Spectrophotometric Determination of Silicon in Gallium Phosphide SIR: During certain fundamental studies o n the solid state behayior of various materials it has been necessary to determine. lattice Si in GaP. The spectrographic method could not be used because the samples to be analyzed \vere thought to contain some unreacted Si. Because of thiq and the limited weight of the samples available it appeared that the analytical method of choice nould be the photometric molybdenum blue method (3). This method fails in the presence of appreciable amounts of phosphate ( 2 ) . Because no waj. waq found to overcome this difficulty it was concluded that a preliminary separation of most of the phosphate would be necessary. This might be accomplished by the anion exchange separation used by Sussman and Portnoy for the isolation of silicate from chromate-treated waters (4). This has proved to be true and a s . a result it has been possible to
Table I. Ion Exchange Separation of Silicate and Phosphate
Si
HC1 added,
P added,
Added,
ml.
mg.
rg.
Pg.
20.0 60.0 20.0 40.0 20.0 60.0 40.0 20.0
19.0 57.0 19.2 38.1 19.1 57.3
0.25 0.25 0.50 0.50 0.75 0.75 1.00 0.25 0.50 0 75 1 00
0 50 0.50 0.50 0.50 0.10
25 25 25 25 25 76 100
125 125
Found,
16.0 19.0
57.1 38.2 15,l 19.1 57.2 10.0 10.0 12.0
60.0 40.0
40.0 20.0 60.0 40.0
40.0 40.0
Table II. Analysis of Composite Samples of Silicate and Gallium Phosphide
Si
GaP added,
10 10 25 25 50 50
75
m-
ir3
100 100 100
2036
Si added,
Si
found,
found,
rg.
PJg.
corrected
20.0 40.0 60 0 20 0 60 0
17 3 34.4 51 4 17 3 51 1 36 3 53 0
20 0
52 0 31 2 49 2 17 0
GO 0 39 7 57 6
40 0
60 0 20 0
60 0 40 0
GO 0 20 0 40 0 60 0
17 6
33 4 45 8
ANALYTICAL CHEMISTRY
rg. . . .
59 5
42 4 61 8 20 4 19 6
38 8 50 3
develop a satisfactory method for the determination of 0.001to lyGof soluble Si in GaP. Since this work was completed, it was discovered that the anion exchange separation of silicate and phosphate had been described by -4ndersson ( 1 ) .
end glass column and to prevent hydrolysis of certain metals that may be present. The recovery of Si in the proposed method is not quite complete because, the amount of washing of the column was curtailed to provide for maximum sensitivity in the spectrophotometric determination. EXPERIMENTAL To establish optimum conditions for the exchange separation, synthetic Apparatus. .~SION EXCHAXGE mixtures of silicate, phosphate, and CoLunm. Prepare a 2- x 17-cm. HC1 in 10-ml. volume were eluted into column with weakly basic XG3-X a 100-ml. volumetric flask containing 20- to 50-mesh chloride form anion enough HC1 to make a total of 1 ml. in exchange resin which has been processed from Dowex 3-X-4 resin by all cases. The Si wab then determined BioRad Labs of Berkeley, Calif. spectrophotometrically with a calibraSupport the resin on a bed of granular tion graph prepared from mixtures of S i c to reduce the Si blank frorp the 1 ml. of HC1 plus suitable amounts of glassware. Before initial use and after silicate. Representative data given in each analysis remove unwant,ed anions Table I show that the recoveries of Si by eluting, at about 1 ml. per minut,e, are about 57, low except in those in30 ml. of KH,OH ( I l ) , 60 nil. of stances where the acidity of the influent water, 30 ml. of HCl (1 9), and three exceeds about 0.75 ml. of HC1 per 10 consecutive 20-ml. portion:: of water. Procedure. PREPARATION O F CALIml. or when the amount of P present B R A T I O S GRAPH. Carry each sample exceeds about 7 5 mg. through the entire calibration or The degree of retention of P by the sample analysis individually, working resin is influenced by the acidity of the as rapidly as possible to minimize influent and by the amount of P present. polymerization of silicate. Heat Up to a point, the higher the acidity gently a mixture of 3 ml. of freshly and the more P present, the less comprepared HCl-HN03 (4 1) plus plete the retention, as shown by the suitable amounts of sodium silicate yellon color of the phosphomolybdic (0' to 50 pg. of Si) for 5 minutes. Destroy the HSOs with the minimum acid in the color development. Khen amount of formic acid (3). the acidity of the influent exceeds about When foaming ceases and the solution 0.75 ml. of HCI per 10 ml., or when is water white, remove the cover and more than about 75 mg. of P are present, boil down on a flame to 1.0- to 1.5-ml. the retention appears to increase. volume. Cool quickly, add 1 ml. of The optimum acidity for the ion 10% ammonium tartrate solution exchange separation appears to be (w./v.) and 8 ml. of water. Transfer about 0.5 ml. of HC1 per ml. At this to the ion exchange column and elute acidity it has been possible to retain a t 2 ml. per minute unt'il the level of quantitatively as much as 50 mg. of P the li uid reaches t,he resin level. Discar2 the effluent. Continue the on a freshly prepared resin column. elution a t about 5 ml. per minute with However, experience indicates that the 10-ml. portions of water until a total retention is not consistently good. of 55 ml. of effluent is obtained in a Evidence indicates that the ability of a 100-ml. volumetric flask. Inalyze for column to retain P deteriorates on use. Si by the molybdenum blue method, Tests also show that the retention is taking the spectrophotometric readings influenced by the technique used in a t 765 mp or 815 mp immediately after cleansing the resin. The best retention dilution to volume ( 3 ) . is obtained when the resin is nashed A i ~ . ~OF~ SAIIPI,~:. ~ s ~ s B y heating gentlJ-, dissolve ul, to 0.1 gram of 150quickly by the TH,OH and HCl mesh sample containing not more than Table I qhow that, at the recomabout 40 pg. of Foluble Si in 3 ml. of mended acidity of 0.5 nil. of HC1 per 1). Continue as HC1-HS03 (4 10 nil as much aq 60 p g . of Si can be directed above, allowing the solution adequately separated from a. much as to stand for 30 secondv after addition P. Thus, even though the 75 mg. of of the H2S04to destroy any phosphoretention of P may not be complete molydic acid that may be present. when 75 mg. of I' are present, the amount of 1' that accompanies the Si is DISCUSSION OF A N I O N EXCHANGE not enough to interfere in the Si deterSEPARATION OF SILICATE AND PHOSPHATE minat ion. Analysis of Gallium Phosphide. The ion exchange separation of silicate necauqe the pernii-;ible acid concenand phos1)hate can be made from either tration in the anion evrhanpe ieparaneutral or dilute HC1 solutions. but tion of silicate and pho-phate 1s the ])resenre of ~ o m cacid is desirable rather limited, Ga, when pre reaction ( 2 ) . For p ( t , 2, erf[@(t, ~ ~ ) ~ = / 2 I] and Equation 2 becomes erf(pTr1/2) = In this region, T, should he a constant, independent of current density, given by
+
+
0.2275
7,
= __
P2
(3)
Figure 1 shows a plot of t / / ~ us. , pZ7, calculated from Equation 2. Xfter t, and T, have been determined n p e r i mentally and the ratio has been calculated, the corresponding value of B 2 ~ rmay be taken from Fiqure 1. h similar plot mith the ordinate inverted has been shown by Testa and Reinniuth (If). EXPERIMENTAL
A11 reagents were reaqent grade or equivalent. Solutions were made ut) b adding neighed amounts of hydroxy[ VOL. 3 6 , NO. 1 0 , SEPTEMBER 1 9 6 4
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