Thompson et a2. (13). Two broad absorptions due to NH4+ (8) appear on the low-wavenumber side of the OH band in the 3100-2600 cm-1 region when NH4N03 is dissolved in the water (B, Figure 8), and these become more prominent when the spectra are solvent-compensated (C, D, Figure 8) and then scale-expanded (Figure 9). There is, however, some distortion of the 3 0 5 0 - ~ m -band ~ because of the negative OH band. The distortion decreases as the solute concentration is decreased, but this also decreases the solute band. Consequently, as the liquid layer is necessarily thin as well as dilute, the signal-to-noise ratio decreases. With the present technique, the lowest concentrations a t which N-H and C-H bands (similar experiments were made with aqueous methanol, Na citrate, etc.) were observed were near 0.1M. (13) W. K. Thompson, W. A . Senior, and 6. A . Pethica, Nature (London). 211, 1086 (1966).
The results thus show that it is feasible to obtain spectral information near or even at the position of one of the strong water absorptions. I t must be kept in mind, however, that the presence of solutes causes the water bands to change in absorption, so that distortions result. Such distortions may also affect quantitative results by causing base lines to fluctuate. The distortions decrease in intensity with decreasing solute concentration. However, because their intensities and frequencies are a function of the nature of the solute, they cannot be precisely compensated.
Received for review March 1, 1973. Accepted May 10, 1973. Support by Contract No. N00014-67-A:0467-0023 from ONR is gratefully acknowledged.
Spectrophotometric Determination of Total Amounts of Polythionates (Tetra-, Penta-, and Hexathionate) in Mixtures with Thiosulfate and Sulfite Tomozo K o h a n d K a z u o Taniguchi D e p a r t m e n t of Chemistry. Faculty of S c i e n c e , Tokai University, Hiratsuka-shi. Kanagawa. Japan
A sensitive determination of total amounts of tetra-, penta-, and hexathionate in mixtures with thiosulfate and sulfite is described. The method is based on the formation of thiosulfate equivalent to the polythionates, and on the spectrophotometric measurement of an excess of iodine for the thiosulfate formed. The factors affecting cyanolysis of pentathionate were examined, including a brief examination of cyanolysis of tetra- and hexathionate. The method could be successfully applied to the determination of these sulfur compounds mixed in various ratios with a deviation of = k O . O l umole.
In recent years, there has been a growing interest in the chemistry of polythionates, thiosulfate, and sulfite, and sensitive and accurate determination of these sulfur compounds in mixtures has become increasingly desirable. In previous papers (1-5), photometric methods for the determination of tetra-, penta-, and hexathionate, based on the formation of thiocyanate from polythionates, have been described. It has been possible to increase the sensitivity by approximately sixty-fold for the determination of polythionates by extracting an ion-pair between the cation of methylene blue and the anion of thiocyanate formed from the polythionates with an organic solvent (6). A method (7) was devised by the authors for the determination of polythionates in mixtures with each other, based 0. A . Nietzel and M. A . DeSesa. Anal. Chem.. 27, 1839 (1955). P. J. Urban, Z. Anal. Chem.. 180, 110 (1961). T. Koh, Bull. Chem. SOC. Jap.. 38, 1510 (1965). T. Koh and I. Iwasaki, Bull. Chem. SOC.Jap., 38, 2135 (1965). T. Koh and I . Iwasaki, Buil. Chem. SOC.Jap., 39,352 (1966). T. Koh. N. Saito, and I. Iwasaki, Ana!. Chim. Acta. 61, 449 (1972). (7) T. Koh and I . Iwasaki, Bull. Chem. Soc. Jap.. 39, 703 (1966)
(1) (2) (3) (4) (5) (6)
2018
on the determination of different amounts of thiocyanate formed by the cyanolysis of polythionates in the presence and absence of cupric ions. However, no similar consideration has been given to the determination of total amounts of polythionates in mixtures with one another. Polythionates react with cyanide according to the following Equation:
S,06*-
+ (X - 1)CN- + HZO SO4*- + 2HCN + (X - 3)SCN- + S203'-
(1)
where X = 4 , 5 , and 6. The present paper is concerned with the determination of total amounts of polythionates when tetra-, penta-, and hexathionate are simultaneously present. The method is based on the formation of thiosulfate equivalent to the polythionates, and on the spectrophotometric measurement of an excess of iodine for the thiosulfate formed. A procedure for the analysis of mixtures of polythionates, thiosulfate, and sulfite is also described.
EXPERIMENTAL Apparatus. Spectrophotometric measurements were made with a Shimazu Model QV-50 spectrophotometer with 10-mm quartz cells. pH measurements were made with a Hitachi-Horiba Model M-5 pH meter. Temperatures were regulated by a Taiyo Model M-1 thermostat. Reagents. All other chemicals used, besides polythionates, were of an analytical grade and were used without further purification. Polythionates. Potassium tetrathionate was prepared as described by Stamm e t al. ( 8 ) , and potassium pentathionate and potassium hexathionate as described by Goehring and Feldmann (8) H Stamm, M . Goehring, and U . Feldmann, 2. Anorg Chem 250, 226 (1942).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973
Allgem.
(9). The polythionates so obtained were recrystallized and then dried at room temperature before storage a t -10 f 2 "C. The water physically adsorbed on the polythionates and the water of crystallization of pentathionate were estimated by the Karl Fischer method. It was confirmed that the polythionates were pure enough for the present purpose; the purity of the polythionates was estimated by determining the total potassium and sulfur contents ( 3 - 5 ) . Standard polythionate solutions were stored at 5 f 2 "C. Standard Tetrathionate Solution (1.0 x M).Dissolve 151.3 mg of potassium tetrathionate (water content 0.0770) in redistilled water and dilute to 500 ml. Standard Pentathionate Solution (1.0 X M).Dissolve 186.8 mg of potassium pentathionate (water content including water of crystallization 10.4870) in redistilled water and dilute to 500 ml. Standard Hexathionate Solution (1.0 X M).Dissolve 183.8 mg of potassium hexathionate (water content 0.2970) in redistilled water and dilute to 500 ml. Standard Thiosulfate Solution. Dissolve sodium thiosulfate pentahydrate in freshly boiled and cooled redistilled water containing a small amount of sodium carbonate, and standardize by iodometry a week after preparation. Prepare working solutions by suitable dilution. These standards were used to ascertain stoichiometry and completion of the reaction of polythionates with cyanide. Sodium carbonate was added as a stabilizing agent. Standard Sulfite Solution. Dissolve sodium hydrogen sulfite in freshly boiled and cooled redistilled water, and standardize by iodometry. Prepare working standards by appropriate dilution. Standard Iodate-Iodide Solution. Add 50.0 ml of 1.0 X 10-2N iodate obtained by diluting 0.1N standard iodate to a solution containing 0.2 g of sodium carbonate and 72.2 g of potassium iodide, and dilute to 500 ml to give a 1.0 X 10-3N iodate in 0.87M iodide solution. Iodide Solution (0.87M). Dissolve 72.2 g of potassium iodide in a redistilled water containing 0.2 g of sodium carbonate and dilute to 500 ml. Sodium Cyanide Solution. Dilute a 1M cyanide solution as required. Formaldehyde Solution. Prepare working solutions by suitable dilution of formalin. Buffer Solutions. Mix acetic acid (0.5 or 1N) with sodium acetate (0.5 or lM), sodium dihydrogen phosphate (0.2M) with sodium hydroxide (0.2N), and sodium monohydrogen phosphate (0.5M) with sodium hydroxide (0.5N),respectively, in various ratios in order to obtain buffer solutions of p H as required. A buffer solution ( p H 7.0) used in procedures A-I, B-11, and B-I11 was prepared by mixing 50 ml o f sodium dihydrogen phosphate (0.2M) with 29.63 ml of sodium hydroxide (0.2N), and a buffer ( p H 7.2) in procedure A-I1 by mixing 50 ml of sodium dihydrogen phosphate (0.2M) with 35 ml of sodium hydroxide (0.2N). Procedures. Procedure A . For Total Amounts of Polythionates (Tetra-, Penta-, and Hexathionate) in Mixtures with One Another. Prepare two reaction mixtures in 25-ml volumetric flasks as follows. Procedure A - I . Cyanolysis of Polythionates. Add 2.0 ml of 0.1M sodium cyanide, 3.2 ml of phosphate buffer solution of pH 7.0, and then a 10.0-ml aliquot of sample solution containing polythionates to a 25-ml volumetric flask. The p H of the solution is thereby brought to 8.6. Place the flask in a thermostat a t 40 "C for 30 min, to convert the polythionates completely to thiosulfate. Procedure A-II. Correction for Unstoichiometric Cyanolysis of Pentathionate. Add 1.0 ml of phosphate buffer solution of pH 7.2 and a 10.0-ml aliquot of the sample solution to a 25-ml volumetric flask. Place the flask in a thermostat at 40 "C for 45 min. Finally, add 1.5 ml of 0.5M formaldehyde to each reaction mixture, and allow to stand for at least 1 min to mask the excess of cyanide completely. To these mixtures, add 3.0 ml of 5 N acetic acid and 2.3 ml of 1.0 X 10-3N standard iodate in 0.87M iodide solution, then fill the flasks to the mark with redistilled water. Mix the contents well, and measure the absorbance o f the solution of iodine-iodide complex against distilled water at 350 n m (IO) within 30 min after the addition of standard iodate-iodide solution. Prepare a n iodate-free reagent blank by adding 2.3 ml of 0.87M iodide in place of 2.3 ml of 1.0 X lO-3N standard iodate in 0.87N iodide solution and by treating it exactly the same, and correct it (9) M Goehring and U Feldmann, Z Anorg Chem 257, 2 2 3 (1948)
T a b l e I. Effecto f Air-Oxidation of iodide in Acid Mediaa Absorbance 5N
HOAc 3 0 ml
0 5N H2S040 8 ml
pH33
pH 3 3
lodatelodatefree 1.0 X free 1 ox Time, reagent Reagent 1 0 - 4 M reagent Reagent 10-4M blankC S Z O ~ * - ~ min blank blankC S 2 0 3 z - c blank 3
0.003
5
0.003 0,011 0.016 0.018 0.024 0.031
10
20 30 45 60
1.163 1.166 1.168 1.168 1.169 1.172 1.178
0.657 0.657 0.659 0.660 0.662 0.666 0.670
0.004 0.008 0.016 0.018 0.021 0.027 0.035
1.165 1.166 1.169 1.171 1.174 1.178 1.184
0.656 0.658 0.660 0.664 0.669 0.672 0.675
Treated as in procedure A-I, b u t photometric measurements were made at the indicated time after the addition of standard iodate-iodide solution. b Sulfuric acid was added in place of acetic acid in procedure A . Iodate-free reaaent blank subtracted.
(see Table I) for all absorbances measured in the present study. Procedure B. For Mixtures of Polythionates, Thiosulfate, and Sulfite. Prepare four reaction mixtures in 25-ml volumetric flasks as follows. Procedure B-I. Determination of Polythionates and' Thiosulfate in Mixtures. Treat two 10.0-ml aliquots of sample solution containing polythionates, thiosulfate, and sulfite as in procedures A-I and A-11, respectively, and then add 1.5 ml of 0 . 3 4 formaldehyde to each mixture and allow it to stand for 1 min. Procedure B-II. Determination of Thiosulfate in Mixtures. Add 3.2 ml of phosphate buffer solution of p H 7.0, a 10.0-ml aliquot of the sample solution, and 1.5 ml of 0.5M formaldehyde to a 25-ml volumetric flask; then allow the mixture to stand for a t least 1 min to mask the sulfite completely. Procedure B-III. Determination of Thiosulfate and Sulfite in Mixtures. Add 3.2 ml of phosphate buffer solution of p H 7.0 and a 10.0-ml aliquot of the sample solution to a 25-ml volumetric flask. Finally, add 3.0 ml of 5 N acetic acid and 2.3 ml of 1.0 X 10-3N standard iodate in 0.87N iodide solution to each mixture; then fill the flasks to the mark with redistilled water. Mix the contents of the flasks well, and measure the absorbance of the solution of iodine-iodide complex against distilled water a t 350 nm ( I O ) .
RESULTS AND DISCUSSION Cyanolysis of Polythionates. A series of standard solutions of thiosulfate and polythionates (X = 4, 5 , and 6) were treated as in procedure A-I. The resulting graphs are shown in Figure 1. According to Equation 1, when one mole of the polythionates undergoes cyanolysis, one mole of thiosulfate is formed. Therefore, if polythionate is converted to thiosulfate stoichiometrically and completely, the calibration graphs of tetra-, penta-, and hexathionate should coincide with that of thiosulfate when plotted as molar concentrations. Figure 1 shows that tetra- and hexathionate were quantitatively converted to thiosulfate according to Equation 1, but pentathionate was not stoichiometric. In previous papers (3, 5, 6, 7) in which the thiocyanate obtained was photometrically determined, the above discrepancy was not found. In order to establish conditions under which pentathionate, as well as tetraand hexathionate, was stoichiometrically converted to thiosulfate, various factors such as temperature, reaction time, and pH were investigated. A lower absorbance for pentathionate could occur if it underwent alkaline decomposition to yield thiosulfate ( 2 1 , 12) as shown in Equation 2: (10) T. Ozawa, Nippon Kagaku Zasshi. 87, 573 (1966). ( 1 1 ) M . Goehring, W. Helbing, and I . Appel, Z. Anorg. Chem.. 254, 185 (19471. ( 1 2 ) M . Goehring, U. Feldmann, and W. Helbing, Z. Ana/. Chem.. 129, 346 (1 949)
ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973
2019
1.2
1.0
o'6
i
B
0.8
n
m
a
c
?
02 0.6
VI
n
a 0
0.4
0.5
1 1.5 2 ml of 0 5 M formaldehyde
3
Figure 3. Effect of amount of formaldehyde on masking t h e ex-
cess of cyanide
0.2
Concentration of thiosulfate was 1 0 X 10-4M,0 , no cyanide and formaldehyde as in procedure 6-111, 0 cyanide was added as in procedure A- 1
0 X 18M Concn of S,d,-, S,Oi, &($-,and S,G-
Figure 1. Calibration graphs of thiosulfate and polythionates Solid line, procedure A - I ; dotted line, procedure A-11; 8 , s40e2-; 0
0.6
-
0.4
-
0.2
-
S2032-:
0,
, sso6'-: 8 ,s 6 0 6 ' -
s a
g
)
VI
D Q
0 '
' 4
I 5
6
7
8
9
10
11
PH
Figure 2. Effect of p H on masking of excess cyanide with form-
aldehyde Concentration of thiosulfate was 1.0 X 10-4M, and 1 . 5 ml of 0 5M formaldehyde was employed, 0 , no cyanide and formaldehyde as in procedure B-1 I I ; 0, cyanide was added as in procedure A-I
'2Sj0,*-
+ 60H-
+
5S203*- 3H,O
(2) Therefore. the reaction of pentathionate with cyanide was carried out a t various pH values below 8.6. A lower absorbance for pentathionate, lower than the expected value, was observed even at a p H level of 6.1, which indicates partial alkaline decomposition of pentathionate. Moreover, the reaction of pentathionate did not go to completion even in 15 hr a t p H 6.1 because the lower the pH of the solution, the slower the rate of cyanolysis. The lower absorbance for pentathionate was thought to be caused by its thermal decomposition at 40 "C. in addition to alkaline decomposition. Therefore, pentathionate solution was allowed to stand for 30 min at various temperatures and pH levels, and then treated as in procedure A-I. In all cases the reaction solutions were buffered to p H 8.3-8.8 ( 5 ) , which is the optimal range for the reaction of 2020
=
both tetra- and hexathionate with cyanide. Pentathionate solution was stable a t 10 "C over the p H range 7.6-8.8; no thermal and alkaline decomposition occurred. At 20. 30, and 40 "C, the higher the p H and the temperature of the pentathionate solution were, the lower the absorbance was. From a consideration of these experimental facts, it was concluded that the discrepancy for pentathionate is caused by alkaline and/or thermal decomposition besides cyanolysis. However, the calibration graph for pentathionate gave good reproducibility under the conditions of procedure A-I over the p H range 8.3-8.8. Masking of Excess Cyanide with Formaldehyde. The cyanide used for the cyanolysis of polythionate consumed the iodine completely, as a result of its reaction with the iodine. Accordingly, a n attempt was made to mask the excess cyanide with formaldehyde. Figure 2 shows how the cyanide could be masked by varying the p H of the mixture solution. The absorbance was in exact agreement with the theoretical value over the p H range of 6.8-10.9. The p H of the mixture solution was brought to 10.3 when 1.5 ml of 0.5M formaldehyde had been added as in procedure A-I. Figure 3 shows how the cyanide could be masked by increasing the amount of formaldehyde. In all cases, the solutions were buffered t o p H 6.8-10.9. It can be concluded from Figure 3 t h a t 0.75 ml of 0.5M formaldehyde solution was sufficient for masking the cyanide completely, and that the amount of formaldehyde did not have any effect on this method. Consequently, 1.5 ml of 0.5M formaldehyde was used in the present study. Masking time of excess cyanide with formaldehyde was investigated. The reaction of cyanide with formaldehyde was virtually instantaneous and went to completion in 0.7 min. Hence, the mixture was allowed to stand for a t least 1 min after the addition of formaldehyde. The mixture solution must be made acidic after the excess cyanide was masked, so the stability of formaldehyde cyanohydrin in acid media was examined. It was observed that this species is very stable; even after 4 hr, no measurable change was found. Effect of Iodide Concentration on Formation of Iodine-Iodide Complex. It is obvious that the formation of iodine-iodide complex may be affected by the concentration of iodide. In order to establish the optimal amount, the experiment was followed with various concentrations of iodide. Figure 4 shows the increase of the absorbance for the complex with an increase in the concentration of iodide. The absorbance remained constant over the range
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 1 2 , OCTOBER 1973
1
I
0.6
QI
u
0.4
0.8 -
C rd
n
b
* n
0.2
0.6 -
4
0.4
-
0
2
1
3
4
"
5
rnl of 5 N acetic acid
t
Figure 5. Effect of amount of acetic acid
1
Treated as in procedure A-I except for the amount of acetic acid; concentration of thiosulfate was 1 0 X 10-4M
0
Q02 0.04 0.06
0.08
0.10
Table Ill. Determination of Total Amounts of Polythionates in Mixtures
0.1 2
Concn of iodide, M Figure 4. Effect of iodide concentration on formation of iodine-
iodide complex
Taken. umoles
s406'-
Treated as in procedure A-I except for the amount of iodide; 0 , reagent blank; 0 . 1.0 X 10-4M S2032-
sS06'-
s6062-
0.05 0.05
2.00
2.00 0.10 0.20 0.70 0.80 1 .oo
0.05 0.70 1.75 0.80 0.10
0.05
0.80
0.50
0.50
2.00
Table I I . Stability of Tetra-. Penta-. and Hexathionate under Conditions of Procedure A-1 I
Time, min 5 10 20
0.40 2.00 0.10 0.60 1.80
30 40 45 50 60 70
1.0 x 2.0 x 2.0 x 1.0 x 2.0 x 1.0 x 10-4~ 10-4~ 10-4~ 10-4~ 10-4~ 10-4~
1.164 1.165 1.164 1.164 1.165 1.165 1.164 1.165 1.164
1.166 1.165 1.164 1.164 1.165 1.166 1.164 1.164 1.163
1.1 65 1.165 1.151 1.149 1.140 1.130 1.128 1.122 1.118
1.165 1.164 1.148 1.134 1.116 1.099 1.091 1.084 1.071
1.166 1.165 1.164 1.164 1.165 1.163 1.162 1.155 1.142
,180 0.60 0.10 0.10 1.80
0.10 0.60 1.80
1.165 1.165 1.164 1.164 1.165 1.162 1.159 1.138 1.126
0.05-0.12M iodide. Consequently, 2.3 ml of 0.87M iodide solution, which gave 0.08M when the final volume was 25.0 ml, were used in procedures A and B. Solution of Iodine-Iodide Complex. Obviously, iodide is subject to air-oxidation in acid medium. The absorbance of iodine-iodide solution was measured after standing in a medium of acetic or sulfuric acid for various periods of time. The experimental results are given in Table I. The solution gave constant absorbances within the experimental error over a 30-min period in acetic acid medium and over a 10-min period in sulfuric acid medium, respectively. Higher absorbances then ensued because of the airoxidation of iodide. Acetic acid was used to make the mixture acidic in the present study. Effect of Amount of Acetic Acid. It is well known that iodine-iodide complex is formed in acid medium from an iodate-iodide mixture. Figure 5 shows that 1.0 ml of 5N acetic acid was sufficient for the complete liberation of iodine and the absorbance remained constant over the volume range of 1.0-5.0 ml. Consequently, 3.0 ml of 5N acetic acid was used to make the solution acidic.
Total amount 0.40 2.00 0.40 2.00 0.40 2.00 1.90 1.20 1.90 1.90 1.90 1.90 1.20 1.90 2.10 2.10 2.10 1.60 2.05 1.70 1.70
0.40 2.00 0.40 2.00
Absorbance
Found, urnoles
0.05
1.80 0.10 1.80 0.60 0.1 0 0.05 2.00 0.80 0.10 0.20
0.40
2.00 0.41 2.01 0.41 2.00 1.90 1.20 1.91 1.91 1.90 1.90 1.21 1.90 2.10 2.10 2.11 1.61 2.04 1.70 1.70 2.00
Determination of Total Amounts of Polythionates in Mixtures. As stated above, pentathionate was not stoichiometrically converted to thiosulfate according to Equation l. Therefore, it was impossible to determine the total amounts of the polythionates in mixtures with one another by only procedure A-I. Accordingly, a study was made to establish conditions (procedure A-11) under which pentathionate gave exactly the same lower absorbance than the reagent blank as the lower absorbance observed for pentathionate in procedure A-I, and under which tetra- and hexathionate gave the same absorbance as the reagent blank, by varying pH, temperature, and reaction time when cyanide was not employed. Table I1 shows that tetra- and hexathionate gave the same absorbances as the reagent blank over intervals of 70 and 50 min, respectively. Pentathionate gave the same lower absorbance as that observed for pentathionate in procedure A-I, in 45 min. Therefore, a calibration graph for total amounts of the polythionates in mixtures can be
ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973
2021
Table I V . Analysis of Mixtures of Polythionates. Thiosulfate. and Sulfite Taken, pmoles
Found. pmoles
Total
Total amount Of
0.07 0.50
0.07
0.50
0.07 0.50
0.21 1.50
0.20 0.30
0.20
0.20
0.50
0.50
0.20
0.10 0.30
0.60 0.90 1 .oo
obtained by adding a difference in absorbance between the reagent blank and the polythionate in procedure A-I1 to the absorbance measured according to the treatment of procedure A-I. A number of known mixtures were analyzed using procedure A. Typical results, shown in Table 111, demonstrate good recoveries of total amounts of added polythionates. The method was extremely accurate, and the deviation for the amount of these three polythionates separately or in mixtures never exceeded hO.01 @mole. Analysis of Mixtures of Polythionates, Thiosulfate, and Sulfite. The absorbance obtained by procedure B-I corresponds to the sum of tetra-, penta-, and hexathionate and thiosulfate in the mixtures, sulfite being masked by the formaldehyde added for removing the excess of cyanide. The absorbance obtained by procedure B-11, where cyanolysis of polythionate was not carried out, corresponds to only the amount of thiosulfate. The absorbance
0.75
0.30 0.40 1 .oo
0.10 0.15 0.25 0.25
0.50 0.50
s~o6~-
s2032-
SO&
0.20 1.51 0.61
0.30
0.75
0.40
0.90 0.99
0.51 0.51
0.1 1 0.14 0.24 0.24
0.99
of procedure B-I11 corresponds to the sum of the thiosulfate and twice the sulfite in the mixture solution. Moreover, the calibration graphs for the three polythionates, thiosulfate, and sulfite were in full agreement with one another when plotted as equivalent concentrations. Therefore, the following equations can be obtained: (S,0e2-) = I - 11, ( S 2 0 3 2 - ) = 11, and (SO32-) = (I11 - II)/2 where I, 11, and I11 indicate the absorbances obtained by procedures B-I, B-II, and B-111, respectively. These sulfur compounds in mixtures could be readily determined from the calibration graph for thiosulfate in Figure 1. The results, Table IV, show that the above technique can be accurately applied to the determination of polythionates, thiosulfate. and sulfite mixed in various ratios. Received for review March 12. 1973. Accepted May 10, 1973.
Automated Rapid Scan Instrument for Spectroelectrochemistry in the Visible Region Eugene E. Wells, Jr. Electrochemistry B r a n c h , Naval Research Laboratory, Washington, D.C.20375
APPARATUS An instrument which uses a computer to control and collect data from a rapid scanning visible spectrophotometer, and which simultaneously operates a potentiostat to control generation of electrochemical intermediates is described. The 370-700 nm spectral range may be swept in as little as 5 msec, and by using real time control to zero the spectral background repetitive spectra may be signal averaged to give an ultimate sensitivity of absorbance unit. For peak monitoring at fixed wavelength, data may be taken as soon as 20 psec after the application of the reaction initiating potential step.
The power of in situ spectroelectrochemistry lies in the hope of identifying both the type and amount of redox species produced in an electrochemical step. Our interest in the solution kinetics of shortlived electrochemically generated intermediates has led us to develop instrumentation designed for both rapid scanning, to obtain spectra, and extreme sensitivity, to monitor intermediate concentrations a t very short times a t fixed wavelengths. 2022
T h e basic design of the monochromator is due to Strojek, Gruver, and Kuwana ( I ) . This design was chosen over a number of others offering rapid scan (2) because it has one shot as opposed to free running, capability, because the wavelength can be easily controlled by either a digital source or an analog waveform generator, and because it has dual beam operation, which reduces substantially the frequency demands of the final amplification stages and makes possible dynamic base-line correction. The monochromator consists of a galvanometer mirror movement which reflects the beam from the entrance slit SI onto the grating, G, via a lens, L (Figure 1).If a current is applied to the galvanometer coil. the mirror assumes a new angle, changing the angle of incidence of the beam on L and, hence, also on G because of the refraction of L. Thus the spectrum sweeps across the exit slit, Sa, and the wavelength passed changes in a manner related to the current flowing through the galvanometer coil. Although the mirror is not in phase with the driving waveform. which is normally a ramp, no evidence of nonlinearity in the wavelength axis has appeared, and the phase problem is obviated by calibrating the instrument as described below a t the desired rate of scan. (1) J . W . Strojek. G . A . Gruver, and T . Kuwana, A n a / . Chem.. 41, 481 (1969). (2) G . C. Pimental, Appi. Opt.. 7,2155 (1968)
ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973
