Indirect Ultraviolet Spectrophotometric Determination of Silicon LOUIS TRUDELL' and D. F. BOLTZ Department of Chemisfry, Wayne State University, Detroit 2, Mich.
b An indirect spectrophotometric method for traces of silicate is based on the ultraviolet absorptivity of molybdate originating from molybdosilicic acid. After extraction of the heteropoly acid with a pentanoldiethyl ether solution, the molybdosilicic acid is stripped with a basic buffer solution, The absorbance of the aqueous molybdate solution is measured at 2 3 0 or 2 1 0 mp. Phosphate, arsenate, and iron(ll1) interfere. The optimum concentration range is 0.06 to 0.5 p.p.m. of silicon when spectrophotometric measurements are made at 2 3 0 mp using 1-cm. cells. The molar absorptivity is 59,000 liters per mole-cm. at 2 3 0 mp.
M
and silicate ions form yellow molybdosilicic acid, which is extractable with certain oxygencontaining organic solvents (10). The determination of silicon has been based on the measurement of the color of either the molybdosilicic acid (2, 3, 6, 6) or the heteropoly blue of silicon, which is obtained by the selective reduction of the molybdosilicic acid (I). The fact that decomposition products of molybdophosphoric acid were used in a sensitive ultraviolet spectrophotometric method for the determination of phosphate (7) suggested the desirability of a similar investigation of molybdosilicic acid. The essential steps in the procedure as ultimately developed are: the formation of molybdosilicic acid, the extraction of this heteropoly acid with a pentanoldiethyl ether extractant, the stripping of the molybdosilicic acid with a basic buffer solution, and the measurement of the ultraviolet absorptivity of the molybdate ion. OLYBDATE
EXPERIMENTAL
Apparatus. The absorbance measurements were made in 1.000-cm. silica cells using either a Cary 14 recording spectrophotometer or a Beckman D U spectrophotometer equipped with a n ultraviolet accessory set. Solutions. STANDARD SILICATESoLUTION. Dissolve 3.15 grams of so1 Present address, Department of Chemistry, Macomb County Community College, Warren, Mich.
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ANALYTICAL CHEMISTRY
dium silicate, Na&Os.9Hz0, in distilled water and dilute t o 500 ml. Standardize this solution gravimetrically. After standardization, use a microburet to transfer sufficient silicate solution to a 500-ml. volumetric flask so that on dilution to the mark the solution contains 0.0100 mg. of silicon per ml. MOLYBDATE SOLUTIOA-.Dissolve 50.0 grams of ammonium molybdate, (XH& Mod& 4H20, in distilled water and dilute to 500 ml. BUFFER SOLUTION. Dissolve 53.5 grams of ammonium chloride and 70 ml. of concentrated ammonium hydroxide in distilled water and dilute to 1 liter. All chemicals were reagent grade and all aqueous solutions were stored in polyethylene bottles to avoid silica contamination. Recommended General Procedure. Weigh, or measure by volume, a sample containing u p to 0.05 mg. of silicon and treat i t so t h a t i t is present as soluble silicate. Add 1.0 ml. of 1 to 1 hydrochloric acid and dilute to approximately 45 ml. Add 2.0 ml. of 10% ammonium molybdate. Let stand for 5 minutes, dilute to 50 ml., and let stand again for another 5 minutes. The p H of this solution should be approximately 1.4. Transfer the solution to a 125-ml. separatory funnel and add 7 ml. of 1 to 1 hydrochloric acid. Rinse the emptied container with 20 ml. of 5 to 1 ethyl ether-1-pentanol extractant mixture, transfer the rinsing to the separatory funnel, and swirl i t with the acidified aqueous solution for about 90 seconds. Withdraw the lower aqueous phase into a 125-ml. Erlenmeyer flask and transfer the organic extractant to the original flask for temporary storage. Pour the aqueous phase back into the separatory funnel, rinse the Erlenmeyer flask with another 20 ml. of extractant mixture, and add the rinsings t o the aqueous phase in the funnel. Swirl the funnel for about 90 seconds, withdraw and discard the lower aqueous phase, and add to the funnel the first portion of the extractant which had been temporarily stored. Swirl the funnel to collect the water droplets into one globule and discard. Wash the extractant three times with 25-ml. portions of 1 to 10 perchloric acid, first using the first two portions of the acid to rinse out the container that held the first extractant portion, thus assuring complete transfer of the extractant to the funnel. Each washing consists of
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swirling the funnel for about 45 seconds and discarding the lower phase. Remove excess molybdate by washing the extract with 1 to 10 perchloric acid. Carefully wash the tip of the funnel with a stream of distilled water to assure complete removal of excess molybdate ions. Transfer 30 ml. of the ammonium chloride-ammonium hydroxide buffered solution to the funnel, swirl for about 1 minute, and withdraw the lower aqueous phase into a 100-ml. volumetric flask. rldd another 15 ml. of the ammonium chloride-ammonium hydroxide buffered solution to the funnel, again swirl for 1 minute, and add the lower phase to the flask containing the first portion of the buffered solution. Swirl the flask to collect all of the remaining droplets of water. Dilute the solution in the 100-ml. volumetric flask to the mark with distilled water and mix well. The p H of the final aqueous solutions should be about 9. Let the solution stand for 30 to 60 minutes and then measure the absorbance a t 230 and/or 210 mp against a reagent blank in matched 1.000-cm. silica cells. Measurement a t 230 mp is recommended because of the high absorbance of the reagent blank solution at 210 mp. Prepare the reagent blank by following this same procedure, but take distilled water, instead of an aliquot of silicate solution, as a sample. Refer the absorbance values to a standard calibration graph obtained by using standard silicate solutions. EFFECT OF SOLUTION VARIABLES
Silicon Concentration. Figure 1 shows t h e ultraviolet absorption spectrum of a solution containing t h e molybdate resulting from t h e decomposition of the molybdosilicic acid. Silicate ions a t t h e less t h a n 1-p.p.m. level do not contribute t o t h e ultraviolet absorbance. Conformity to Beer's law was observed for 0.02 to 0.5 p.p.m. of silicon. The optimum concentration range was 0.06 to 0.5 p.p.m. of silicon at 230 mp and 0.03 to 0.3 p.p.m. at 210 mp based on Ringbom plots. Figure 2 shows a comparison of sensitivities when absorbance measurements are made a t 230 and 210 mp. This indirect ultraviolet method is 2.5 and 4.5 times as sensitive, when absorbance measurements are made a t 230 and 210 mp, t ~ 9 the heteropoly blue procedure of Bolts and Mellon ( I ) .
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- 20I
WAVELENGTH, m y
Figure 1. Absorption molybdate extract
spectrum
of
Buffer extract of molybdosilicic acid (0.5 p.p.m. of Si) vs. reagent blank soluiiaii
Acidity Prior to Extraction. A p H of 1 to 5 was found to be satisfactory for the formation of molybdosilicic acid (8). Previous investigations have shown that molybdosilicic acid, once formed in weakly acidic solution, is stable in strongly acidified solutions (9). The pH a t which molybdosilicic acid was formed in this work was about 1.4. The effect of acidity of the molybdosilicic acid solution on the extent of extraction by the 5 to 1 diethyl ether1-pentanol extractant was investigated by using 0.1, 0.3, 0.5, 0.7, and 1.ON hydrochloric acid solutions. The absorbance readings a t 230 mp for solutions containing 0.4 p.p.m. of silicon a t these acidities were 0.155, 0.570, 0.790, 0.790, and 0.790, respectively. Hence, maximum extraction is achieved in the 0.5 to 1.ON range, with 0.7N being selected for the recommended procedure. The effect of higher acidities was not investigated. Molybdate Concentrtttion. By using 0.4 p.p.m. of silicon and 1.5, 2.0, and 2.5 ml. of the molybdate solution per 50 ml. of solution, absorbance values of 0.750, 0.755, and 0.755 were obtained, with the same volumes of molybdate solution used in preparing the reference so-
Table l. Effect of Diethyl Ether-lPentanol Ratios on Absorbance of Reagent Blanks Ratio of diethyl A , at 230 mp ether-1-pentanol (OS. H2O) 0.10 0.699 0.25 0,597 0.67 0.469 1 .oo 0.432 1.50 0.367 2.33 0.319 4.00 0.240 5 .oo 0.135
lution. T h e intermediate volume of 2.0 ml. of molybdate solution was chosen as sufficient. Extractant. An appropriate extractant should extract the molybdosilicic acid, should not extract the excess molybdate required for complete formation of the molybdosilicic acid, should exhibit a cut-off wavelength below 210 mp, and should have slight solubility in aqueous buffer solution if exhibiting appreciable absorptivity in the lower ultraviolet region. Wadelin and Mellon found chloroform to be an effective diluent to minimize the extraction of excess molybdate in the extraction of molybdosphosphoric acid with 1-butanol and indicated that 1pentanol was an effective solvent for molybdosilicic acid, 265 mp being the shortest usable wavelength (10). After preliminary tests using various solvents and diluents, it was concluded that 1-pentanol was the most efficient solvent for molybdosilicic acid with minimum extraction of the excess molybdate, the addition of diethyl ether as diluent diminished the solubility of the 1-pentanol in the aqueous buffer solution to a tolerable level, and if the ratio of diethyl ether to 1-pentanol exceeded 5 the efficiency of the extraction of molybdosilicic acid decreased. Table I shows the diminution of the absorbance of the reagent blank solution as the relative amount of diethyl ether in the extractant is increased. Thus, the 5 to 1 diethyl ether-1-pentanol mixture was selected as the appropriate extractant. On the basis of the data cited in Table 11, two extractions with 20-ml. portions of the extractant were deemed sufficient to ensure maximum recovery of molvbdosilicic acid with a minimum experiments. Stability. The final aqueous buffered solutions were stable for 24 hours, when solutions contained 0.08 and 0.40 p.p.m. of silicon. For both silicon solutions there is a slight increase in absorbance within the first hour, b u t no change in the 1-hour and 24-hour values. Conformity to Beer's law was observed for a series of solutions allowed to stand for 8 weeks. These findings indicate that the final buffered solution of molybdate should be allowed to stand a t least 30 minutes and preferably 60 minutes before absorbance measurements are made. Diverse Ions. A study was made to determine the permissible amounts of various ions t h a t may be present without interfering with the determination of 0.2 p.p.m. of silicon. S o attempt was made to determine the effects of ion concentrations larger than 500 p.p.m., since this concen-
""I
I
" 0
01
0 2
P P M
of
03
SI
0 4
0 5
Figure 2. Sensitivity of absorbance measurements at 230 and 2 10 mp
tration is extremely large compared to the concentration of silicon that was present. Errors less than *37. of the mean transmittance value were considered negligible. The cations tested were added as chlorides, perchlorates, or sulfates and the anions
Table
II.
Effect of Extraction Technique (0.4 p.p.m. Si)
extractions
No. of
M1. of extractant per extraction
Absorbance
1 2 2 3
20 10, 10 20. 20 20; 10, 10
0.475 0.620 0.775 0.800
Added as
P.p:m.
permitted 500 500 500 500 500 10 1 500 500 300 500 0 0
500 250 500 500 500 50 10 200 20 250 0 500 250 100
VOL. 35, NO. 13, DECEMBER 1963
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were added as ammonium; potassium, or sodium salts. Table I11 summarizes the results of this study. The interference of phosphate, arsenate, and iron(II1) is an especially serious limitation of the method. KOattempts were made t o remove phosphate or arsenate by a preliminary extraction with ethyl acetate by the DeSesa and Rogers method (4), or t o circumvent the deleterious effect of iron(II1). -1Ithoug.h less than 100 p.p.m. of either the fluoborate or the borate ion exhibits negligible absorbance a t 230 mp, the effect of these ions on the specificity and efficiency of extraction of molybdosilicic acid was not studied. Therefore, their possible deleterious effect on the extraction process would ha1 e to be ascertained before this method could be applied to samples in which the solubilization of silicon had been achieved by the fluoride method.
Reproducibility. An indication of t h e precision of this procedure was ascertained from the results of 12 samples, each containing 0.2 p.p.m. of silicon. These samples gave mean absorbance values of 0.392 and 0.792 at 230 and 210 mp, respectively. The standard deviations were 0.006 absorbance unit, or a relative standard deviation of 1.5%upport in the form of an SSF Summer Re;;earch
Fellowship for High School Chemistry Teachers. LITERATURE CITED
(1) Boltz. D. F.. Mellon. RI. G.. IXD. ’ ESG.CHEU.,AXAL. ED.i9,8T3 (1947). (2) Case, 0. P., Ibzd., 16,300 11944). (3) De Sesa, 11.A , Rogers, L B., .4h.4~. CHEY.26,1278 (1954) (4) Ibid., p. 1381. ( 5 ) Jolles, A4.,Seurath. F.. Z . anaew. Chem. 11,315 (1898). 16) Judav. C.. Meloche. \ , Iinudson. H. IT.. V. IV.! ISD:ESG.CHEII.;hs.;~. ED. 12; 270 (1950). (7) h e c k , C. H., Boltz, D. F., A 4 s a ~ . CHEU.30,183 (1958). (8) Milton, R. F., dnaly.?t 76,431 (1951). 19) Strickland. J. D. H.. J . din. Chern. SOC.74.872 il952) ~
RECEITED for review Soveiiiber 21) 1962, Accepted September 13, 1963. Division of Analytical Chemistr)., l 4 k d Meeting, ACS, Atlantic City, S . J . Septeniberjl062.
Visible Absorption Characteristics of the Bis-(2,9dimet hy I- 1,lO- phena nt hroline)- a nd Bis- (4,4’,6,6‘tetramethyl-2,2‘-bipyridine)-Copper(I) Ions J.
R. HALL, M. R. LITZOW, and R. A. PLOWMAN
Chemisfry Department, University o f Queensland, Queensland, Australia
b Spectrophotometric studies have indicated that the absorbing species in solution for the determination of copper using 2,9-dimethyl- 1 , I 0-phenanthroline (dmp) and 4,4’,6,6’-tetramethy1-2,2’bipyridine (tmb) are the bis complex ions, [Cu(dmp)~]+ and [Cu(tmb)n] +, respectively. Solutions of the pure compounds, [Cu(ligand)s]X where X = CI, Br, I, Nos, and CI04, have spectral characteristics in agreement with the earlier studies. In general, however, the solutions conform to Beer’s law only when a large excess of the ligand i s added. Deviations from Beer’s law in the absence of excess ligand are attributed to dissociation of the [Cu(ligand)*]X complexes to the corresponding monochelate species. These ligands coordinate to many transition metals and their apparent specificity for copper in extraction procedures i s probably due to complexes of other metals having wavelengths of maximum absorption well removed from the A,, values for the copper complexes, or much lower molar absorptivities.
R
have been published about the use of 2,9-dimethyl-l,lO-phenanthroline (dmp; trivial name, neocuproine) for the determination of EPORTS
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ANALYTICAL CHEMISTRY
copper (8,12). n’hile the reagent is regarded as specific for copper, i t is not \Tell known that the ligand forms complexes with a number of transition metal ions. An analogous reagent, 4,4‘,6,6’tetramethyl-2,2‘-bipy~idine(tmb), has also been suggested for the determination of copper (9). This article describes the absorption behavior of solutions of pure compounds containing the bis(dmp)- and bis(tmb)-copper(1) ions and reports that some complexes may be found that interfere with the determination of copper using these reagents. EXPERIMENTAL
Apparatus. A Unicani SP500 spectrophotometer was used for the absorption measurement.. The wavelength and absorbance *calm were checked against standard solutions, d a t a for which are available from the IT. S. Department of Commerce ( 1 1 ) . Reagents. The compounds CuX t m b , where X = C1 and Rr. and [ C ~ ( t n i b ) ~ ] nSh, e r e X = S O n and Clod, were prepared by rrduction of an aqueous solution of the corresponding copper(I1) complex with hydrazine sulfate. The copprr(I1) compleves were isolated by method5 similar to those previously described for the analogous dmp complekeq ( 6 ) . CuItmb was obtained by the reaction of Cu-
Cltmb with excess sodium iodide dissolved in acetone. The compounds [Cu(tmb)n]X.HzO, where X = C1 and Br, n-ere prepared,by refluxing an ethanol solution of the corresponding mono(tmb) complex and tmb, while [ C ~ ( t n i b ) ~was ] I formed by adding cuprous iodide to an ethanol solution of excess tmb. The compounds lICl,tmb, where M = Fe, Co, and S i , n-ere prepared by addition of tmb to exce;s metal chloride dissolved in methanol (for FeC12) or ethanol (for CoClzand SiCI,). All the compounds were analyzed for metal, carbon, hydrogen, and nitrogen. The samples used for the absorption experiments were analytically pure. The solvents were Baker analyzed reagent chloroform and l n a l a r isoamyl alcohol. The solution, n-ere examined in matched, stoppered I-cm. cell.?. RESULTS A N D DISCUSSION
The procedure generally adopted for the determination of copper is to reduce an aqueous solution of copper(I1) to copper(I), t,hen add esceij neocuproine, and finally extract n i t h i;oamyl alcohol (I+$) or chloroform (5). The extracts show maximum absorption in the visible a t about 455 m p (molar absorptivity E = 7950 i 100) and the absorbing species has been determined spect’rophotometrically ( 1 4 ) to lie [Cu(dmp),]+.
