2342
Anal. Chem. 1984, 56, 2342-2345
Microdetermination of Silicic Acid in Water by Gel-Phase Colorimetry with Molybdenum Blue Kazuhisa Yoshimura,* Masako Motomura, and Toshikazu Tarutani Department of Chemistry, Faculty of Science, Kyushu University 33, Hakozaki, Higashiku, Fukuoka 812, Japan
Tsugio Shimono Resources and Environment Protection Research Laboratories, NEC Corporation, Miyazaki, Miyamaeku, Kawasaki 213, Japan
The blue specles of molybdoslllclc acld Is strongly adsorbed on Sephadex gels. About 70% of the specles In a 100-cms sample solutlon Is concentrated In 0.20 g of Sephadex 0-25 wkhln half an hour. A sensltlve method, gel-phase colorlmetry, based on the dlrect measurement of llght absorptlon by a gel phase, whlch has adsorbed the blue specles, has been developed. The optlmum condltlon for color development In the gel phase has been examlned on the bask of the procedure In ASTM D859-80 or JIS K0101. The colored gel beads are packed Into a 5mm cell, the absorbances at 805 and 450 nm are measured, and the absorbance dlfference Is used for the determlnatlon of traces of slllclc acid. By use of the present method, the amounts of slllclc acld at the parts-perbllllon level or lower could be determined for lndustrlal waters of hlgh purlty.
Electronic grade industrial water of high purity is used as a washing agent in the production of integrated circuits (IC). Because purity of the water affects the product yield and reliability of IC, its quality must be very carefully controlled. The content of silicic acid has been generally monitored to control the quality of the pure water. Nowadays, the production of higher integrated circuits demands pure water of higher quality and results in the necessity of trace determination of silicic acid. Recently, ASTM Committee D19 proposed that the total content of silicic acid should be controlled to under 5 pg/dm3 (1). However, it is difficult to determine such traces of silicic acid by analytical methods hitherto used. A more sensitive analytical method is desired. Sephadex gels, which consist of dextran cross-linked by epichlorohydrin, show high affinity for some oxo anions. Complex formation of oxo anions with the hydroxyl group of the dextran matrix is considered to be responsible for the reversible adsorption. This property has been utilized to concentrate boron, vanadium, and molybdenum selectively from natural waters and rocks (2-4). Heteropoly acids are also strongly adsorbed on Sephadex G-25 (5). The blue species of molybdosilicic acid is no exception and is used for determination of traces of silicic acid absorptiometrically (6). If the gel-phase absorbance can be directly and quantitatively measured after sorption of the colored species, this gel-phase colorimetry may be much more sensitive than the conventional solution method. Solid-phase absorptiometry has already been developed as ion-exchanger colorimetry and applied to the direct determination of transition metals a t parts-per-billion levels in water (7,8). The method for silicic acid has been reported using anion-exchange resin of molybdate form (9). However, the resin has high background absorbance and the sensitivity is not high enough to fulfill the demand mentioned above. Waki and Korkisch (10) have developed ion-exchanger ul0003-2700/84/0356-2342$01.50/0
traviolet spectrophotometry using Sephadex ion exchanger, which has a low background even in the ultraviolet region. Therefore, the combination of the selective adsorbability of the molybdenum blue on Sephadex gel and the direct measurement of gel-phase absorbance may afford a high sensitive method for traces of silicic acid in water.
EXPERIMENTAL SECTION Reagents. High-purity water was prepared by using subboiling distilled water, obtained by distillation of doubly distilled water with a commercially available subboiling distillation apparatus (Fujihara Factory). The subboiling distilled water was distilled with a Teflon apparatus. Nitrogen gas washed with sulfuric acid was introduced into the water heated at about 200 OC, which sent the water vapor effectively into the cooling bath. The distillation rate was about 30 cm3/h. This water was stored in a clean Teflon bottle. All reagents used were of analytical grade. Standard silicic acid solution (1000 ppm for silicon) was supplied by Wako Fine Chemicals as sodium metasilicate solution in 0.2 mol/dm3 sodium carbonate. Reducing agent solution was prepared by mixing a 50 cm3 acid solution containing 0.5 g of l-amino-2-naphthol-4-sulfonic and 2.0 g of anhydrous sodium sulfite and a 120 cm3 solution containing 20.0 g of sodium hydrogen sulfite and diluting to 200 cm3. Sephadex G-25 (Medium) was purchased from Pharmacia Fine Chemicals. Apparatus. Absorbance measurements were done with a Nippon Bunko spectrophotometer, Model UVIDEC-320, or a Varian spectrophotometer, Model Cary 210. The sample cell is shown in Figure 1. The 5-mm space is made by inserting a quartz spacer into an ordinary 10-mm quartz cell which has two small holes at the bottom. The sample cell makes the packing of the colored gel beads easier because the equilibrated solution flows out through the small holes and the gel beads settle down more quickly. An acrylic resin spacer is also used to make certain the entire light beam strikes only the packed area. Procedure for Gel-Phase Colorimetry of Silicic Acid. To a 100-cm3water sample containing 0.01-4 pg of silicate silicon in a poly(ethy1ene) container, 2 cm3 of 50% (v/v) hydrochloric acid solution and 8 cm3of 5% (w/v) ammonium molybdate solution were added. The solution was allowed to stand for 5 min and was mixed with 3 cmaof 10% (w/v) oxalic acid solution. After 1 min, 2 cm3 of the reducing agent solution, 10 cm3 of the hydrochloric acid solution, and then 0.20 g of the gel were added. The mixture was stirred for 30 min, the colored gel beads were allowed to settle, and then the gel was transferred to the 5-mm cell by means of a pipet. The cell was set in an ordinary holder in the spectrophotometer. The absorbances at 805 and 450 nm were measured with air as reference. The net absorbance of the blue species of molybdosilicate adsorbed on the gel at 805 nm was obtained from the difference between two absorbances (AA)minus the corresponding difference for the blank (AA(for the blank)). A perforated metal plate of absorbance 2.0 was used in the reference beam to balance the light intensities. Distribution Measurements, To 100-cm3water samples containing various amounts of silicic acid, the reagents were added in the same manner as that described above. The solution was 0 1984 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
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Table I. Distribution Ratio of the Blue Species of Molybdosilicic Acid
quartz spacer(5 mrn)
D, dm3/kg
lo6 (initial concn), mol/dm3 1.42 1.71 2.28 2.85 4.27 5.69 8.54
1.34 1.31 1.26 1.29 1.10 1.00 0.850
1 5 mrnP Flgure 1. The gel holder for absorbance measurementsof gel phase.
0
5 Volume of masking agent
10
(cd)
Figure 3. Effect of the concentration of masking agent on color development: masklng agent, 10% (w/v) oxalic acid; sample, 2.5 pg of SI, 100 cm3; gel, Sephadex G25, 0.20 g.
Wavelength
(nm)
Flgure 2. kbsorptlon spectra of the molybdenum blue specles of slllclc acld: solM lines, solution spectra (cell, 10 mm), (A) 1.2 mg of Sl/dm3, (B) blank; dotted lines, gel spectra (cell, 5 mm) (gel, Sephadex G25, 0.20 g; solutlon, 100 cm3), (C)2.5 pg of Si, (D) 1.0 pg of SI,(E) blank.
allowed to stand for 10 min and then 0.20 g of the gel was added. The mixture was stirred for 30 min. After the equilibrated solution was filtered through a glass-fiber filter paper (Toyo GAlOO),the concentration of molybdosilicic acid of the solution was determined by the conventional solution method. The distribution ratio of molybdosilicic acid, D, was calculated by means of the equation D = [(mol of molybdosilicate adsorbed)/(kg of dry gel)]/ [(mol of molybdosilicate)/(dm3 of solution)]
RESULTS AND DISCUSSION Absorption Spectra of the Molybdenum Blue Species. Absorption spectra of the blue species of molybdosilicic acid are shown in Figure 2. The yellow species of molybdosilicic acid is very stable. Kircher and Crouch (11) have reported the conditional formation constant of the p-isomer to be in the order of 1030. The reduced blue species of the isomer is also stable and, therefore, almost all the silicate in a sample solution reacts with molybdate to form the blue species. The gel-phase spectra are similar to that of the solution except that absorption maxima for these spectra separated by about 10 nm. The similarity of the spectra shows that the species adsorbed on the gel is the same as that in the solution. In addition to the absorbance at the absorption maximum of the blue species in the gel phase (805 nm), the absorbance in the range where the blue species has no absorption (450 nm) is measured in order to compensate for the background due to the gel. The use of the absorbance difference (PA)
makes the solid-phase colorimetry highly precise (7). Adsorption Isotherm of Molybdosilicic Acid. Distribution measurements were carried out by using solutions containing silicic acid in a linear calibration range of the solution method, in which the chemical species of the blue complex is not changed. The measured distribution ratios obtained are listed in Table I. By assuming the uniform complex formation of the blue species with the gel, constant adsorption capacity, and no interaction among the blue species adsorbed, the data are fitted by a linearized Langmuir-type isotherm. At low coverage, the value of D reached a constant value, 1.5 X lo3,which may give a linear calibration curve to the gel-phase colorimetry. The adsorption capacity is 0.076 mol/kg of dry gel, and the formation constant is 1.9 X lo4 dm3/m01. Size exclusion may affect the adsorption of the blue species. The values will be further discussed elsewhere. Optimization of Conditions. Molybdate and Acid Concentration. Silicic acid reacts with molybdate to form the /3-isomer of the yellow complex in 0.1 mol/dm3 acid concentration. The amounts of molybdate and acid suggested in ASTM or JIS were added. Oxalic Acid Concentration. Oxalic acid was generally used to eliminate the interference by phosphate. The presence of a high concentration of the masking agent suppressed the color development of molybdosilicate (Figure 3) and therefore 3 cm3 of 10% (w/v) oxalic acid solution was added. Amount of Reducing Agent. Molybdenum yellow complex is converted to the blue complex by reduction with 1amino-2-naphthol-4-sulfonic acid. The absorbances of the molybdenum blue were constant for the addition of the reducing agent of 1-4 cm3 (Figure 4),whereas the absorbance for the blank increased with increasing the amount of the reducing agent and the absorption spectra were different from those of the blue species because of reduction of isopolymolybdates. The addition of 2 cm3 of the solution was adopted. The solution is stable for 5 days if stored in a refrigerator. Hydrochloric Acid Concentration. Optimum acid concentration for reduction of the yellow species is higher than 0.6 mol/dm3 (12). Adsorbability of the blue species on
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984 Table 11. Effects of Foreign Ions on the Determination of Silicic Acid"
i
species added
molar ratio (foreign ion/Si)
P(V)
0
2
AdV) Ge(1V) Al(II1) Ti(1V) Fe(II1)
6
4
Volume of reducing agent
+
10
10.4
+4
25.3
+153
10
10.0
1
18.2 10.2 10.2 9.1 9.8 9.9 9.7 8.9 10.0 10.0
0 +82 +2 +2
100
l0OC
Figure 4. Effect of the concentration of reducing agent on color development: reducing agent, 0.25 % (w/v) l-amlno-2-naphthol-4sulfonic acid 1% (w/v) Na2S03 10% (w/v) NaHSO,; sample, 2.5 pg of Si, 100 cm3; gel, Sephadex 025, 0.20 g.
+
lOOOC 10
Mn(I1) Cu(I1)
re1 error, %
lob
10 10
(crn3)
amt of Si found, pg/dm3
100 l0OC lO0OC
-9 -2 -1
-3 -11 0 0
100-cm3sample solution containing 1.00 pg of Si. *Without the masking agent. Deionized by use of a cation-exchange resin column (16 X 25 mm, AG 50W-X2-Ht (100-200 mesh)). Table 111. Determination of Silicic Acid Contents in Pure Water Samples (n = 3)
samplea A B 0
10
C D-1
20
Volume of (1.l)HCI
D-2
(cm3)
Figure 5. Effect of the concentration of hydrochloric acid on color development: sample; 2.5 pg of Si, 100 cm3; gel, Sephadex G25, 0.20 g.
1
1.01
D-3 D-4 E-1
E-2 E-3 E-4
concn, pg/dm3 1.2 f 0.1
0.6 f 0.1 0.3 f 0.1 2.7 f 0.1 2.9 i 0.1 1.2 f 0.08 0.3 f 0.05 0.8 f 0.1
0.8 h 0.2 0.6 f 0.3 0.8 f 0.1
"A, distilled water with a Pyrex glass vessel; B, subboiling distilled water with a quartz vessel; c, deionized water with a Milli-Q system (Millipore); D, water for the IC producing processes; E, water for thermal power generation circulating in the plant.
0
20
60
40
Tlme for stirring
(min)
Figure 6. Time dependence of color development: of Si, 100 cm3; gel, Sephadex G25, 0.20 g.
sample, 2.5 pg
Sephadex G-25 may depend on acidity of the solution, in analogy with oxo acids of boron, vanadium(V), and molybdenum(V1) (2-4). Figure 5 shows that the addition of 10 cm3 of the hydrochloric acid solution gives a good result. Stirring Time. The effect of the stirring time on adsorption of the blue species on the gel is shown in Figure 6. The blue species is adsorbed within 30 min of equilibration. The stirring time was fixed a t 30 min. Stirring and standing for longer than a few hours gives a slightly increased AA for the blank. It is recommended that AA be measured a t a constant time interval after color development. Temperature. The absorbance of the adsorbed blue species was nearly constant in the temperature range 10-30 O C . Calibration. The calibration curve is linear and may expressed by AA = 0.0285C + AA (for the blank), where C is the concentration of silicate silicon, in pg/dm3. Nearly the same results were obtained with two spectrophotometers, and AA (for the blank) = -0.049 f 0.0026 (n = 3) with the Nippon
Bunko spectrophotometer and -0.022 f 0.0013 (n = 4)with the Varian spectrophotometer. The sensitivity of the gel-phase colorimetry is 36-fold higher than the conventional solution method using the 10-mm cell. The larger the sample volume, the higher the sensitivity (13). For the present system, the value of D is approximately 1.5 X lo3 at low ranges. The sensitivity may be 1.6-fold higher for the 200-cm3sample and 2.5-fold higher for the 500-cm3 sample than that for the i00-cm3 sample. It is difficult to obtain water that is completely free from silicic acid and to make sure of the purity of water, beause there are no effective concentration methods for silicic acid. Distilled water of subboiling distilled water with a Teflon vessel is, therefore, regarded as one that is completely free from silicic acid. Effects of Foreign Ions. The effects of foreign ions are shown in Table 11. Almost all ions except for germanate did not cause more than 5% error when present in up to 10 times the concentration of silicic acid. Iron and copper interfered when present in a 100-fold ratio to silicic acid. The interference could be eliminated by passing sample waters through an acrylic resin column packed with a cation-exchange resin. Determination of Silicic Acid in Pure Waters. The present method was applied to the determination of silicic acid in pure water (Table 111). The analytical results were reproducible to about f O . l pg/dm3 on repeated runs. The water for the IC production contained less than 2.9 pg/dm3 of silicate silicon. A content lower than that proposed by the ASTM
Anal. Chem. 1904, 56, 2345-2349
Committee could easily be determined with high precision. The content of silicic acid in water for a high-pressure boiler of a thermal power generator was maintained a t a constant level during circulation in the plant. In the case of the gel-phase colorimetry, concentration of the molybdenum blue species and the color development take place simultaneously. As a result, silicic acid a t parts-perbillion or lower levels can be determined in a conveniently short time. Within 90 min, six samples could be analyzed. Registry No. Silicic acid, 7699-41-4; water, 7732-18-5.
LITERATURE CITED (1) ASTM Committee D19.02.03.03, Draft 8, Feb 18, 1983. (2) Yoshimura, K.: Kariya, R.; Tarutani, T. Anal. Chim. Acta 1979, 109, 115-121.
2345
(3) Yoshimura, K.; KaJI, H.;Yamaguchi, E.; Tarutani, T. Anal. Chim. Acta 1981, 130. 345-352. (4) Yoshimura, K.; Hlraoka, S.; Tarutani, T. Anal. Chlm. Acta 1982, 142, 101-107. (5) Yoza, N.: Matsumoto, H.; Ohashi, S. Anal. Chim. Acta 1971, 54, 538-541. (6) ASTM D859-80 (I983), “Standard Test Methods for Silica in Water”; JIS KO101 (1981), “Testing Method for Industrial Water”. (7) Yoshimura, K.; Waki, H.;Ohashi, S. Talanta 1976, 23, 449-454. (8) Yoshimura, K.; Nigo, S.; Tarutani, T. Talanta 1982, 29, 173-176. (9) Tanaka, T.; Hiiro. K.; Kawahara, A. Bunseki Kagaku 1991, 30, 131-1 34. (IO) Waki, H.;Korkisch, J. Talanta 1983, 30, 95-100. (11) Klrcher, C. C.; Crouch, S. R. Anal. Chem. 1982, 54, 1219-1221. (12) Umezaki, Y. Nlppon Kagaku Zasshi 1961, 10. 1353-1356. (13) Yoshimurq, K.; Ohashi, S. Talanta 1978, 25, 103-107.
RECEIVED for review April 30,1984. Accepted June 18,1984.
Spectrophotometric Determination of Amines with p-Chloranil Robert E. Smith* and William R. Davis The Bendix Corporation, Kansas City Division, P.O. Box 1159, Kansas City, Missouri 64141
Prlmary, secondary, and tertlary amlnes, both aliphatic and aromatlc are shown to react with p-chloranll In dioxane/b propanol (1:4, v/v) to produce a blue to purple color. Most amlnes tested also react wlth p-chloranil in other organlc solvents. The colored compound formed can be stable for 8 h at room temperature, dependlng on the amlne and the solvent used. Several organlc compounds (phenol, epoxles, alkynes, and non-amlne nltrogens) are 8hown to not Interfere wlth the determination of amlnes. A few appkatlons of amlne determlnatlons In real samples are discussed. The reactlon product Is Identmed as monoamlne qulnones where the amlne dlsplaces one chlorine of the p-chioranll. Tertiary amlnes react to form water-soluble, surface actlve quatenary amlnes. Detectlon llmlts vary wlth each amlne, but for N,N,N’,N’tetramethylethylenediamlne, the detection llmlt Is 0.05 mg.
A method was required to determine tertiary amine catalysts used in the synthesis of alkynes. Ideally, the method would be rapid and reproducible and require little or no instrumentation. Hopefully, a solution of the alkyne being synthesized could be added to a test solution. If a tertiary amine were present, the test reagent would turn a bright color. A test reagent which has been shown to react with a variety of amines is p-chloranil (i.e., 2,3,5,6-tetrachlorobenzoquinone). Sass et al. (1)described the formation of a green color when tertiary amines in toluene reacted with p-chloranil on a boiling water bath. More recently, Ibrahim et al. (2) described the formation of a blue color when certain tertiary amine type tranquilizers and antidepressants reacted with p-chloranil in a dioxane-ethanol mixture (1:4, v/v). The tertiary amines that they studied did not react with p-chloranil in toluene. Taha and El-Kader (3) described the use of p-chloranil as a spray reagent for tertiary N-ethyl drugs on thin-layer chromatograms. Other workers have reported reactions between tertiary amines and p-chloranil in a variety of solvents (4-10). Some of these reports, as well as others (21-19) have shown that a number of primary and secondary, aliphatic and aromatic amines also react with p-chloranil. Again, a variety of solvents and reaction conditions were used in these studies. 0003-2700/64/0356-2345$01.50/0
The present work was performed to better characterize the use of p-chloranil as a spectrophotometric reagent for the determination of amines. It is demonstrated that p-chloranil does react with a variety of amines in a dioxane-2-propanol mixture (1:4, v/v) to produce a blue or purple color. This method is shown to be ideal for a quick spot test in monitoring the purity of alkynes. The effects of solvent on the reaction of amines with pchloranil and upon the stability of the colored complexes are also reported. A few applications for the determination of amines are discussed, The reactions involved in developing the purple color are identified.
EXPERIMENTAL SECTION Ultraviolet (UV) and visible spectra were measured with a Perkin-Elmer Model 320 spectrophotometer. Infrared spectra were recorded with a Nicolet Model 7199 Fourier transform infrared (FTIR) spectrometer operating at 2 cm-’ resolution. Nuclear magnetic resonance (NMR) spectra were obtained with a Varian FTBOA NMR at either 79.542 MHz (‘H NMR) or 20.000 MHz (13C NMR). Amines were determined by adding 2 mL of a 0.5% solution of p-chloranil in 1,4-dioxane to 8 mL of amine in 2-propanol at room temperature. After an appropriate time (see Table I), the absorbance at the optimum wavelength in the visible region (Amm) was measured. Synthesis and Purification of Amine-Quinones. Isobutylamine- and Morpholine-Quinones. To 40 mg (0.16 mmol) of p-chloranil in 8 mL of dioxane, either 6.4 mg (0.09 mmol) of isobutylamine or 8.2 mg (0.09 mmol) of morpholine in 32 mL of CH2C12 was added. The mixtures were stirred for 1 h at room temperature and transferred to a rotoevaporator to remove the solvent under vacuum. The residues were redissolved in CH2Clz and applied to a preparative scale silica TLC plate. The plates were developed in CH2Clp In each case, a dark purple band was separated and scraped into a 50-mL beaker. The purple compounds were redissolved in CH2Clp The CH2C12was evaporated under a stream of nitrogen and some of the purple solid was pressed into a KBr pellet for IR analysis. Some of the morpholine-quinone was redissolved in acetone-$ for NMR analysis. Tributylamine- and Tetramethylethylenediamine-Quinones. To 40 mg of p-chloranil in dioxane, either 29 mg (0.16 mmol) of tributylamine or 20 mg (0.16 mmol) of tetramethylethylenediamine in 32 mL of 2-propanol was added. The solvent was stirred for 30 min at room temperature and removed by rotoevaporation in vacuo. The residues were redissolved in 2-propanol and applied 0 1984 American Chemical Society
