used in these runs is 2 X 10-6M. The use of higher metal concentrations would undoubtedly extend the range to higher ligand levels. All determinations were performed either by slope (Figures 1 and 2) or one point determinations (Figure 3). Most of the experiments have been performed with 1-cm silica cells. The use of IO-cm cells provides greater sensitivity. It is interesting to note that the useful range of concentration for 1,lO-phenanthroline is an order of magnitude smaller than simple equilibrium considerations based on Cu(I1)phenanthroline data would predict. We have no explanation for this behavior at the present, unless the phenanthroline complex of Cu(1) is more stable than that reported for Cu(1I).
Examples of curves used for determination are shown in Figures 1, 2, and 3. CONCLUSIONS
We have demonstrated the suitability of the Cu(I1)-catalyzed autoxidation of ascorbic acid for the development of a rapid, simple, and sensitive means of determining a wide variety of complexing agents.
RECEIVED for review August 7, 1967. Accepted March 2 5 , 1968. Work supported by Edgewood Arsenal through Contract DA18-035-AMC-744(A).
Use of Metal Ion Catalysis in Detection and Determination of Microamounts of Complexing Agents Catalimetric Titration of Cyanide Ion Horacio A. Mottola’ and Henry Freiser Department of Chemistry, Unizersity of Arizona, Tucson, Ariz. 85721
A new and widely applicable approach in the development of methods for the detection and determination of trace levels of complexing agents by modifying the rate of metal-ion catalyzed redox reactions is presented. The metal ion catalyst is used as a titrant for a solution containing the complexing agent as well as an indicator reaction mixture. Catalysis becomes effective only after the ligand has become complexed so that a dramatic change is observed with a slight excess of the catalytic titrant. Using as indicator reaction the autoxidation of L-ascorbic acid catalyzed by traces of Cu(lI), low concentrations of cyanide ions (lO-7M level) have been determined in a simple and reproducible fashion. The end point i s reached when one mole of copper has reacted with two moles of cyanide ion. The effect of various parameters affecting the titration are discussed. The effect of some interferences has been considered. THEPRACTICAL VALUE of metal-catalyzed oxidation-reduction reactions for the determination of minute amounts of complexing agents has been recently demonstrated ( I , 2). This paper presents a new analytical approach which permits simpler and faster determinations. In this method, readily adaptable to automation, the metal ion catalyst is used as a titrant. The possibility of using catalytic reactions as an end point indication had been first pointed out by Yatsimirskii (3), who coined the term catalimetric titration, but has been practically neglected. The autoxidation of L-ascorbic acid catalyzed by traces of Cu(I1) provides a versatile and sensitive indicator reaction for 1 Present address, Department of Chemistry, Oklahoma State University, Stillwater, Okla. 74074
(1) H. A. Mottola and H. Freiser, ANAL.CHEM., 39, 1294 (1967). (2) H. A. Mottola, M. S. Haro, and H. Freiser, ibid., 40, 1263 (1968).
(3) K. B. Yatsimirskii and T. J. Fedorova, Proc. Acad. Sci. USSR, 143, 143 (1962).
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ANALYTICAL CHEMISTRY
the detection and determination of several complexing agents with different donor atoms (2). The determination of CNat pH 6.5 using the methods described earlier (2), was found to be of limited utility because it was applicable in a very narrow concentration range. These observations, in agreement with earlier observations at higher concentrations and by different techniques (4), suggested the use of a new titrimetric approach in which increments of Cu(I1) solution would be added to the indicator reaction mixture containing CNuntil a [CUIlevel is reached at which the reaction becomes fast enough to be easily followed. The amount of titrant required to reach this point would be proportional to the cyanide content of the sample, provided that the copper cyanide formation is complete and rapid (relative to the rate of the autoxidation). EXPERIMENTAL
Reagents and solutions as well as general precautions have been described earlier ( 2 ) . Most titrations were performed in a carbonate-bicarbonate buffer of pH = L0.50 ( p = 0.01). All determinations were made at room temperature, 23-5 “C. Apparatus. The photometric cell (40 mm x 60 mm x 50 mm) was constructed from 3-mm Plexiglas sheets sealed with epoxy resin. The path length was approximately 42 mm. The cell contained two quartz windows made from 1-mm transparent fused quartz microscope slides (Thermal American Fused Quartz Co.), 35 X 25 mm in size. The cell is similar to one described by Headridge (5). Stirring was accomplished by means of an air-driven magnetic stirrer (Chemical Rubber Co.) located underneath the cell compartment and using a ‘/*-inchTeflon-covered magnetic stirring bar in the cell. The titration assembly was composed of a Beckman DU spectrophotometer equipped with hydrogen lamp and photo-
s. Butt and M. Hallaway, Arch. Biochem. Biopltys., 92, 24 (1961). ( 5 ) J. B. Headridge, “Photometric Titrations,” Pergamon Press, New York, 1961, p 31. (4) V.
A IOXT
12.0
4.0
20.0’
28.0
44.0
36.0
CiN-,x I O - ~ M
Figure 2. Working curve for catalimetric titration of cyanide ions Experimental conditions: Titrant: [Cu(II)] = 1 X 10-3M; rate of adding titrant: 0.0048 ml/min. ; ascorbic acid concentration: 4 X lO--bM, pH = 10.50 (carbonate-bicarbonate, p = 0.01). Recorder setting: 5 volts/division
U
25 seconds
,0020 rnl CU(II)
I~IO-~M
-~ m2A1 - ~[HzA]
I Figure 1. Titration curves for 8.0 X lO-’M CNA . Recorder setting at 1 volt/division. B. Recorder setting at 5 volts/division. Ascorbic acid concentration: 4.2 X 10-sM, pH = 10.50 (carbonate-bicarbonate, p = 0.01). Rate of adding titrant: 0.0048 ml/min. The origin of curve B has been offset to permit comparison of the curves
multiplier, a titration chamber, and a Model 20-2A F. L. Moseley Co. recorder. The entire DU tube amplifier was disconnected and replaced by a high quality operational amplifier. The original slide wire was retained. The recorder was connected in place of the null-meter. The titration compartment was a modified Beckman 4400 cell compartment in which a flat platform to hold the photometric cell was placed instead of the standard 100-mm cell holder. An aluminum cell compartment cover was made with a metal tube fastened to it, positioned to support a micrometer buret (0.2-1111 total capacity, 0.0002 smallest division, 0.5 % accuracy, Roger Gilmont Instruments, Inc.). Strips of felt were glued around the cell compartment and cell cover to provide light-proof seals. All metal parts were painted with Black Velvet Coating 101-C10 (3M Co.) baked at 60 “C for 5 hours. Procedure. A 2.0-ml volume of a concentrated carbonatebicarbonate buffer (22.3 g NaHC03 and 80.6 g Na2C08 per liter of solution) was transferred to the photometric cell. Then the sample to be titrated was pipetted into the cell, followed by water to give a volume of 48.0 ml, and finally, 2.0 ml of a solution wlO-3M in ascorbic acid. The solutions were mixed before titration. The sample was titrated by addition of a Cu(I1) solution at an appropriate rate (a 1 X 10-3Msolution and rates of 0.0040-0.0120 ml/min were found suitable.) The cyanide content in the sample was calculated from a working curve (Figure 2). RESULTS AND DISCUSSION
The catalimetric titration described in this paper is based on the detection of the point at which variation of the concentration of ascorbic acid becomes appreciable. The rate of the reaction may be written as:
dt
where
+
k = ki kn[Cu(II)] (2) [HzA] = ascorbic acid concentration, and kl and kz characterize the rates of the uncatalyzed and catalyzed reactions, respectively. Instead of determining a series of k values at different Cu(I1) concentrations which would be time- and sample-consuming, a semi-automatic photometric titration was employed. In this case, by adding Cu(I1) at a constant rate, Le.,
(3)
d[Cu(II)]/dt = constant we may write
--d[HnAl
a:
d[CU(II)l
-
=
kL[HtA]
+ ~~[CU(II)][HTA](4)
dt
or rearranging -d[ln[HzAll
=
kl’
+ k2’d[Cu(11)]2
(5)
Since [H2A] absorbance, a plot of log A us. [CU(II)]~ should consist of two straight line segments (one horizontal) crossing at the end point. The results obtained in different titrations were plotted in this manner. Intersecting straight lines were obtained in agreement with Equation 5 (Figure 3). The end point could be located either from a plot of log A DS. [Cu(II)] or, more conveniently, by extrapolation of the two linear segments of the titration curve (Figure 1) which gives adequate reproducibility. Using log A us. [CU(II)]~ plots for end point determination, the end point break appears at a [CN-]/[Cu(II)] ratio close to 2. Actually, 20 separate determinations at CN- concentrations of 2 X 10-7 to 40 X IO+ and at a recorder setting of 5 volts/div gave an average [CN-]/[Cu(II)] ratio of 2.08, and 9 determinations at CN- concentrations of 4 X 10-7 to 24 x 10-7 and at a recorder setting of 1 volt/div gave an average ratio of 2.11. Titrations with [Cu(II)] = 1 X 10-1 at 0.0048 rnl/min with 8.0 X 10-6M CN- gave a [CN-]/[Cu(II)] of 2.20 from log A us. [Cu4+12. VOL. 40, NO. 8, JUlY 1968
0
1267
Table I. Effect of Some Foreign Species [Titrations at room temperature; 5 V/div span; 0.0020 ml Cu(I1) 1 X 10d8M/25sec, pH = 10.50 ( p = O.Ol)] CN- added CN- found Foreign species x 1 0 - 7 ~ x 1 0 - 7 ~ Error, % x 10-6M 8.0 7.50 -6.2 Br- (as NaBr) 8.0 7.20 -10.0 4.0 8.0 8.00 8.0 7.54 -5.8 C1- (as NaC1) 8.0 +6.2 8.50 4.0 8.0 7.90 -1.3 8.0 7.80 -2.5 I- (as NaI) 8.0 -11.0 7.11 8.0 8.60 +7.5 4.0 8.0 8.3 $4.0 SCN- (as KSCN) 8.0 7.75 -3.0
Inasmuch as ascorbic acid is in great excess over cyanide (more than 1000 times) copper(I1) is probably instantly reduced by ascorbic acid so that complexation occurs between CN- and Cu(1). The following set of reactions may be written : CU(I1)
+e
+ CNCUCN + CNCuf
-
CU(1) CuCN Cu(CN)z-
which would lead to the observed [CNl/[Cu(II)] ratio. Titrant addition rates ranging between 0.002 and 0.01 2 ml/min of 1 x 10-3M Cu(I1) have been used to titrate 8.0 x lO-7M CN- under experimental conditions similar to those used to obtain the working curve. At the lower rates (0.0020 and 0.0040 ml/min) the [CN-]/[Cu(II)] obtained from log A us. [Cu2+]2 are approximately 2.0. As the addition rate increases markedly the [CN-]/[Cu(II)] ratio decreases as would be expected in view of a faster build up of catalyst with a resulting loss of “resolution” in the recording. With all other factors constant, different recorder settings (10 to 0.5 volts/div) do not affect the [CN-]/[Cu(II)] ratio obtained from log A us. [ C U ~ + ] ~ . It must be mentioned, however, that the empirical end point determination, as illustrated in Figure 2, yields ratios different from log A us. [Cu(II)12 plots. These ratios are a function of [CN-1, voltage span of recorder, rate of adding titrant, amount of ascorbic acid. With increasing concentration of ascorbic acid in the range from 1 to 10 x IO-SM, the empirically determined end point appears later. This results from using a % T recording scale, whereas the change of concentration is linearly related to absorbance. A sharper break occurs at the higher ascorbic acid concentrations. A working curve obtained by titrating different amounts of CN- (4.0 to 24.0 X 10-7M) at a concentration of ascorbic acid of 9.1 x lO-5Mgave a line parallel to the one shown in Figure 2, with a displacement of 0.0100 ml above but with the same slope. Thus, increasing the ascorbic acid concentration does not improve sensitivity but the curves seem somewhat better defined. The time required for titration, however, is increased. From a practical viewpoint, using the experimental conditions reported here, 4.5 X 10-5M
1268
ANALYTICAL CHEMISTRY
1.00
M O
5.00
7.00
9.00
11.0
130
22.0
26.0
CURVES A B.C
2.00
6.00
10.0
14.0 CURVE 0
18.0
CU(Il)
xlot4
Figure 3. Typical plot of log absorbance us. [CU(II)]~ Experimental conditions as described in Procedure. Curve A : No cyanide added; Curve B: 2.00 X lO-7M cyanide; Curve C: 4.00 X lO-’M cyanide; Curve D ; 8.00 X lO-’M cyanide. r = [CN -I/KuOI)I ascorbic acid seems the most appropriate for the purposes of titrating CN- at the IO-’M level. Ten determinations out of ten samples containing 8.0 X 10” CN- gave an average end point of 0.0279 ml of 1 x 10-3M Cu(I1) with a standard deviation of ctO.001 (4%). Table I summarizes the results obtained in the presence of bromide, chloride, iodide, and thiocyanate as possible interferences. These species do not interfere appreciably (experimental error under 10 %) when present in thousand fold excess of cyanide. CONCLUSIONS This work demonstrates that catalimetric titrations are readily adaptable to the quantitative determination of traces of complexing agents. The approach, applicable to any ligand capable of rapid formation of stable complexes with the metal catalyst, has been used to determine traces of cyanide (10-7Mlevel). The method, simple and reproducible, based on the inhibitory effect of CN- on the Cu(I1)-catalyzed autoxidation of L-ascorbic acid (pH 10.50) is very rapid, generally requiring no more than a few minutes per sample. Chloride, bromide, iodide, and thiocyanate can be present in at least 1000-fold excess over cyanide. The use of kinetic, rather than equilibrium parameters as end point indicators, is capable of more general application and merits further attention. RECEIVED for review August 7, 1967. Accepted March 25, 1968. Work supported by Edgewood Arsenal through Contract DA18-035-AMC-744(A).
