V O L U M E 28, N O . 5, M A Y 1 9 5 6 of tryptophan in 5 ml. of phosphate buffer ( p H 1.38) for 4 hours at 118’ C. All samples were diluted 12.5 times with sodium citrate-hydrochloric acid buffer ( p H 1.80) before reading in the photofluorometer. D a t a representing the average of 5 separate analyses were plotted and a linear relationship was shown to exist between the tryptophan concentration and the fluorescence intensity developed under standard condit,ions. A standard error of f3.07,for 12 y of tryptophan was calculated. The error of estimate was essentially constant over the ent’ire range, up to 20 y of trypt,ophan. There was no difference in the reactions of 1,D- alld DL-tryptophan. Sornially the fluorescence intensity of a 2 to 25 dilution of the sample was determined. The fluorescent materials could be extracted with isobutyl alcohol with no loss of fluorescence, thus providing a means for concentrat,ing these products and nxiking the method more sensitive. This extraction technique is similar t o t h a t applied in the analysis of thiamin by the thiochi,ome nitsthod (1), LITER.ATURE CITED
(1) .kssoviation of Vitamin Chemists, Inc., “Methods of Vitamin d a a a y . ” 2nd ed., p. 111, Intersrience, S e w York, 1951. ( 2 ) Friedman, L., Kline, 0. L., J . Biol. C‘heni. 184, 599 (1950).
887 (3) Gordon, M., Mitchell, H. K., Ibid., 180, 1065 (1949). (4) Graham, W. D., Hsu, P. Y., hIcGinnis, J., Science 110, 215 (1949). (5) Haney, H. N., hL. 9. thesis, Kansas State College, hfanhattan, Kan., 1952. (6) Lingane, J. J., A N A L .C H m f . 19, 810 (1947). (7) Naillard, L. C., Compt. rend. 154, 66 (1912). (8) Miller, B. S., Seiffe, J. Y., Shellenberger, J. A . , Miller, G. D., CereaZ Chem. 27, 96 (1950). (9) llohammad, A., Fraenkel-Conrat, H., Olcott, H. S., Arch. Biochem. 24, 157 (1949). (10) lloore, S., Stein, W. H., J . B i d . Chem. 176, 367 (1948). (11) Ibid., 192, 663 (1951). (12) Portner, C., Hogl, O., Anal. Chim. Acta 8, 29 (1953). (13) Schram, E., Dustin, J. P., lloore, S., Bigwood, E. J.. I b i d . , 9, 149 (1953). (14) Spies, J. R., Chambers, D. C., ASAL. CHEM.20, 30 (1948). (15) Ibid., 21, 1249 (1949). (16) Ibid., 22, 1209 (1950). (17) I b i d . , p. 1447. RECEIVED for review August 2.5, 1955. -4ccepted February 20, 1956. Division of Analytical Chemistry, 128th Meeting, ACS, Minneapolis, hlinn.. September 1955. Contribution 262, Department of Flour and Feed hlilling Industries, Kansas Agricultural Experiment Station, Manhattan, Kan. Taken from a thesis presented t o the Graduate School of Kansas State Collega b y Gerald D. Miller in partial fulfillment of the requirements for the degree of master of science.
Determination of Nickel in Oxidized Films on Nickel Metal B. D. BRUMMET and R. M. HOLLWEG’ Edison Laboratory, Thomas A. Edison, Inc., W e s t Orange,
A rapid spectrophotometric method for the deterniination of nickel in oxidized films on nickel metal has been developed. The solvent used, 0.5% potassium cyanide, dissol\es the oxidized film but not the nickel metal. The method is sensiti\e to niicrogram quantities of nickel.
T
HE formation of films of oxidized nickel on nicLe1 metal has been studied by several investigators. I n most cases the film was measured by either \veight changes or x-ray or electron ditrruction. The latter method requires removal of the film from the base metal. Techniques for stripping oxidized films from h a c metal have been developed hy Evans and Stockdale ( 1 I > Phelps, Gulbransm, and Hirkman ( S ) , and \-ernon, Wormx-eIl>and S u r s e (6). Both chemic-al and electrochemical methods have been employed. Thin films of nickel hydroxide can be electroplated on nicakel metal from a borate-buffered solution of nickel sulfate ( 2 1 . h study of the electrochemical characteristics of films of this type mndc it necessary to determine the amount of nickel in the film. K h r n n-orking with thin films on nickel metal, very small amounts of the base metal whirh may dissolve cause large errors in the film analysis. Therefo1.e: the solvent used must be selective for the oxidized film. .\ number of acids of varying concentrations were tried, but in all cases some of the base metal was dissolved. A 2 % solution of formic acid was found most suitable of those tested. The oxidized film dissolved readily in this solution while the base metal dissolved very slowly. However, there was some difficulty in determining when the film was completely dissolved. Some of the common nickel complexing agents were also tried. An aqueous solution of potassium cyanide was found to be specific for the oxidized film, provided all ot,her osygen was excluded from 1
Present address, Chas. A. Pfizer Co. Inc., New York, S . Y .
N. J. the solution. I n addition to its specificity, it was useful in the analy& of solutions containing microquantities of nickel. Prior to the use of the cyanide method, the nickel dissolved by the formic acid was determined spectrophotometrically with dimethylglyoxime ( 4 ) . This method v a s not applicable to solntions of nickel cyanide, but ultraviolet absorption by the nickel cyanide complex solved the problem simply and satisfactorily. NICKEL DETERMINATIOK
The ultraviolet absorption spwtrum of a 0.5% solution of potassium cyanide containing nickel ion is shown in Figure 1. There are three absorption peaks, which occur a t 208, 268, and 286 mp. A much neaker absorption peak appears a t 310 mG. A calibration curve was prepared for the absorption peak at 268 nip; a Beer’s law curve was obtained in the range from
202
u ,
0 2 0 / 210
220
230
240
250
2 6 0 210 280 290 W A V E LENGTH, mu
I 300 310
320 330
340 U O
Figure 1. Absorption spectrum of nickel in 0.5% potassium cyanide solution
888
ANALYTICAL CHEMISTRY
2 x 10-7 t o 25 X 10-7 gram of nickel per milliliter. This offers a sensitive method of analysis in t h e part per million range. SELECTIVITY OF POTASSIUM CYANIDE ;Z nickel screen approximately 21/2 by 25/* inches was used to determine the selectivity of aqueous potassium cyanide for the oxidized film. T h e screen was first cleaned in concentrated hydrochloric acid and washed with distilled \yater. I t was then
I ICURVE 6ooj f
v
0
~
0
IO
20
I
30
'
-
CURVE I
I
I
40 SO 6 0 70 80 M I N U T E S IN SOLUTION
I SO
I 100
I 110
I I20
Figure 2. Amount of nickel dissolved in 0.5% potassium cyanide solutions under various conditions 1. Screen removed from aqueous potassium cyanide into nitrogen atmosphere, then dipped into fresh, nitrogen-flushed potassium cyanide solution 2. Screen cleaned in concentrated hydrochloric acid and air-dried, then placed in nitrogen atmosphere, and dipped into nitrogen-flushed potassium cyanide 3. Screen cleaned in concentrated hydrochloric acid and air-dried, then dipped in potassium cyanide which had been nitrogen-flushed in contact with air 4 . Screen cleaned in concentrated hydrochloric acid, then dipped into potassium cyanide with nitrogen flush a. Screen cleaned in concentrated hydrochloric acid and dipped into potassium cyanide saturated with oxygen 6. Screen cleaned in concentrated hydrochloric acid, then dipped into potassium cyanide with oxygen bubbling over screen
dipped in potassium cyanide solutions for varying lengths of time and the amount of nickel dissolved was determined. The hydrochloric acid dip, followed by a water wash, n as repeated after each potassium cyanide dip. T h e amount of nickel dissolved by successive dips increased u p t o a contact point of 1 minute. Longer contact periods failed t o dissolve more nickel. T h e surface of the metal appeared to have a soluble film which could be removed by the cyanide solution, after which nothing more was dissolved. The conclusion was reached t h a t a fairly reproducible film of olide or hydroxide was formed on the nickel when it was exposed t o air. -4dditional tests showed that, when the sample was moved as rapidly as possible from the first potassium cyanide solution t o the second, there was still an appreciable amount of nickel dissolved. T h e rate of film formation was rapid. T h e experiment was then repeated with a nickel screen and a solid nickel plate in a nitrogen atmosphere t o avoid exposure t o oxygen. With both samples nickel was found in the first solution after 2 minutes' contact, but not in the second, third, or fourth aolutions. It was concluded t h a t the nickel found in the first solution amounted t o the air-oxidized film on t h e nickel Because
the samples were not exposed to air between the second, third, and fourth solutions, no oxidation could occur and, therefore, no nickel was dissolved. Continued work on the determination indicated t h a t the amount of nickel which dissolved JYas sensitive to the amount of oxygen present in the potassium cyanide solution. T o clarify this point, several experiments were performed, starting with solutions free of dissolved oxygen and continuing t o solutions saturated with oxygen. The dattaobtained are shown in Figure 2. These experiments show t h a t elemental nickel is insoluble in a 0.5% potassium cyanide solution if oxygen is not present in the solution. I n order t o prove conclusively the usefulness of ?he method, it should also be shown t h a t an oxidized film is completely dissolved in the potassium cyanide solution. This is difficult because the stoichiometry of oxidized films is indefinite, a faet which makes it difficult t o prepare a film containing a known amount of nickel. Although a known sample cannot be prepared, complete solution of the oxidized film in potassium cyanide eolution was demonstrated by sho\yiiig in the following way that the base metal \vas free of appreciable quantities of oxidized film after the potassium cyanide dip. A sample was prepared from 16 strips of nickel metal, 3 mils thick and 1 by 3 inches; this offered a large surfitce area. .4pproximately 10 t o 12 mg. of oxidized film was plated from a borate-buffered nickel sulfate solution. The film n-as then dissolved in potassium cyanide and the electrode thoroughly washed with water, dried, and weighed. I t was then reduced in a hydrogen atmosphere for 1 hour at 950" C. and again weighed. The change in weight resulting from the hydrogen reduction \vas f 0 . 2 mg., which is approximately the experimental error for samples of this type. If the oxidized film had not been completely removed by the potassium cyanide dip, there would have been an appreciable loss in byeight resulting from the hj-tlrogen reduction. PROCEDURE
Aqueous potassium cyanide, 0.5%, was flushed 10 miiiutw with nitrogen gas. T h e sample mas immersed in the solution foi 2 t o 5 minutes, depending on the film thickness, and t h e nitrogen flush was continued during t h e contact period. T h e resulting solution was diluted with 0.5% potassium cyanide t o a concentration suitable for analysis and the absorbance as measured at 268 mfi, using 0.5% potassium cyanide for a blank. DISCUSSION
When elemental nickel is exposed t o air, an oxidiLed film very rapidly forms on the surface. This film is soluble in aqueous solutions of potassium cyanide. T h e oxidized film deposited electrolytically on nickel metal from a borate-buffered nickel sulfate solution is also completely soluble in aqueous potasqium cyanide. Both of these films can be dissolved without dissolving the base metal if oxygen is excluded from the solutions. For very accurate work a nitrogen box can be used t o provide a nitrogen atmosphere. However, good results can be obtained hy flushing the solution with nitrogen before addition of the sample and then continuing the flush during the contact period. This method may have general applicability t o the study of corrosion rates of nickel metal. LITERATURE CITED
(1) Evans, U. R., Stockdale, J . , J . Chem. SOC.1929,2651. Halssinsky, hl., Quesney. AI., C o m p f . rend. 223, 792-4 (1946). (3) Phelps, R. T., Gulbransen. E. 4.,Hicknian, J. W., IND.€sa. C H E M . , A N A L . ED.18, 391 (1946).
(2)
Porcelain Enamel Institute, Quality Development Committee, Washington. D. C., Bull. T-16. ( 5 ) T'ernon, W. H. J., Wormwell, F., Nurse, T. J., J . Chem. SOC.
(4)
1939, 621. RECEIVED for review .4pril 6 , 19.55.
Accepted February 23, 1956.
