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 ) .
Nitroso-R Salt Solution. Recrystallize t h e commercially available salt from water as directed b y Andrew and Gentry (4).Dissolve 1 gram of the pure dried salt in 100 ml. of water and store in a glass-stoppered brown bottle. The solution is stable for several months if kept in the dark. Wash Solution. Mix 100 ml. of water plus 100 ml. of ammonium hydroxide plus 15 ml. of perchloric acid (70%). PREPARATION
OF
CALIBRATION CURVE
Dilute 0, 3.0, 6.0, 9.0, and 12.0 ml. of standard cobalt solution t o 25 nil. in a 125-ml. conical flask. Add 5 nil. of buffer solution follon-ed b y 2 ml. of nitroso-R solution. Heat just to boiling on a flame and then allow to remain just below the boiling point for about a minute. Add 5 ml. of nitric acid (1 a), heat just to boiling on a flame, and then cool to 25" C. in a cold nater bath. Dilute to 50 ml. in a volumetric flask and immediately measure photometrically in a 1-em. absorption cell a t 515 mp using distilled water as the reference.
+
purple. If the end point is overrun, neutralize to the desired color with sodium hydroxide solution (10% w./v.). Remove the paper, add 5 ml. of buffer and 2 ml. of nitroso-R, and continue as directed in Preparation of Calibration Curve. If the solution appears t o darken progressively during the photometric measurement or if the solution in the volumetric flask becomes cloudy, allow it to stand for 3 or 4 minutes, filter through a dry Whatman No. 42 paper, and repeat the photometric measurement. Carry a reagent blank through the procedure but use 5 ml. rather than 10 ml. of perchloric acid (1 2).
+
DISCUSSION
To prevent cobalt from accompanying the nicke! in the hexamminoperchlorate separation, it is necessary to convert to the cobalt state with a n oxidant such as hydrogen peroxide, brumine, or ammonium persulfate.
Table 1.
Analysis of Synthetic Nickel Samples
ANALYSIS
Cobalt,
OF SAMPLE
Dissolve 0.500 gram of the sample in 5 ml. of nitric acid (1 1) in a 125-ml. conical flask by heating gently. Boil momentarily t'o expel brown fumes. Add 15 ml. of water plus 5 ml. of citric acid solution (10% w./v.) and heat just to boiling. Remove to the bench, add 2 nil. of hydrogen peroxide (3% solution), swirl. add 20 ml. of ammonium hydroxide, allow to stand for about 30 seconds, and then add 10 ml. of perchloric acid (1 2). Cool to 20" t o 25" C. in a cold mater bath. Filter on a Buchner funnel, with the aid of suction, using a double layer of 5.5-em. JVhatman S o . 42 filter paper. Kash down the inside walls of the 125-ml. flask with 10 ml. of cool wash solution (20' to 25" C.). With the suction operating, pour in one stroke onto the precipit,ate so as to wet all of the latter. Discard the precipitate. Transfer the filtrate to a 200-nil. conical flask, add 2 or 3 small crystals of silicon carbide, and boil to a volume of about 25 ml. Place the flask in a cold w t e r bath and add 3 grams of pellet sodium hydroxide plus 2 drops of hydrofluoric acid. Boil until the solution no longer smells of ammonia and then reduce the volume t>o15 ml. Add 10 nil. of water plus a small piece of Congo paper and neutralize carefully with nitric acid until the color of the paper turns t o reddish
Added 10.0
+
+
60.0
120.0 10.0 30.0
60.0 90.0
100 0
y
Found 1o.oa 60.00
120.00 9.9
reasonable amounts of nickel, copper, and the hydroxide group metals (Table 1). To obtain complete color development of the cobalt-nitroso-R compound after separation of the cobalt from the nickel, i t is necessary t o destroy the cobaltic ammine by boiling with sodium hydroxide. Because of the attack on the glassnare during this boiling, a little hydrofluoric acid must be added to prevent the precipitation of silicic acid during the subsequent nentralization. Perchloric acid does not present :my difficulties in the photonietric determination ( 2 ) but occasionally, especially if more than 3 grams of sodium hydroxide is added, a precipitate will appear in the solution a t the time of the photometric measurement. The proposed method can probably be used for the determination of small amounts of cobalt in high-nickel steels, provided that the bulk of the iron is removed by a n ether evtractioii before the Precipitation of the nickrl. Moreover, a highly sensitive nictliod for the determination of traces of magnesium in nickel has been deyeloped in which the nickel and cobalt frum a 1-gram sample are removed hy the hexamminoperchlorate separation and the magnesium in the filtrate is then determined by the 8-quinolinol method ( 5 ) .
30.0
60.5 90.0 99.5
cl R b 10.0 60.0 59.9b 90. O b 90.0 120. 5 b 120.0 No citric acid added. * 1 mg. each of Fe, Ti, Mn, Si, Cu, Mg, and 0.1 mg. each of Zn, Cr, Ca, and P added. 5
JIicrogram quantities of such metals as iron, titanium, aluminum, manganese, and silicon can be tolerated in the separation, but a n appreciable amount of the cobalt is lost by coprecipitation on the hydroxide precipitates when milligram quantities of these metals are removed along with the nickel. Fortimately, this loss can be prevented b y complexing the metals with enough citric acid to prevent their precipitation. By using an excess of nitroso-R and b y making the photometric measurements at 515 nip it is possible to determine the cobalt in the prrsence of
EXPERIMENTAL
Several 0.5-gram samples of very pure. cobalt-free nickel were dissolved I ) Various aliin nitric acid (1 quots of standard cobalt solution plus, in some instances, aliquots of standard solutions of other metal impurities normally found in nickel were added. All the metal ions added were in their normal valence states. The mixtures were then analyzed for cobalt. The results obtained are shown in Table I.
+
LITERATURE CITED
(1) Am. Soc. Testing Materials, Phil-
adelphia, Pa., "ASTM Methods of Chemical Analysis of Metals," p. 245, 1956. ( 2 ) Ibid., p. 274. (3) Andrew, T. R., Gentry, C. H. R., Metallurgia 60, 69 (1959). (4) Ibid., p. 72. ( 5 ) Luke, C. L., ANAL. CHEM.28, 1443 (1956). (6) Zbid., 30, 359 (1958). RECEIVEDfor review February 1, 1960. Accepted March 15, 1960.
VOL. 32, NO. 7, JUNE 1960
837
