGH
834
ANALYTICAL CHEMISTRY
ZrO(OH),
&0
0
-
+ &OH
SO, No
II
I/
0
0 -2rO
ll
I
I
I
I
t 2 H,O
/I
0
0
0-ZrO
SO,
No
c7 06
c5 w
0 3
Ci
-
-
/
/-
'os:5,/C 5
3'5
-ME
PII h C r P S
y
04
a CC m
2
03
C 2
Figure 1. Effect of development time on color intensity of zirconyl-alizarin preformed-lake
plastic bottle with a close-fitting cap. This solution contains 10 y of Fper ml. Zirconium-alizarin reagent, sulfuric acid (40 ml., 36N) is cautiously added to approximately 400 ml. of water and allowed to cool. Kext 120 ml. of 12N hydrochloric acid are added, while stirring, and the solution allowed to cool again. Then 0.30 gram of zirconium salt (ZrOC12.SH20) and 0.35 gram of sodium alizarin sulfonate are each dissolved in approximately 100 ml. Both solutions are filtered of water. and added t o the acid mixture. Filtering off insoluble residue at this stage avoids clogging u p the filter paper a t the final filtration. After dilution to 1000 ml. n i t h water the reagent is thoroughly mixed and allowed to stand for 48 hours. Colloidal suspensions are removed by filtering through S & S KO. 576 or equiralent filter paper, and the reagent is stored in a dark stoppered bottle. Procedure. Pipet a n aliquot of t h e steam distillate containing not more than 180 y of fluorine into a 100-ml. volumetric flask. Carefully run in 5 ml. of the zirconium-alizarin reagent from a buret. Dilute to the mark with water. Mix well. T o a series of four 100-ml. volumetric flasks, add from a IO-ml. buret 0, 4, 8, and 10 ml. of the standard fluoride solution. Add exactly 5 ml. of the zirconium-alizarin reagent and dilute to the mark n ith 1%-ater.Mix well. Allow a t least 2l/2 hours for color development and measure the absorbance a t 520 nip, using distilled water as reference. Plot the absorbance of the standard solutions against micrograms of fluorine per 100 1111. on linear graph paper and read off the fluoride content of the samples from the curve obtained. Because interferes with the zirconium-alizarin reagent, all glassware should be scrupulously clean. The use of detergents containing phosphates or polyphosphates should be avoided; cleaning is best effected with chromic acid. RESULTS AND DISCUSSION
Sensitivity and Effect of Development Time on Color Intensity. Zirconium oxychloride a n d sodium aliz-
Figure 2. Standardization curves for fluoride determination using zirconyl-alizarin preformed lake
3 2
c
IC
23
___
to
4c
5c
6:
-_ ~
ac
9c
M I C R 3 S R A M S C C F L U O F UE
Figure 3. Effect of perchloric acid on absorbance of zirconyl-alizarin preformed lake
arin sulfonate were combined in a solution containing hydrochloric and sulfuric acids. Calibration curves established with solutions containing 1.75 mg. of sodium alizarin sulfonate per ml. ( p H = 1.0) showed maximum sensitivity with zirconium-alizarin ratios 1 to 1 and 1 to 0.9 (molar). Outside these limits and with decreasing lake concentrations, deterioration in sensitivity was observed and departure from the Beer-Lambert law became evident. At high zirconium-alizarin ratios reproducibility was poor, whereas increasing reagent concentration at the optimum ratios produced colors too intense for spectrophotometric measurements. Because of the longer color development time necessary a t higher hydrogen ion concentrations, no attempt was made to work below p H 1.0. The preformed lake possessed good stability, showing less than 1% change in absorbance 12 months after i t was prepared; calibration curves in the range 0 to 180 y of fluorine per 100 ml. gave very good agreement with the Beer-Lambert Ian- and showed no deterioration in sensitivity. The effect of development time on color intensity is shown in Figure 1, for four fluoride levels. A rapid change occurs in the first hour of color develop-
Table I.
Determination of Fluorine in Synthetic Samples
Zirconyl-Alizarin Thorium-AlizLake arin Lake FFFFound, Added, Found, y Dev. y Dev. Y 15 15.0 0 . 0 15.5 f 0 . 5 25 24.5 - 0 . 5 25.5 $ 0 . 5 35 33.0 + 2 . 0 38.0 $ 3 . 0 45 43.0 + 2 . 0 47.0 + 2 . 0 55 54.0 -1.0 56.0 $1.0 75 74.0 -1.0 75.5 + 0 . 5 85 85.0 0 . 0 8 5 , 5 +0.5 Std. dev. 1.2 1.5
ment. After 2 hours, however, the rate of change a t the 80-and 100-7 levels is much lower and can be ignored for the smaller concentrations. A minimum time of 21/* hours is, therefore, adequate. Figure 2 illustrates calibration curves for three development intervals. Absorbances read after 5 hours indicate a small change in the slope of the curve, but the consequent loss of sensitivitl- is of no practical importance; after 21 hours there is only a slight shift in the same direction. VOL. 32, NO. 7,JUNE 1960
835
Table II. Determination of Fluorine in Steam Distillates from Vegetation Samples
Zirconrlalizar jn 5 0 4 0
Fluorine Found, y Thoriumalizarin Dev. 6.5 +1.5 6.0
11 0
14.0 5.0 9.0 1 5
0 64 0 55 0 60 5 64 0
62.0
4 8 1 59
0 0 0
81 5 81 5 84 0 85 0 86 0 80 0 85 5 81 0 81 0 82 0 82 0 80 5 Std. dev.
65.0 59.0 59.5 65.5 77.0 78.5 86.5 85 5 82 0 81 0
88 5 82.0 84.0 84.5 84.5 79.0
+2.0 +3.0
+1.0 +1.0 C0.5 +3.0 +2.0
+4.0 -1.0
+l.5 -4.5 - 33 . 00
$2.5 0 -4.0
-3 0
-2.0 +3.0
$2.5 +2.5 -1.5 2 6
Effect of Free Acid and Ambient Temperature. Despite careful tem-
perature control during steam distillation of fluoride-bearing materials, some acid passes over into t h e distillate. Usually, however, this is small,
averaging 0.5 ml. of 0 . 0 5 s acid per 100 ml. of aliquot. Because of t h e low working p H of the zirconiumalizarin procedure (pH = 1.0) changes in hydrogen ion concentration brought about by such small amounts of acid are insignificant I n consequence, if temperature control is maintained during distillation, neutralization of the free acid becomes unneccesary. This is depicted in Figure 3, n here arc plotted calibration curves in the presence of three concentrations of perchloric acid, Equivalent to 1 , 5 , and 10 nil. of 0.05n‘ acid per 100 ml. of distillatc. The error introduced by 1 ml. of free acid is negligible. Comparison of rcsults of actual determinations obtained in the piesence of free acid n ith those found in a separate aliquot after neutralization shoned no greater effect than would be predicted from Figure 3. Tables I and I1 compare results on synthetic and vegetation samples with corresponding d u e s obtained by a thorium-alizarin lake procedure (pH = 2.30 f 0.02) similar to the one proposed by Icken and Blank ( 2 ) , aftei ashing with sodium and lithium carbonates. a fixative developed in thi3 laboratory,and tested on simples supplied by Stanford Research Institute. The ziiconiuni method required no p H adjustment. During this investigation small fluctuations in absorbance were observed for solutions of identical fluoride concentraI
tions, read at different times after the same development period. Because the rate of color development appears to be a function of the ambient temperature, for highest precision each group of fluoride determinations should be accompanied by a series of standards. However, as the calibration curve is strictly linear. only four points need be plotted. ACKNOWLEDGMENT
The author is indebted to Ilenri Shehyn of this laboratory for advice and comments during the course of this work and to Aluminium Laboratories Ltd. for permission to publish. REFERENCES
(1) Bumsted. H. E., Wells, J. C.. SAL.
CHEM.24, 1595 (1952). 12) Icken. J. X I . . Blank. B 11.. Ibid.. 25, 1741 (1953). (3) Liebhafsky, H. A , , \Tinslow, E. H., J . Am. Chem. SOC.6 0 , 1776 (1938). (4) Remmert, L. F., Parks, T. D., ~ X A L . CHEM,25,450 (1953). (5) Rowley, R. J., Grier, J. G., Parsons, R. L., Ibid., 25, 1061 (1953). (6) Sandell, E. B., “Colorimetric Determination of Traces of Metals.” Interscience, ?;ew Tork, 1944. (7) Willard, H. H., Winter, 0. B., 1x0. EYG.CHEM , -4SAI.. ED.5 , 7 (1933). RECEITED for review September 21, 1959. accepted January 25, 1960. Outline of paper presented at Boyce Thompson Institute Meeting, February 1959. \
,
Photometric Determination of Traces of Cobalt in High-Purity Nickel C. L. LUKE Bell Telephone laboratories, Inc., Murray Hill, N. 1.
b The sensitivity of the photometric nitroso-R method for the determination of small amounts of cobalt in nickel has been increased by separating the cobalt from a large sample of the nickel before attempting the photometric determination. Microgram quantities of cobalt can be quantitatively separaied from nickel by converting the cobalt to a stable cobaltic ammine and then precipitating the nickel as hexamminoperchlorate. The proposed new method is suitable, as written, to the determination of 0.001 to 0.02% of cobalt in high-purity nickel. By increasing the sample size and the light path in the photometric measurement, it should b e possible to determine as little as 0.000 1%.
836
ANALYTICAL CHEMISTRY
T
H E IKCREASING USE Of hlgh-pUrity nickel in the vacuum tube industry has necessitated the development of more sensitive analytical methods for the determination of the trace impurities present. I n particular, a new method for cobalt is needed, as none of those in use a t the present time possess sufficient sensitivity ( 1 , s ) . The direct photometric nitroso-R method for cobalt in nickel (I) has proved to be reliable and is attractive because of its reasonably good selectivity The method, holyever, is not very sensitive because of the limitation on sample size. I t seemed that a simple solution to the problem of increasing the sensitivity would be to separate the cobalt from a large sample of the nickel
before attempting the photometric determination. Xs a result of recent ivork in the deterniiriatioii of chroniium in nickel (6) it appeared probable that the separation could he accomplished by first conyerting the cobalt to a 1-ery stable cobaltic ammine and then removing the nickel by precipitation as hexamminoperchlorate. This has been confirmed and it lins thus been possible to increase the sensitivity of the nitrosoR method markedly n-ithout any decrease in accurary. REAGENTS
Standard Cobalt Solution, 10 y of
cobalt per ml. ( I ) . Sodium Acetate-Acetic Acid Buffer
Solution ( I ) .
