ANALYTICAL CHEMISTRY
1582 0.09412. The indicator blank is normally negligible, but it should be checked.
(5) Hillebrand, W. F., Lundell, G. E. F.,Bright, H. A., and Hoffman, J. I., “Applied lnorganic Analysis,” 2nd ed., pp, 180-1, Wiley, S e w York, 1953. (6) Koenigs, E., and Greiner, H., Ber. deut. chem. Ges., 64, 1049-56
SU*MMMARY
(1931). (7) Kolthoff, I. >I., and Sandell, E. B., “Textbook of Quantitative Inorganic dnalysis,” 3rd ed., pp. 5 2 2 4 , hlacmillan, Yew York, 1952. (8) Stone, K. G., unpublished results. (9) Tropsch, H., Monatsh. Chem., 35, 775-9 (1914).
4-Aminopyridine is a solid, monoprotic base which is suitable as a standard for acidimetry when methyl red is used as the indicator in the titration. LITERATURE CITED
(1) Albert, A., J . Chem. SOC.,1951, 1376. (2) Camps, R., Chem. Zentr., 7 3 11, 647-9 (1902). (3) Farr, H. V., Butler, A. Q.,and Tuthill, S. M., ASAL. CHEM.,23, 1534-7 (1951). ( 4 ) Fossum, J. H.. Rfarkunas, P. C., and Riddick, J. A., Ibid.. 23, 491-3 (1951).
(IO) Wibaut, J. P., Heraberg, S.,and Schlatmann, J., Rec. trav. chim., 73, 140-2 (1954). RECEIVED for review February 10, 1956. Accepted June 24, 1955. Abstracted from M.S. thesis submitted b y Clayton E. Van Hall in J u n e 1954 to Graduate Faculty of Michigan State University. Presented before the Division of Analytical Chemistry, 127th Meeting, ACS, Cincinnati, Ohio.
Complexometric Titration of Indium KUANG LU CHENG’ Department of Chemistry, University of Connecticut, Storrs,
This investigation was undertaken to develop a rapid and accurate method for the titration of small amounts of indium. Indium may be titrated directly with (ethylenedinitri1o)tetraacetic acid using l-(2-pyridylazo)-2-naphthol as an indicator at pH 2.3 to 2.5 or pH 7 to 8 with an accuracy within 0.5%. At pH 2.3 to 2.5, alkali metals, alkaline earth metals, aluminum, and manganese do not interfere. A t pH 7 to 8, copper, zinc, cadmium, nickel, silver, mercury, and other metals which form very stable complexes with cyanide do not interfere if cyanide is added. Iron may be masked by the addition of fluoride. The common anions such as chloride, sulfate, nitrate, perchlorate, fluoride, tartrate, and citrate do not interfere. Bismuth, lead, gallium, and tin interfere.
A
C O M l I 0 4 method for the determination of indium in-
volves weighing the precipitate as indium sesquioxide after ignition of the hydroxide a t llOOo to 1200’ C. An aniperometric method ( 7 ) and a flame photometric method (6) have been proposed. Recently, Flaschka and others (2-4) have reported the titration of indium by (ethylenedinitri1o)tetraacetic arid (ethylenediaminetetraacetic acid) using eriorhrome black T as the indicator and a lead solution for back-titration at pH 10. I n the course of an investigation of the reaction of metals n i t h 1-(2-pyridylazo)-2-naphthol( 1 ) it appeared that this azo dye might be used as an indicator in the complexometric titration of indium. This paper describes a simple and quick method for the direct titration of indium by (ethylenedinitri1o)tetrpacetic acid using 1-(2-pvridylazo)-2-naphtholas the indicator. The titration is carried out a t pH 2.3 to 2.5 using acetic acid as a buffer. Alkali metals, alkaline earth metals, aluminum, and manganev(I1) do not interfere with the titration, The heavy metals which form strong complexes with cyanide do not interfere if the titration is made in the presence of cyanide a t p H 7 to 8. Jentzsch and others ( 5 ) developed a simple scheme for the separation of indium. When this separation is followed by a complexometric titration, the result is a rapid and widely applicable analytical method for indium. REAGENTS
(Ethylenedinitri1o)tetraacetic acid solution, 0.01M. Approximately 3.72 grams of the disodium salt of (ethylenedinitri1o)tetra1
Present address, Westinghouse Electric Corp., E a s t Pittsburgh, P a
Conn.
acetic acid were dissolved in water and diluted to 1 liter. This solution waa then standardized against the standard indium solution using the procedure recommended in this paper. Standard indium solution, 0.01M. An accurate weight of 1.1 to 1.2 grams of indium metal (99.95%) in a small amount of hydrochloric acid was heated on a hot plate and diluted to 1 liter with water. The pure indium metal was obtained from the Indium Co. of America. Indicator solution, 0.01%. Approximately 0.001 gram of 1-(2pyridylazo)-2-naphthol was dissolved in 10 ml. of methanol. The indicator solution is very stable. The preparation of the azo dye has been described ( 1 ) . Sodium hydroxide, 1 s . All other chemicals used were of reagent grade. PROCEDURE
Titration at pH 2.3 to 2.5. The solution containing 0.05 to 0.2 millimole of indium in a 200-ml. beaker was diluted to approximately 50 ml. and neutralized with 1.V sodium hydroxide until a slight white precipitate was formed. Two milliliters of glacial acetic acid were added to dissolve the precipitate. The solution was titrated with (ethylenedinitri1o)tetraacetic acid solution after addition of 2 drops of the indicator solution. The end point from red to pure yellow was very sharp. Titration at pH 7 to 8. I n the presence of copper, zinc, nickel, and other metals which form strong complexes with cyanide, the titration of indium in alkaline medium using cyanide to mask the interference is recommended. The solution containing 0.05 to 0.2 millimole of indium in a 200-ml. beaker n-as diluted to approximately 50 ml. and adjusted to pH 7 to 8 using acetic acid and ammonium acetate after the addition of a suitable amount of potassium cyanide and approximately 1 gram of potassium sodium tartrate. The solution was titrated with (ethylenedinitri1o)tetraacetic acid solution after addition of 2 drops of the indicator solution. The end point was also from red to pure yellow. The calculation may readily be made according to a 1 to 1 ratio of the indium-(ethvlenedinitri1o)tetraacetic acid complex DISCUSSION
Effect of pH. Flaschka and Amin (3, 4) used eriochrome black T as the indicator, which requires a p H of 10 for detecting the end point. I t was found, by the author, that indium was still strongly complexed by (ethJ-1enedinitrilo)tetraacetic arid a t pH 2 and that indium could be titrated by (ethylenedinitri1o)tetraacptic acid a t a wide range of pH between 2 to 10 using 1-(2pyridylazo)-2-naphthol as an indicator. KO sharp end point was obtained when the solution was adjusted to pH 1.5 or below because the indium-indicator complex is not stable at that low pH. I t was also impossible to titrate indium at very high pH because the formation of indium hydroxide caused a slow end point and because the indicator itself is an acid-base indicator; it changes
1583
V O L U M E 2 7 , NO. 1 0 , O C T O B E R 1 9 5 5 Table 1. Titration of Indium in Presence of Foreign ;\letals 3Ietal Added, 31g.
Taken
Indium, hlilliinole Founda
0.0520 0.1040 0.1560 0,2080
0.0522 0,1040 0.1568 0.2070
0.0520 0.0520 0.0520 0,0520
0.0520 0.0520 0.0520 n . 0520 0,0520 a
0.0522 0.0520 0.0521 0.0523 0.0520
0.0518 0.0521
0,0521 0 0523
Error, 4 38 00 13 48 -0 38 0 00 -0 19 -0 58 0 00 -0 18 T O 1') TO 0 -0 -0
+o -0
19
5s
rlrerage of triplicates.
color from yellow to pinkish a t pH above 11. The selection of p H for the titration depends upon the presence of interfering metals. Interferences. \Then eriochrome black T is used as the indicator in the complexometric titration of indium a t p H 10, interferences from aluminum, manganese, alkaline earth metals, and iron would be expected because they form colored complexes with the eriochrome black T indicator. Furthermore, the titration should be made in the boiling solution. It was found that these metals did not form colored complexes with 1-(2-pyridylazo)-2naphthol and were not strongly complexed by (ethylenedinitri1o)tetraacetic arid in the acid medium. Therefore, indium may be titrated with (ethylenedinitri1o)tetraacetic acid using 1-(2-pyri-
dylazo)-2-naphthol as the indicator in the presence of these metals at pH 2.3 to 2.5. When the titration is made a t pH 7 to 8 xnd suitable amounts of c p n i d e are added, indium may be titrated in the presence of copper, zinc, nickel, cadmium, cobalt, silver, mercury, and other metals which form very stable complexes with cyanide. Some tj-pica1 results are also shown in Table I. +4slow end point was encountered if the titration was made in the absence of tartrate a t high pH. Iron(II1) interference could be eliminated if the titration tvas made a t pH 7 to 8 and 1 gram of potassium fluoride, 1 gram of potassium sodium tartrate, and a small amount of potassium cyanide were added. The coninion anions such as cthloride, sulfate, nitrate, perchlorate, fluoride, tartrate, and citrate did not interfere. Bismuth, lead, gallium, and tin interfered. Accuracy. By using a microburet, an accuracy of 0.570 or tietter was obtained for 0.05 to 0.2 millimole of indium. LITERATURE CITED
Cheng, K. L., and Bray, R. H., ANAL.CHEM.,27, 782-5 (1955). Flaschka, H., and Abdine, H., Mzk~ochzm.Acta, in press. Flaschka, H , and Amin, A. M., Ibad., 1953, 410-12. Flaschka.. H... and Amin. A. LI.. 2. anal. Chem.. 140. 6-9 (1953). Jentssch, D., Frotscher,' I., Schwerdtfeger, G.,' and' Sarfek, G'., I b i d . , 144, 8-16 (1955). (6) hfeloche, V. W., Ramsay, J. B., Mack, D. J., and Philip, T. V., ASAL. CHEM.,26, 1387-8 (1954). (7) Nimer, E. L., Hamm. R. E., and Lee, G. L., Ibid., 22, 790--3 (1950).
(1) (2) (3) (4) ~, (5)
RECEIVED for reriew hIay 5 ,
1955.
Accepted July 13, 1955.
Flame Photometric Determination of Silver in Cadmium and Zinc Sulfide Phosphors A. 0. RATHJE Chemical Products Works, General Electric Co., Cleveland,
Because silver is a common activator for sulfide-tj-pe phosphors, 3 rapid method for its determination was desired. This paper describes a flame photometric method for silver which is both rapid and accurate. No separations are required. The interferences from other ingredients in the sample have been studied and methods developed for overcoming these interferences.
N
EARLY all the flame photometric methods reported in the
literature have been for the determination of alkali and alkaline earth metals. Extensive coverage has been given to the determination of these elements in a wide variety of materials and t o the interferences caused by other metals and anions in the samples. With the advent of photomultiplier tubes and improved burners, the sensitivity of the flame photometer has been greatly increased, making possible the determination of small amounts of many other elements. The rather low detectioii limit reported for silver ( 4 ) made such a determination feasible in the sulfide-type phosphors. Silver is commonly used as the activator in zinc sulfide and zinc cadmium sulfide phosphor-, in a concentration on the order of approximately O.Ol'%. This paper presents a rapid method for accurately determining small amounts of silver in these phosphors without troublesome separations. APPARATUS AND REAGEWTS
Beckman Model DU spectrophotometer equipped u ith a Model 9200 flame photometry attachment, hydrogen-oxygen burner, and photomultiplier attachment.
Ohio Zinc sulfide, silver-free, may be made by precipitating zinc from a solution of zinc sulfate (conforming to ACS specifications) with hydrogen sulfide, washing with water, and drying a t 105" C. Cadmium sulfide, silver-free, may be made by precipitating the cadmium from a solution of ACS grade cadmium sulfate with hydrogen sulfide, followed by washing with water and drying a t 105" C. Standard silver solution, 50 y of silver per ml. Dissolve 0.1575 gram of ACS grade silver nitrate in 2 liters of distilled water. Sodium chloride solution,' 100 y of sodium per ml. Dissolve 0.2542 gram of ACS grade sodium chloride in 1 liter of distilled water. Magnesium chloride solution, 1.0 mg. of magnesium per ml. Dissolve 8.36 grams of ACS grade magnesium chloride hexahydrate in 1 liter of distilled water. EXPERIMENTAL
Silver exhibits two relatively strong flame emission lines, one occurring at 328.1 m9 and the other a t 338.3 mp. Of the two, the former is somewhat less intense and has a higher flame background; hence, the line a t 338.3 mp was chosen for all experimental work. Under the conditions employed the flame emission is directly proportional to the silver concentration in the range 0 to 500 y of silver per 100 ml. of solution (Figure 1). Use of Acetone to Increase Sensitivity. Previous workers have shown the effect of many organic solvents on flame emission (1-3, 5). I n an attempt to increase the sensitivity for eilver, a number of organic liquids miscible with v,-ater, including acetone, methyl ethyl ketone, methanol, ethyl alcohol, and ethylene gly-
