Anal. Chem. 1987, 59, 787-789
787
Spectrophotometric Determination of Traces of Silicon in Water after Collection as Silicomolybdenum Blue on an Organic-Solvent-Soluble Membrane Filter Issei Kasahara,* Ritsuko Terai, Y u k a r i Murai, Noriko Hata, Shigeru Taguchi, a n d Katsumi Goto Faculty of Science, Toyama University, Toyama 930, Japan
A rapid and simple preconcentratlon technlque, based on collecting trace elements on a membrane fllter and dlssdvlng the membrane fllter In an organlc solvent, has been applled to the spectrophotometrlc determlnatlon of traces of slllcon In water. Slllcon, In the range of 1-14 pg as SO2 In 50 mL of water sample, Is converted Into a-slllcomolybdlc acld and treated wtth a reductant solution containing tln( IV), L-ascorblc acld, and oxallc acld. The slllcomolybdenum blue formed Is collected on a nitrocellulose membrane filter In the presence of n -dodecyltrlmethylammonlum bromlde. The membrane and the slllcomolybdenum blue are dissolved In 5 mL of dlmethylformamlde (DMF), and the absorbance due to the silicomolybdate In the DMF Is measured at 740 nm agalnsl a reagent blank. Phosphate In concentratlons similar to that of slllcaie Interferes, but moderate concentrations of arsenate and anlonlc surfactants and high concentrations of sodium chlorkle do not Interfere. The detection lknn Is 0.13 pg of SO2 In 50 mL of sample on a 3a, bask.
High-purity water is essential to the production of highperformance integrated circuits and to the generation of steam in thermal and nuclear power plants. A careful control of water quality is required in these industries. Silicon is one of the major impurities, the concentration of which has to be carefully controlled. An upper limit of 20 pg/L as Si02 is prescribed for high-pressure boilers and an even more stringent limit is set for the water used for production of large-scale integrated circuits. Accurate measurements of dissolved silicon may be required also in some biological, geochemical, and marine studies. Spectrophotometric methods for the determination of dissolved silicon in natural water are usually based on the formation of a yellow heteropoly acid (1)with molybdate and the reduction of the yellow acid to the more light-absorbing blue form (2-8). Many reductants have been studied for the selective reduction of silicomolybdic acid to molybdenum blue to improve the precision and the detection limit, but the determination of microgram-per-liter levels of silicon in water is difficult without preconcentration. Solvent extraction of silicomolybdenum blue is a common practice used for prebut it is time-consuming and tedious. Other concentration (9), common techniques include adsorption on and desorption from hydrophobic adsorbents. We have devised several methods for the determination trace elements based on this principle (10-17). Recently, we have presented a simpler and versatile preconcentration technique, which is based on collecting trace elements on a membrane filter and dissolving the membrane in an organic solvent. Thus, traces of phosphorus were collected on a membrane filter as phosphomolybdenum blue, which was then dissolved in a small volume of dimethyl sulfoxide (MeaO) together with the fiiter, and the phosphorus was determined from the absorbance of the Me2S0 solution (18, 19).
The present investigation was undertaken to apply a similar technique to the preconcentration-spectrophotometric determination of silicon in water. Silicon was collected as silicomolybdenum blue on a nitrocellulose membrane filter in the presence of n-dodecyltrimethylammonium bromide (C,,-TMAB), and the membrane filter and the molybdenum blue are dissolved in a small volume of N,N-dimethylformamide (DMF). The absorbance due to the silicomolybdate in the DMF was measured at 740 nm against a reagent blank. EXPERIMENTAL SECTION Reagent Solutions. All reagents used were the purest grade chemicals. All solutions were prepared with the distilled water that had been obtained with a still equipped with a Teflon tube cooling pipe and stored in a Teflon bottle. Ammonium Molybdate Solution. Dissolve 22 g of ammonium molybdate tetrahydrate in 100 mL of water. Standard Silicon Solution. Fuse 500 mg of silicon dioxide with approximately 5 g of anhydrous sodium carbonate in a platinum crucible. After cooling, dissolve the contents in water and make up to 1.0 L with water. This solution contains 500 mg/L of Si02. Acetate Buffer Solution. Dissolve 20 g of sodium acetate trihydrate in water, add 35 mL of glacial acetic acid, and dilute to 100 mL with water. n -Dodecyltrimethylammonium Bromide (C12-TMAB) Solution. Dissolve 0.40 g of Cl,-TMAB in 100 mL of water. If necessary, remove any silicon present as an impurity in the following manner (20): Add 1 mL of 18% aluminum chloride hexahydrate solution to 100 mL of the C12-TMABsolution, adjust the pH to 8.5 with aqueous ammonia, stir for about 30 min, and filter off the aluminum hydroxide formed. This procedure was repeated twice. Reductant Solution. Prepare as described by Smith and Milne (4). Prepare a tin(1V) chloride solution as follows: Dissolve 25 g of anhydrous tin(1V) chloride (fuming liquid) in water and dilute to 125 mL. Prepare an oxalic acid solution as follows: Dissolve 34 g of oxalic acid dihydrate in 400 mL of water. Add 20 mL of the tin(1V) chloride solution and 0.8 g of L-ascorbic acid to 80 mL of the oxalic acid solution and mix well. Prepare the combined reductant solution daily. Membrane Filter and Filter Holder. Toyo TM-2 membrane filter (25 mm in diameter, 0.45 pm pore size, nitrocellulose membrane) was used. Other nitrocellulose membrane filters may be used equally satisfactorily. A Toyo KG-25 filter holder (effective filtration area 2.0 cm2) was used. Recommended Procedure. Place 50 mL of sample solution, containing less than 14 pg of SiO,, into a clean Teflon flask. Add 2 mL of acetate buffer solution and 2 mL of molybdate reagent. Set aside for 15 min and then add 10 mL of reductant solution. After 20 min to allow for full color development, add 1 mL of CI2-TMAE3solution. Filter the solution through a membrane filter with suction to collect the molybdenum blue formed. The full power of a water aspirator can be applied. Wash the membrane filter with about 10 mL of water. Dissolve the filter in 5 mL of DMF, and measure the absorbance of the DMF solution at 740 nm against a reagent blank. RESULTS AND DISCUSSION Choice of a Reductant Solution. Most spectrophotometric methods for traces of silicon in water are based on the
0003-2700/87/0359-0787$01.50/00 1987 American Chemical Society
788
ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987
Table I. Comparison of Different Reagents for the Reduction of Silicomolybdic Acid to Molybdenum Blue in the Presence of C,,-TMAB
reductant
form of silicomolybdate
nm (0.4 mg/L SiO,)"
l-amino-2-naphthol-4-sulfonic acid (2)
p
white precip
n $
0.649 0.241 0.322 0.228 0.360 high blank
SnCl, + L-ascorbic acid ( 4 ) SnClz (8) p(methy1amino)phenol sulfate (6) L-ascorbic acid (9) SnC12+ L-ascorbic acid (8, 9) SnC12 ( 4 , 8 )
p $
p n
.e -A-A-A
0
abs at 740
e
c-
-9
0-0
A--c
H 4
uw 0.8 z d m p:
Sample volume, 25 mL; amount of DMF, 5 mL.
White p r e c i p i t a t e f o r m s .
1.O
Flgure 2. Color intensity and stability of silicomolybdenum blue in different solvents: amount of SiO,, 13.7 pg; volume of solvent, 5 mL; (A) methylcellosolve, (e)DMF, (C) Me,SO.
1
1 .o
-
0.8
a 0 h
0.6
c 0.6 -2
C1 d B z DMAC
w
U
x
C
TMAB
0.4 CT
CI2TI.IAB
0
m
