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 p H 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 p H 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 was 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, 7 8 2 - 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
(1) (2) (3) (4) ~, (5)
(1950).
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-
1584
ANALYTICAL CHEMISTRY
Table 1.
Effect of *4cetoneConcentration on Flame Emission of Silver at 338 M p
Bcetone, M1./100 MI. S o h .
0 15 25 40 50 250 y Of silver in each solution. for all solutions. a
Relative Luminosity at 335 M M
30a 45 50 GO 71 Instrument control setting. identic r l
col, were added t o the sample solutions. Of these, the acetone and methyl ethyl ketone caused the greatest enhancement of dver emission. Blthough t h e latter was somewhat better in this respect, acetone was chosen because it is more readily availablr and less expensive. Table I shows the effect of varying the acetone concentration on the relative luminosity a t 338 mp, -
g
cn
1
O
100
z
2
0
r
firing, i t is difficult to remove residual flus completely by washing with water. I n the samples analyzed, the sodium content was generally around 0.040% while the magnesium content approsimated 0.12%. The effect of this sodium and magnesium on the luminosity readings a t 338 mp was Jtudied. Both these elements increase the luminosity a t 338 mp, as shown in Tables 11 and 111. A4nexamination of the data shows that the increased readings a t 338 mp are due not t o any enhmcement of silver emission by these elements but rather t o an increase in the background radintiori. llagnesium has a neak emission band throughout the 338-1np region and, hence, its presence tends to increase the iiiteiiiitj- reading a t the position of silver emission. ' The effect of sodium on the background radiation is shown more clearly in Table 11-. Although sodium exhibits a relatively weak emiGsion line a t 330 mp, the radiation from this line falls off sharply and, at the slit n-idth used (0.2 mm.) should be down to zero at about 332 nip. Hence the increase in background radiation iq due to some other cause, probably to continuum interference from the sodium. (The rather high readings a t 330 mp are due largely to a very strong flame background in the region of 300 t o 330 mp. This flame background tapers off rapidly a t 330 mM but is still moderately strong a t 338 mp, although the zero suppressor may be used to cancel out the background.) Thi- interference from sodium and magnesium can be comperisnted for in two ways. If the concentration of these t a o elements in the sample is knon-n from previous analyses (also by a flame photometric procedure), the exact amount of each may be added to the blank and t o the standard solution. If these concentration8 are unknoffn, compensation may be made by measuring the light intensity close t o the line on both sides and subtracting the interpolated intensity a t 338 mp. Such a procedure is justified in this case, as the background rndiation in this region is more or less continuous or bandlike.
'table 11.
Sodium Concn., P.P.M.
5 5
c!
5
5
MICROGRAMS SILVER PER 100 lvlL Figure 1. Flame intensity at 338 mp us. silver concentration
5
0 Adjusted adjiiztnlent. a
Relative Luminosity a t 338 J I p
10 15 90 20 97 20 5 to zero by means of zero suppressor switch and dark currm:
Table 111.
I n each case, the relative luminositj- reading is a net reading compared against a blank containing the same amount of acetone but no silver. This eliminates the effect of background radiation, which changes seriously with a change in the aretone-water ratio. The background radiation in each case is adjusted t o zero by employing the zero suppression switch incorporated in the photomultiplier attachment. This allows the use of the entire transmittance scale for silver intensity readings. The presence of acetone in the sample solutions cawei slightly greater fluctuations in the flame intensity than normal with a water solution. The intensity readings change rather quickly once the solution is poured into the sample beaker, probably because of volatilization of acetone which causes a change in the water-acetone ratio. Hence it is very important that the volumetric flask containing the sample solution be tightly stoppered prior t o the determination and that the luminosity reading be taken immediately after the solution is poured into the sample beaker. Because of this volatility interference, 30 ml. of acetone rather than 50 was added for each 100 ml. of solution, even though 50 ml. gave somewhat greater sensitivity. Effect of Sodium and Magnesium. Sodium chloride and magnesium chloride are commonly used as fluxes in the firing of sulfide phosphors. Although the phosphors are washed after
Effect of Sodium on Silver Emission
Silver Concn.. P.P.M.
Effect of Magnesium on Silver Emission
Relative Luminosity: 335 mp 335 mu 341 nip 00 0 0 0 112 30 7 119 60 13 124 12 1 . 0 22 132 20 150 31 141 30 a Adjusted to zero by means of zero suppressor switch and dark current adjustment. Silver Concn.. P.P.N.
Magnesium Concn., P.P.M.
p
Effect of Zinc,and Cadmium. Small changes in the ratio of zinc to cadmium have a negligible effect on the silver emission. However, it is advisable to prepare standards having the approuimate zinc and cadmium concentrations found in the sample. Solution of Sample. Hydrochloric acid proved t o be very effective in dissolving the samples, giving clear solutions requiring no filtration. The small amount of silver chloride formed is readily soluble in the large excess of hydrochloric acid employed, owing t o the formation of a complex argentichloride. Nitric acid is less satisfactory, as free sulfur is always formed which must be filtered off and may tend to adsorb or occlude some of the silver. The amount of hydrochloric acid used is not critical. Con-
V O L U M E 27, NO. 10, O C T O B E R 1 9 5 5 Table IV. \!ave
1585
Effect of Sodium on Flame Background
Length, h‘fr
No sodium
330 331 332 335 338
82
Relatir e Luminosity” 20 {).p.m. sodium 1-7;
44
31
33
4u
4?
?
10
’?
5 0
341
Differenee
If both the sodium and magnesium concentrations in the sample are known from previous analyses, the proper amount of each may be added to the blank and to the standard. Such a procedure would eliminate the need for a correction. Determine the silver concentration of the sample by comparing the net relative luminosity of the sample with that of the standard. Within the range employed the silver luminosity is directly proportional to the silver concentration.
1
3
No silver present. Solutions contain only water, hydrochloric acid, acetone. and sodium chloride.
2 . typical set of data together with the method of calculatiori is shown heloa : Liiiiiinosit)- ReadingSam lple Standz
\Tare Length, nip
ct.ntr:itions from 10 to 30y0 h y volrinie gave almost identic21 intensit! readings for solution? containing 8 p.p.m. of silver. PROCEDURE
\l’eigh into a 100-ml. borosilicate glass beaker an amount of of silver the sulfide phosphor which will contain 50 to 500 (generally 1 to 5 grams). Add 5 ml. of distilled water. then 30 nil. of concentrated hydrochloric acid, and cover immediateljwith a 9-cm. borosilicate glass cover glass. Add several sniall glass beads to prevent bumping later. Place the sample on a steam plate until completely decomposed, then place on a wire gauze over a low flame and boil gently for several minutes until the solution is perfectly clear. After cooling the beaker in a tray of cool water, quantitatively transfer the contents into a 100-ml. volumetric flask containing 30.0 ml. of acetone, dilute to the mark with water, and mix well. e ,
Table 1’. Recover) of Silver b y Flame Photometric hlethod bsmple“
ZnCdS
Silver Added,
*,
55 325 110 200 100 ~~
a
Silver Found, y
GI 332 120 212 102
“a Error
+-‘: 2 1 9 1 t0.0 f2.O
5 grams of sample used in each case.
AIensure the relative luminosity of the solution at 338 m p . Chitrol settings will vary with different instruments. Hov ever, the following conditions wete found to be optimum fot this investigation and may serve as a guide: oxygen pressure, 10 pounds; hydrogen pressure, T pounds; slit width. 0.20 nini.: sensitivity control a t counterclockwise limit; selector switch at 0.1 : photomultiplier a t full sensitivity; zero suppression switch a t KO.1 position. Allow the instrument to “’ilarm up” for 10 to 15 minutes before taking measurements. Keep the shutter open throughout all subsequent procedure and make all luminosity measurement9 immediately aftei placing the solutions in the sample beakers. Place in atomizing position a L‘blank”solution carried through the entire procedure simultaneously with the unknown and containing approximately the same amount of zinc and/or cadmium sulfide, hydrochloric acid, and acetone. Set the transmittance control a t 0 % T and balance the needle by rotating the dark current control knob. Subsequently, determine the relative luminosity on a “standard” solution of the same composition but also containing 500 y of silver. Finally, obtain the reading on the sample solution. Rinse the burner frequently with di3tilled water between readings. Repeat all readings on flesh wniples to be sure they are reproducible.
338 335 341 ~~
92 I1
100
8
2
Iuteriioluted reading at 338 nip
3 1
Increa\e in background due to sudiiiru a n d iuapnesiam 8 - 2 = 6 Haniiile ryading. corrected for sodiiirii a r i l 1 i i i n background 92 - 6 = 81; v n t . .80 X 500 = -133 I00
rit first it might appear that a background correction should also be made for the standard-i.e., thnt the reading of 2 should be subtracted from the reading of 100 on the standard to give a net reading of 98. However, this n-ould be incorrect. The backgronrid of the blank has already been adjusted to zero by ine:iii~of the zero suppressor. Because the blank and the standa r d c.ont:iin no sodium or magnesium and are alike in every respect except silver, the reading of 100 is due entirely to silver. The low readings on the standard a t 325 and 341 nip are not due to any additional background but rather to a “tailing off” of the silver emission line. This efl’ect should be the same on the sample. Therefore, the increase in intensity on the sample at 3X5 arid 311 mp is due to sodium and m:ignesium. The increawd bnckgroiind should be subtracted onl!. from the sample reading at 338 Ill/.&. RESULTS
In order to test the accurac,y of the method, recovery studies were iiiude on both zinc sulfide and zinc cadmium sulfide phosphors. Known amounts of silver, ranging from 55 to 460 -,, were added t,o silver-free samples of these phosphors, which contained approximately 0.04% sodium and 0.12% magnesium. The samples were then carried through the entire procedure and t.he silver was determined. The results are listed in Table V. For these analyses the largest deviation was only 12 y, indicating that the accuracy is entirely satisfactorj-. The results are reproducillle within 2 or 3 scale divisions corresponding to 10 to 15 y of silver. The relative error is greatest for low silver conc.entr:itions, so that a practical limit of 5 to 10 p.p.m. in the phosphor Paniples should be set. The chief advantage of this method is its speed. The time required to analyze five to six samples simultaneously, including a.eiyhing and solution of the samples, ip only about 2 hour.. A C K N O I LEDGAlENT
The author is deeply grateful to Gerrit Dragt for hii helpful suggestions in preparing this manusvript. LITERATURE CITED
Hecause the sodium and magnesium generally found in the s:tmples affect the silver luminosity readings, a correction must be made for these impurities. This ma?- be done in the following manner :
(1) A N ~ LCHEM., . 22,1202 (1950).
Obtain the luminosities for the sample and the standard at both 335 and 341 mp and interpolate to obtain the luminositj, nt 338 mp. The difference between the interpolated readings represents that part of the silver luminosity reading due to sodium or magnesium interference and should be subtracted from the reading obtained on the sample a t 338 mp to give the net silver luminosity.
’ (1952). (4) Gilbert, P. T., Jr.. Becknan Instruments. Inc., Reprint R-56 f 1952). ( 5 ) Parks, T. D., Johnson. H . O., arid Lykken. L., A s ~ L .CHEar., 20,822 (1948).
( 2 ) Bills, C. E., McDonald, F. G., Kiederrneier. 11.C., Ibzd., 21, 1096 (1949).
W.,and Schwartz.
(3) Curtis. G . W., Knauer, H. E., and Hunter, L. E., .18TlI Symposium on Flame Photometry, .Imerican Society for Testing Materials, Philadelphia, Pa., Spec. Tech. Pub. 116, 87-54
R E C E I V E fDo r review September 23, 19.34. Accepted July 1. 1955.
