+
(1 [Ce(IV)]) in the denominator is approximately equal to unity. Consequently, a simplified rate expression which applies over most practical analytical conditions is Rate
+
5.82 X 103[Ce(IV)][As(III)](3.6 X 105[As(III)] 8.46) 1.75 X 105[As(111)]2 21.5[As(III)] 5.ll[Ce(IV)] [‘ITat
+
+
(12) This rate expression applies with high reliability at 0.50M H2S04 and 25.0 “C. The data in Figures 8 and 9 will be useful in predicting rates at other sulfuric acid concentrations and temperatures. However, when using these data for such purposes, it should be kept in mind that the data were obtained under a single set of concentrations of reactants and catalysts. No attempt was made to incorporate H2S04as a variable into the rate expression. It should be re-emphasized at this point that the only information supporting the reaction sequence given above is that it results in a kinetic expression which satisfies the experimental observations. There likely are other sequences which also would satisfy the data. Therefore Equations 2-6 should be viewed as a mechanism which permits a systematic development of a rate expression. Furthermore, it is difficult to evaluate the uniqueness of any of the specific rate constants except kl. This constant is uniquely determined when [As (111)] is much larger than [Ce(IV)]. This study provided no information on the probable nature of the intermediate suggested in Equation 3. Schenk (12) has ___ . (12) G. H. Schenk, J. Chem. Edrtc., 41,32 (1964).
__-
suggested the species I-As(OH)zO as an intermediate in the reaction of macro amounts of iodine and As(II1). There are some interesting contrasts between the iodide catalyzed reaction and the osmium catalyzed reaction studied earlier (13). For the osmium catalyzed reaction, plots of normalized rate us. [As(III)I/[Ce(IV)I or [Ce(IV)]/[As(III)] show no dependence upon absolute concentrations of reactants. Also, the reaction rate is strictly proportional to the osmium concentration throughout the range of conditions examined. There is no dependency on the order of addition of reagents. Under comparable conditions, osmium is about one order of magnitude more active as a catalyst than is iodide. An objective of the present study was to obtain information which would enhance the analytical utility of this reaction. The manner in which these data can be utilized for analytical purposes is illustrated in another report (11). ACKNOWLEDGMENT
Appreciation is expressed to J. Comer and M. McClintock for their valuable assistance in fitting the rate expression to the experimental data.
RECEIVED for review April 9, 1969. Accepted June 23, 1969. Research sponsored by the Air Force Office of Scientific Research under Grant 1212-67.
(13) R. L. Habig, H. L. Pardue, and J. B. Worthington, ANAL. CHEM., 39, 600 (1967).
Analytical Applications of the Iodide and Osmium Catalyzed Reaction between Cerium(lV) and Arsenic(ll1) Pedro A. Rodriguez’ and Harry L. Pardue Department of Chemistry, Purdue University,Lufayette, Ind. 47907 The iodide and osmium catalyzed reaction between Ce(lV) and As(lll) has been utilized for a variety of analyses. Iodide and osmium are determined separately and in mixtures without prior separation using Hg(ll) masking of iodide activity. Iodide is determined in the presence of iodate by taking advantage of the fact that iodate is inactive as a catalyst. Both iodide and iodate are determined by reducing iodate to a catalytically active form using As(lll). Silver(1) and mercury(l1) are determined utilizing their inhibitions of the iodide catalyzed reaction. All analyses are performed at concentrations down to 10-*M with osmium being determined at concentrations down to 3 x 10-loM. Most relative errors and standard deviations are in the range of 2%. YATSIMIRSKII has discussed the applicability of catalytic oxidation-reduction reactions for quantitative analyses based upon kinetic measurements (1). Because of the amplification effect of these reactions, they are sensitive to extremely small 1 Present address, Procter and Gamble Company, Cincinnati, Ohio.
(1) K. B. Yatsimirskii, “Kinetic Methods of Analysis,” Pergamon Press, New’York, N. Y., 1966.
1376
ANALYTICAL CHEMISTRY
changes in the concentration of catalyst. These reactions frequently are used for quantitative analyses in the 10-*M concentration range. A major problem frequently associated with the use of these reactions results from the fact that often more than one species catalyzes the reaction. This can lead to poor selectivity. Some of these difficulties may be overcome by separating the interferring substances, as was done by Surasiti and Sandell in the determination of ruthenium and osmium (2). It has been suggested that a more complete understanding of these reactions than now exists may reveal kinetic differences which can be exploited for simultaneous determination without prior separations (3, 4). Yatsimirskii has reported on the determination of zirconium and hafnium in mixtures by a kinetic method (9,and Margerum and coworkers have reported on the determination of thirty different metal ions in multicomponent mixtures using kinetic measurements on ligand ex(2) C. Surasiti and E. B. Sandell, Anal. Chim. Acta, 22, 261 (1960). (3) H. L. Pardue, Rec. Chem. Prog., 27, 151 (1966). (4) R. L. Habig, H. L. Pardue, and J. B. Worthington, ANAL. CHEM., 39,600 (1967). ( 5 ) K. B. Yatsimirskii and L. P. Raizman, J. Anal. Chem., USSR, English Transl., 18,719 (1963).
change reactions (6-9). The catalytic oxidation-reduction reactions hold the potential for selective analyses of multicomponent mixtures at the 10-8M concentration range without any prior separation. To take advantage of this potential, it is imperative that these reactions be studied systematically and carefully to reveal differenceswhich can be exploited. Results of detailed studies of the osmium and iodine catalyzed reaction between Ce(1V) and As(II1) have been reported (4, 10). This report describes the utilization of information obtained in these studies for a variety of analyses. Although iodine is an effective catalyst for this reaction, iodate shows no catalytic activity. However, As(II1) slowly reduces iodate to a state which has catalytic activity equal to an equivalent amount of iodide. This information was utilized to determine iodide and iodate in mixtures with [I-]/[IO,-] ratios from 1O:l to 1 :10 and the lower concentration component present at 2.7 X 10-8M. Relative errors were typically 1-2Z. Stoichiometric amounts of either Ag+ or Hg2+ completely inhibit the catalytic activity of iodide. Neither ion has any effect on the osmium catalyzed reaction. This information has been utilized to determine iodide and osmium in mixtures with [I-]/[Os(VIII)] ratios between 1O:l and 1OO:l. For example, analysis of a solution containing 6.79 X 10-8M, iodide and 6.82 X 10-l0M Os(VII1) yielded concentrations of iodide and osmium of 6.72 X 10-8M and 7.10 X 10-10 M , respectively. The inhibitory effect of Ag+ and Hg2+on the reaction also was used for the determination of traces of these ions. Mercury(I1) was determined at concentrations between 2.5 x 10-8M and 1.25 X 10-7M with a maximum relative error of 2.5 and Ag+ was determined at concentrations between 2.6 x 10-8M and 2.6 X 10-7M with a maximum relative error of 5 %.
GENERAL CONSIDERATIONS
=
+
5.82 X loa [Ce(IV)] [As(III)I (3.6 X lo5[As(III)] 8.46) [I]Tot 1.74 X lo5 [As(III)12 21.5 [As(III)] +5.11 [Ce(IV)ll
+
Reaction conditions selected for this work were [Ce(IV)] = 7.5 x 10-4M, [As(III)] = 8.09 X 10-3M, [HzSO~]= 0.5M and t = 25.00 j=0.02 'C. Under these conditions Equation 1 reduces to UI-
=
1.21 X l o 4[Ce(IV)] [I]Tot
uos =
+ +
5.75 X 104[Ce(IV)][As(III)] (8.1[Ce(IV)] 25 [As(III)]) 5.97 [Ce(IV)12 5.75 [Ce(IV)l [As(III)] 6.95 [As(III)]z [OSITot
+
(3) For the experimental conditions described above this expression reduces to uoS =
1.97 X lo5 [Ce(IV)I [OS]Tot
(2)
showing that the reaction rate is independent of As(II1) concentration and first order in [Ce(IV)I. (6) D. W. Margerum and R. K. Steinhaus, ANAL.CHEM.,37 222 (1965). (7) D. W. Margerum, J. B. Pausch, G. A. Nyssen, and G. F. Smith, ibid., 41,233 (1969). (8) J. B. Pausch and D. W. Margerum, ibid., 41, 226 (1969). (9) R. H. Stehl, D. W. Margerum, and J. J. Latterell, ibid., 39, 1346 (1967). (10) P. A. Rodriguez and H. L. Pardue, ibid., 41, 1369 (1969).
(4)
The approximation in Equation 4 is good to about 10 in 2M Also, there is a slight acid dependency between 2M HzS04 and 0.5M H2S04 for this reaction. Therefore, Equations 2 and 4 cannot be compared rigorously. However, they serve to illustrate two pertinent points. Because both reactions are first order in Ce(1V) under the experimental conditions, response curves approaching linearity are expected when T us. time is recorded for transmittance between 20 and 60% (10). It is observed that osmium is about 20 times more active than is iodine. Therefore it is quite simple to determine low concentrations of osmium in the presence of high concentrations of iodide, but very difficult to determine low concentrations of iodide in the presence of high concentrations of osmium. The kinetics of the formation of a catalytically active species in the reaction between As(II1) and iodate can be represented approximately as follows: IE UOI~,A~(III= ) kIOa-Aa(II1)
[As(III)] [IO,-]
(5)
where ~ I O ~ - . = A ~1.45 X l/mole-sec. For the measurement conditions described above ([As(III)] = 8.09 X l O - S M ) , this expression reduces to V:~s-Aa(III)
All reaction rates discussed below are in units of moles/ liter-second and initial rate measurements are used throughout. It was shown in a previous report (10) that a ratio of [As (III)]/(Ce(IV)] greater than 2 :1 is necessary to yield proportionality between iodide concentration and reaction rate. The approximate form of the rate expression for this situation is given in Equation 1. VI-
The rate expression for the osmium catalyzed reaction in 2 M H2S04is ( 4 )
= 1.17
x
10-4 [1oS-]
(6)
For a 1 x 10-7M concentration of iodate the rate of production of catalytic species would be about 1 X lo-" moles/litersecond. If it is assumed the analytical measurement requires 20 seconds, a total concentration of about 2 X 10-l0Mcatalyst would be generated. Clearly, iodide in concentrations greater than 10-8 can be determined with negligible interference from iodate if the measurement is made rapidly. On the other hand, iodide determinations at concentrations below 1 X 10-8M would suffer interference from the reduced iodate. Generally, for the stated conditions, iodide can be determined at [1-]/[103-] ratios of 1 :10 or larger, with little or no interference if the rate measurement is completed within 20 seconds after mixing. On the other hand, if the measurement method requires more than twenty seconds then iodide results can be expected to show positive errors unless a correction is applied, as discussed below. Total iodine (iodide and iodate) content of a sample is determined by reducing the iodate with As(II1) and then determining the total iodine by the catalytic method. If the iodate reaction were carried out under the conditions for which Equation 6 applies ([As(III)] = 8 X 10-3M, t = 25 "C) the time required to yield 99 conversion would be about 3.9 X l o 4 seconds. However, by carrying out the reaction at higher As(II1) concentration and elevated temperature, the time for quantitative conversion of iodide to iodate is reduced significantly. The data in Table I illustrate recoveries of iodide from iodate under a variety of conditions. The first four entries in the table demonstrate the small amount of iodide generated in the reaction cell after various periods of time. VOL. 41, NO. 11, SEPTEMBER 1969
1377
Table I. Conversion of Iodate to Iodide by Arsenic(1II) Reaction Rate5 Iodide Recovery, (moles/liter) Temp "C time (min) (moles/Lsec) equivalent ( M ) o t b 1.62 x 8.11 X 1025.0 1C ... ... 1.62 x 10-8 2.7 X 1025.0 lC 5.2 X ' l W 5.9 x 102.2 50 1.2 x 1 0 - J 1.4 x 10-10 1.7 25.0 1.62 X 8.11 X 103.3 x 10-10 4.1 25.0 1OC 2.9 x 1O-sb 1.62 x 8.11 X 10-8 15d 8.3 x 10-ge 0.283 X 1 P 12 2.30 X 10-7 63 5.05 x 1.02 x 1044 83 15d 3 .o x 10-76 5.05 x 2.30 X 1035d 6.5 X lOYe 2.21 x 1096 2.30 x 10-7 94 5.05 x 35d 6.7 x lo-* 2.28 X 10-7 99 2.30 X 109 100 5.05 X 200 5.9 x 10-76 2.01 x 1087 2.30 X 10-7 25 5.05 X Catalyzed reaction rate measured under the following conditions ([Ce(IV)] = 7.5 X lO-'M, [As(III)] = 1.6 X 10-2M,[&SOa] = 0.5M, t = 25.0 "C) after reaction with As(II1). b Equivalent iodide concentration (moles/liter) given by 0.113 X rate. c In reaction cell in the presence of 7.5 X lW4MCdIV). d In sealed ampule. e Equivalent iodide concentration computed on basis of 3.02 fold dilution of reacted sample; [I]Tot = 3.02 X 0.113 X rate. [IOt-I
[As(III)I
z
(1
The last five entries in the table demonstrate the effect of reacting iodate with As(II1) in a sealed ampule at elevated temperatures for various times. Clearly, a reaction time of 35 min at O.OSMAs(II1) and 100 OC (boiling water) is sufficient for complete conversion of iodate to the catalytically active form. The general procedure for determining iodide and iodate in mixtures consists of measuring rates of two solutions, one which has been subjected to reduction by As(II1) and one which has not. The first gives the total iodine concentration [IO3-]), the second gives the iodide concentration and ([I-] the difference gives the iodate concentration. If the iodate : iodide ratio is too large (greater than about 10 :1) or the measurement time is too long (greater than about 20 sec), then errors may result from conversion of iodate to iodide in the cell. A correction can be applied by using the determined value of iodate, the total reaction time, and Equation 6 to estimate the amount of catalyst produced from iodate, and then subtracting this from the determined iodide concentration and adding it to the iodate concentration. Clearly a series of successive approximations could be used; however, for the samples handled in this work, a single approximation was sufficient. There is one other potential source of error in this and the other determinations described here. This results from the fact that in the presence of low As(II1) concentrations Ce(1V) can oxidize iodide to a catalytically inactive form (10). The rate of this reaction has the approximate form given in Equation 7.
+
10,-
UI-,Ce(IV)
=
5 [ce(Iv)] [IITot
(7)
If the Ce(1V) concentration in the cell were made too large, then significant amounts of the catalytic activity could be lost during normal measurement intervals. It is for this reason that the Ce(1V) concentration is kept low relative to the As(II1) concentration. The determination of Ag+ and Hgz+are based upon the fact that both ions react with iodide to yield a catalytically inactive product. (This behavior also is used to deactivate iodide in the presence of osmium and permit the selective determination of osmium.) At the concentration levels considered here (-10-8 MAg+, Hg2+), the rates of these reactions are slow compared to the measurement time. As a result, it is necessary to equilibrate either ion with iodide for at least five minutes prior to determining the excess iodide for Ag+ or Hg2+ determinations. No attempt was made to evaluate quantitative aspects of the kinetics of these reactions. However, the 1378
ANALYTICAL CHEMISTRY
catalytic reaction should provide an effective method of studying these kinetics. No waiting period is necessary when a large excess of inhibitor (>10-5M) is used to deactivate iodide in presence of osmium. EXPERIMENTAL
The measurement conditions for most determinations are as follows: [Ce(IV)] = 7.5 X lO-'M, [As(III)] = 8.09 X 10-3M, t = 25.00 f 0.02 "C. The Ce(IV) and As(II1) solutions are prepared in 0.5M H & 0 4 as described in the previous paper (10). Iodide, iodate, osmium (VIII), As+, and Hg2+ are prepared in deionized-distilled water. All solutions are equilibrated in a water bath at 25.00 "C prior to making any measurements. The solutions are handled with hypodermic syringes with Teflon (Du Pont) needles. The general procedure for the measurement step consists of adding 1.00 ml each of 2.50 X 10-*M As(III), 0.5M HzS04, and 2.32 X 10-3M Ce(IV) to the reaction cell followed by 100 pl of catalyst if the latter is not included with one of the other reagents. The transmittance is measured at 407 nm in a 1.OO-cm cell. Many of the analyses reported here were performed by T us. time. extracting initial rates from recorded curves of Others were performed using automatic rate measuring instrumentation described previously (11). Results were comparable in most cases. Procedures. IODIUE-OSMIUM(VIII) MIXTURES. The reaction rate is measured for the sample without prior treatment. Then a 50-pl injection of inhibitor (7.7 X 10-4M HgC12) is introduced into the cell before addition of the catalytic mixture. The Hg2+ concentration is about two orders of magnitude larger than the highest iodide concentration used in this work. Then the catalytic mixture is added and the resulting rate is measured. The iodide concentration is computed from the difference between the two rates using Equation 2. From the conditions used in this work, ([Ce(IV)] = 7.5 X 10--4M),the iodide concentration in the cell is given by: [I-] = 0.113 (uo -
UBg,+)
The osmium concentration is computed from the rate for the solution containing Hg2+ and Equation 4, using a rate constant determined for the experimental conditions used. For the conditions used in this work, the osmium concentration in the cell is given by [os]
=
4.11
x
10-3
uHpl+
(9)
(11) H. L. Pardue, M. F. Burke, and D. 0. Jones, J. Chem. Educ., 44, 684 (1967).
Table 11. Effect of Hgz+ on the Rates of the Iodide and Os(VII1) Catalyzed Reaction Rate (molesiliter sec) X 10' Error (2) b (molesiliter) x 109 Calca Obsa Calcb Obsb
[Os(VIII)]
[I-]
rl
3.41 3.41 3.41 0.341 0.682 1.71
27.2 40.8 67.9 67.9 67.9 67.9
10.7 11.9 14.3 6.84 7.67 10.2 0 Total rate before inhibition by Hgz+. Rate after inhibition by Hgz+(1.23 X
8.31 8.31 8.31 0.831 1.66 4.16
10.3 12.0 14.1 6.77 7.46 9.88
0.7 2.7 2.9 3.0 3.2 1.0
3.4 2.2 2.9 4.0 1.5 0.5
IODIDE-IODATE MIXTURES.Ampoules to be used for reacting iodide with As(II1) are treated with chromic acid cleaning solution, washed with distilled water, and dried at 110 "C for one hour. A portion of the sample containing iodate is sealed in the ampoule with 0.05M As(II1) in 0.5M HzS04and heated in a boiling water bath for 35 minutes. The ampoule is cooled, opened and its contents are equilibrated to 25.00 "C. Then 1.00 ml of this sample is added to the cell along with 1.00 ml of 0.5M HeS04 and 1.0 ml of Ce(1V) and the rate is measured ( v I - , I o ~ - ) . This rate, after correcting for dilution, is compared to a rate measured for 100 p1 of the sample prior to treatment with As(III), ( V I - ) . The first approximation of the concentrations of iodide and iodate in the cell are computed from Equations 10 and 11. (10)
uI-
- [IOs-ll;ncarexp (1.17 x
lJ-1corr =
2.72 2.62 -3.8 -3.7 4.08 3.99 -2.1 -2.2 ... 0.0 ... 4.08 6.79 6.12 +4.7 -9.9 ... f4.4 ... 6.79 6.79 6.69 +4.1 -1.5 -1.0 6.79 6.72 +4.1 6.79 6.44 +0.6 -5.2 5 Mercury(I1) concentration in cell is 1.23 X 10-5M. b Mercury(I1) concentration in cell increased to 2.3 X 10-4M.
rI03-]uncor= 0.113 (ul-+loa- -
-4.0 +1.9 +4.6 +4.0 +4.2 $0.7
b
If the iodate concentration is found to be greater than about 10 :1 over the iodide concentration or the measurement time exceeds about 20 seconds, then the iodide concentration is corrected using the differential or integrated form of Equation 6 as illustrated in Equation 12.
3.28 3.48 3.41 3.57 3.56 0.355 0.710 1.72
[I-]uncor= 0.113
-3.7 +0.8 -1.4 -1.0 -2.7 -3.2
a
moles/liter) in reaction cell.
Table 111. Analytical Results for Iodide-Osmium(VII1) Mixtures Using Hg2+ Inhibition of Iodidea [Os](moles/liter) [I-] (molesiliter) x 10s x 108 Error ( %) Taken Found Taken Found Os I3.41 3.41 3.41* 3.41 3.41b 0.341 0,682 1.71
7.98 8.47 8.69 0.864 1.73 4.19
Std dev (2)
u1-)
t)
(12)
Only the differential form was used in this work for correction of iodide concentration. Note that it seldom is necessary to apply a correction to the iodate concentration or to use a second approximation for the iodide concentration. SILVER(I)AND MERCURY(II).The Ag+ or Hgz+ sample is equilibrated for about five minutes with a measured excess of iodide. Then the reaction rate (yAi( = or YUgzt) is measured on 100 p1 of the sample. The measured rate is compared to the rate ( V I - ) for an equal volume of iodide with no metal ion added. The metal ion concentration is computed using Equation 13. [Ag+] = 0.113 ( U I - [Hg"]
=
0.113 ~
6 1 -
(13)
UA~+)
- uIigz+)
It should be noted that the multiplier in Equations 8-14 (excluding 12) may vary slightly from one day to the next and should be evaluated each day for highest accuracy. RESULTS AND DISCUSSION
The data in Table I1 illustrate the effectiveness of HgZi in inhibiting the catalytic activity of iodide. It is observed that in all cases, greater than 95 of the iodide is inhibited while the reaction rate due to osmium catalysis is unaffected. Table I11 contains results for the analysis of iodide and Os(VII1) mixtures. All errors are within 5 % . Attention is focused on the fact that osmium is determined down to 3.4 x
Table IV. Analytical Results for Iodide-Iodate Mixtures Using As(II1) Reduction of Iodate [I-](moles/liter) X 108 Taken Found (molesfliter) Measurement Error (Z) -.~~.~ -110-3-- 1 . . , X 108 . . ", Uncorr Corr Taken Found time (sec) I-" 103-* 2.63 ... 13.5 ... 27.4 ... 3.30 2.67 1.41 1.42 27.2 27.2 27.6 27.6 27.2 27.2 Error quoted is for corrected results. No corrections were applied to the iodate data. 2.72 13.6 27.2 2.72 13.6 27.2 27.2 27.2
0
*
0.00 0.00 0.00
...
... ...
... ... ...
-3.3 -0.7 +0.7
... ...
27.0 27.0 27.0 2.70 13.5
27.1 27.5 27.4 2.80 13.7
201 47 24.3 23.9 24.3
-1.8 +3.3
+0.3 +1.9 +1.5 +3.7 $1.4
0.0 +1.4 0.0
...
VOL. 41, NO. 11, SEPTEMBER 1969
1379
Table V. Analytical Results for HgZ+Using Inhibition of Iodidea
x 108
-
Taken 0.00
2.50 5.00 7.50 10.0
12.5 a Iodide
liter-sec) liter) X 107 X 1W 0.00 23.5 0.00 2.54 19.1 5.07 4.90 15.0 9.80 7 45 10.6 14.9 9.75 6.56 19.5 12.3 2.23 24.6 concentration with no Hg2+ added Found
Error
dev
0.0 +1.6
(%) 1.3 0.5
(x)
-2.0 -0.7 -2.5 -1.6
0.4 0.4
0.3 1.5
is 27.1 X 10-8M.
Table VI. Analytical Results for Ag+ Using Inhibition of Iodidea [I-]
Rate Reacted [AS+](molesiliter) (moles! (moles/ x 108 liter-sec) liter) Taken Found X 107 X 108 0.00 0.00 25.5 0.00 2.61 2.53 21.3 2.53 5.22 4.96 19.2 4.96 7.83 7.38 17.1 7.38 10.4 10.0 14.8 10.0 13.0 12.6 12.6 12.6 2.61 24.9 1.92 24.9 Iodide concentration with no Ag+ added is
Error (%)
Std dev
(x)
-3.1 -5.0 -5.7 -4.2 -3.1
1.1 0.9 0.9 0.5 1.2 2.6
-4.6
6.0
I
,
.
27.1 x IO-aM.
10-lOM with an error of 4 %, corresponding to a concentration error of 1.3 X 10-"Mor an absolute error of (3 X 10-3L) (1.3 X l0-l1M/L) = 3.9 X 10-*4moles. In evaluating or planning an extension of these results, it must be kept in mind that osmium is much more active as a catalyst than is iodide. Consequently, it is difficult to determine iodide in equimolar mixtures with osmium. Kinetic data available to date show no conditions under which this difference in catalytic activity can be reduced significantly or reversed. Table IV contains results for the analysis of iodide-iodate mixtures. The first three entries in the table illustrate the effects on the iodide determination of incubation with As(II1). The effects are observed to be minimal. The remainder of the results demonstrate that iodide and iodate in ratios of 10 :1 to 1 :10 can be determined with good accuracy.
1380
ANALYTICAL CHEMISTRY
Tables V and VI contain analytical results for Hg2+ and Ag+, respectively. It is observed that both metals can be determined with good accuracy, the results for Hg2+ being somewhat better than the results for Ag+. At present it is not possible to use this reaction to differentiate between Ag+ and Hg2+. However, it should be possible to determine the total metal ion concentration in mixtures of the two. It was mentioned that the kinetics of the formation of the metal complexes with iodide are slow. It may well be that the rate of these reactions are sufficiently different that they could be differentiated on the basis of kinetic differences. No attempt was made to study these effects quantitatively. It is probable that Os(VII1) could be determined in the presence of I- by utilizing Ce(1V) to oxidize iodide to the catalytically inactive iodate (IO). This could be advantageous if it were undesirable to add Ag+ or Hg2+ to the sample being determined. The effects of Ag+ and Hg2+on iodine were not evaluated. However, it is probable that equilibration of iodine with these ions would not affect its catalytic activity. Because the rates of formation of the inactive complexes of Ag+ and Hg2+with iodide are slow relative to the speed with which reaction rates for the catalyzed reaction can be measured, it is probable that a combination of iodide masking using Hgn+and As(II1) reduction could be used to selectively determine iodide, iodine, and iodate in mixtures of the three. In recent years there have been numerous arguments put forward supporting the use of initial rates as opposed to single point integral methods for analytical purposes (3). The iodide-iodate procedure suggested above illustrates an example in which the initial rate method works very well, but the longer integral methods would fail completely. In the integral methods, a large fraction of the iodate in the sample would be reduced and it would be very difficult, if not impossible, to separate the effects of iodide and iodate. This and other work cited above demonstrate clearly that when good quantitative kinetic data are utilized, selective analyses can be performed using reaction-rate measurements. There is little question that the reliability of the results reported above was enhanced by the use of the stabilized photometer. It is probable that many of the analyses reported here could be performed with a photometer of lesser stability if decreased reliability is acceptable. RECEIVED for review April 9, 1969. Accepted June 23, 1969. Research sponsored by the Air Force Office of Scientific Research under Grant 1212-67.
