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solutions containing 1.0 gram of NHlCl per 100 ml. when high concentrations of iron and acid are present. Analysis of Standard Samples. The applicability of the method was tested on various National Bureau of Standards (N.B.S.) low alloy steels and irons using the recommended 2.0 grams of NHICl per 100 ml., and also 1.0 gram of NH4C1 per 100 ml. in both samples and standards. The results of these analyses are given in Table V. There appears to be little difference in the results obtained using the two different concentrations of ammonium chloride, and most of the results are in reasonable agreement with the certificate values. The results obtained on Sample 4h were consistently low, n hich niay have been due to occlusion of chromium by the relatively large amount (1.79y0) of graphitic carbon which remained undissolved in the acid solution employed. Interferences. Samples containing tungsten yielded low chromium values by the procedure described, presumably due to the occlusion of chromium by the precipitated tungstic oxide. The addition of ammonium chloride to sulfuric-phosphoric acid mixtures (e) which retain tungsten in solution, did not overcome the depressive effect of iron on chromium absorption.
Table V.
Analysis of Standard Samples Chromium, %
Found N.B.S. sample 55b, ingot iron 5j, cast iron 8h, Bessemer steel l O j , Bessemer steel l l l b , Ni-Mo steel 4h, cast iron 156, Cr-Ni-Mo steel 159, Cr-Mo-Ag steel 36a, Cr-Mo steel
2.0 grams
NH4C1/100 ml. 0.003 0.021 n .023 0.068 0.105, 0.110 0.420 0.98 2.38
Advantages of NH4Cl Procedure. When the suggested procedure can be applied, two distinct advantages are realized. These are a considerable increase in sensitivity and elimination of the necessity of preparing and using matched standards. However, since the geometry of burner design and sampling systems varies with different makes of atomic absorption equipment, these advantageous effects may not be realized on all types of instruments. Other Interference Suppressors. A limited investigation of the use of other salts as interference suppressors has indicated t h a t the chlorides of the alkalis and alkaline earths may be
1.0 gram NH4C1/100ml.
Certificate value
0.003 0.021. 0.022 0.019 0.024 0.069 0.108, 0.109 0.435 1.02 2.40
0.002 0.022 0.022 0.023 0.070 0.117 0.429 1.oo 2.41
of utility. Cesium, rubidium, potassium, and calcium chloride appear to be the most effective. Analytical application of the use of these salts for the determination of chromium, however, has not been attempted. LITERATURE CITED
(1) David, D. J., Analyst 86, 730 (1961). (2) Kinson, K., Hodges, R. J., Belcher, C. B., Anal. Chim.Acta 29, 134 (1963). (3) Nostyn, R. A., Cunningham, A. F., ANAL.CHEM.38, 121 (1966).
LUCIENBARNES, JR. Air Reduction Co., Inc. Central Research Laboratories Murray Hill, N. J. 07971
Alkali Titration of Germanium SIR: There are few methods described in the literature for the determination of germanium. Most of these depend upon the absence of chloride, and this is impossible when the sample is soluble only in hydrochloric acid or aqua regia. The sample to be analyzed was a goldgermanium alloy, soluble only in aqua regia, and requiring a distillation of the germanium as its tetrachloride. Grayimetric precipitating agents described in the literature are hydrogen sulfide (8),tannin ( 5 ) , and magnesium sulfate ( 3 ) . Two more reagents ( 4 ) , cupferron and tetraphenylarsonium chloride, precipitate the related element tin. These were also tried without success. The presence of large amounts of chloride seems to interfere in all but the tannin method. Yet with tannin the recoveries of germanium are low unless some chloride is present. At best, the gravimetric methods are s l o ~ ~For , instance, the germanium sulfide and magnesium germanate must sit for 2 days before they can be filtered. The bulky organic precipitates can be filtered n ithin an hour, but they require much nashing to be freed from chlorides. The cupferron precipitate re-
quires a t least 12 washings before there is no reaction of the washings with silver nitrate. Only two volumetric methods were noticed in the literature, the iodometric titration (6) of a thiogennanate, and the alkali titration (1) of a germanic acid-mannitol complex. Both, as described, require a chloride-free solution. In this work, the alkali titration has been adapted to the germanium chloride distillate, which contains much hydrochloric acid and some nitric acid. Because of the nitric acid, the indicators had to be changed. A better pH for the start and finish of the titration was found. If the amount of germanium is over 20 mg., the presence of carbon dioxide does not interfere, except that it means selecting an empirical end point, which is a little before the pH where the potentiometric inflection is greatest. Of course, one cannot boil out carbon dioxide without losing some volatile germanium chloride. EXPERIMENTAL
Reagent. NaOH, 0.02N, standardized against pure germanium was used throughout t h e procedure.
Procedure for Gold-Germanium Alloy. Weigh a sample containing between 20 and 35 mg. of germanium and place it in a flask connected to a condenser. Add 20 ml. of 12F hydrochloric acid and 5 ml. of 16F nitric acid. Allow t h e acids to react a t room temperature for 1 hour. (For pure germanium this time must be 4 hours.) The outlet of the still must be immersed in 75 ml. of distilled water cooled in an ice bath. When the sample is nearly dissolved, gradually heat the acids to boiling. Continue heating until only about 3 ml. of liquid are left in the flask. Once air in the flask has been displaced by steam, the distillate may be sucked back. This may be prevented by opening an air valve, placed near the outlet end of the still, and equalizing the pressure. When the distillation is complete, remove the stopper of the distillation flask, and let the apparatus cool. Disconnect the flask and wash germanium oxide from the sides and tip of the condenser tube with 25 ml. of 6N sodium hydroxide. (If a gravimetric finish is desired, ammonium hydroxide may be substituted.) Finally, wash the condenser tube with water, and it is ready for the next sample. Allow the distillate to cool in VOL. 38, NO. 8, JULY 1966
1085
Table 1.
Titration Results with Pure Germanium
. - ..( i t s ,
1’1 C%f,tl I
m.FL
Ioiittd
19,9
20.3 22.1
22,o 22.3
22.8 25.2
25.I 25.3 2G.0
25.1 25.7 2!) s 96,4
29 ( J
3G.!J Sttl. tlcv.
=
Uev., mg. +0.4 +0. 1 0.0
+0.1 -0.2 -0.3
0 . 1 Illp;
II. Results with GermaniumGold Alloy Sample, gr:lIll? Gc, 5‘0 Dev., %
Downloaded by RUTGERS UNIV on September 8, 2015 | http://pubs.acs.org Publication Date: July 1, 1966 | doi: 10.1021/ac60240a039
0 1834
12 07 12 05 1 1 90 1 1 82
2047 2107 217.3 2420 3079
Mean Rel. std. dev.
12 32 1 1 82 12.00% =
+o
$0 -0 -0 $0 -0
07 05 10 18 32 18
1.58
ice water while the gold solution is washed out of the sample flask for eventual recovery. This cooling prevents loss of germanium chloride, and helps to Iwvent the formation of a colored goltl so1 formcd from traces of gold which soinetinics di5t ’I l over. T o thc cold distillat’r add a few drops of I)lirnolplitlialcin (0.5yoin ethanol) indicator. . \ t M 6.Y sotliuni hydroxide until there is jiwt moiig-h to turn the indirator r d . I t takes about 15 ml. plus tlic 25 nil. s h a d y added to rinse out t h r condtnscr. Iinniediately add 6N hydrochloric acid a drop at a time until the red color diFap1)cars. Kow add five drops of p-nitrophenol indicator (0.1%). If the solution is yellow, add onc morc tlrol) of 6,V hydrochloric acid. Ft.oin a \)uwt ntld 0.02.V sodium hydrositlc until the solution is yellow (pH 5 . 5 ) . At thi-: point ta.ke the initial reading of the buret. Add 1.0 gram of mannitol anti bcgin the actual titration. Whrn the yellow color has returned, add five tli,ol)s of phcnol red indicator (saturatrd in watcr). .\ drcprr ycllow color results. Continue the titration until a clefinit? rcd tint is seen by looking horizontally through the solution (pH 6.8). Take t8hisI m e t reading as the end of the titration. The germaniiim-m~niiitcil conq)lex acts as a monoprot’icacic I. The titration inay also be made with a potcntiometrr or pH meter.
1086
RED
CHANGES
SO.8 -0.5
Table
0 0 0 0 0
PHENOL
ANALYTICAL CHEMISTRY
t ML.
Figure 1.
NAOH
Potentiometric curve
RESULTS A N D DISCUSSION
Choice of Indicators. The brom cresol purple suggested by Cluley (1) is unsuitable because it reacts with the nitrates present. Phenolphthalein was chosen for the rough adjustment because it is stable toward nitric acid and is colorless through the range of the titration. p-Nitrophenol and phenol red were chosen because they are compatible and change a t the right pH. The choice of pH was made after running a potentiometric titration and calculating where the end point should be. The potentiometric curve is shown in Figure 1. The curve shows that the point of greatest inflection is a t a pH of 7.6. A p H of 6.8 is chosen as the end point to compensate for the carbonate that is always present in sodium hydroxide. Interferences. After germanium chloride is distilled, about the only interferences possible are acids of arsenic, boron, and carbon. The aqua regia would take arsenic t o the pentavalent state, which is not volatile under the conditions of the distillation. The amount of boron picked up from borosilicate glass is insignificant (equivalent to 0.05 ml. of 0.02N sodium hydroxide), No attempt was made to remove carbon dioxide; the end point was adjusted for it. This, however, limits the range in the size of the sample, and means that the sodium hydroxide should be standardized against pure germanium carried through the procedure. Accuracy and Precision. The procedure was checked against pure
germanium and germanium-gold alloy, nominally 12% germanium. The results for known amounts of germanium are shown in Table I, and for gerinanium in the gold alloy in Table 11. The gold content of the alloy was foiind to be 88.1%. The mean for the germanium-gold alloy was 12.00% gcimanium. The standard deviation is 0.19%. For pure germanium the standard deviation is 0.4 mg. An advantage of the volumetric method is that the distillation and titration together take only about 3 hours for a single sample. A good gravimetric method may give somewhat more precise results, but it takes 2 or more days of elaped time. LITERATURE CITED
( 1 ) Cluley, R . J., itnalust 76, 517 (1951). (2) Hillebratid, W.F.,Liiiidell, G. E. F., Bright, H. .4., Hoffmm, J. I., “Applied Inorganic Aiinlysis”, p. 300, Wiley, New York (1953). J . A W L Cheni. . SOC.44, (3) RIueller, J. H., 2483 (1922). (4),Nad:din, R. J., “Concentration Tech-
niqries Before Spectrographic Analysis,” Westinghoiise ltesearch Rept. (1961). ( 5 ) Weisler, Alfred, AN.\L. CIIEM.16, 311 (1944). (6) Willard, H. H., Ziiehlke, C. W., Ibid., 16, 322 (1944). J.\MES
F. REED
Pennsylvania State Univemit y McKeesport, Pa. 15132 RESEARCHsponsored by Williams Gold Refining Co. of BiiffFlo, N. Y. Winter Meeting, ACS, Phoenix, January 19GG.
