Multiple Analysis by Differential Pulse Polarography P. Lama Dipartimento di Chimica lndustriale e dei Materiali, Universita di Bologna, Viale del Risorgimento, 4-40134 Since its earliest development, polarography has been a techniaue suitable for analyzing m a w components in a solution wkhout having t o separate or mask them. In classical d.c. polarography, however, when several electroactive analytes are present and many waves develop, one can sometimes meet with difficulties due to partial or total overlapDine of two or more waves. which does not allow a reliahle measurement of the relevant diffusion currents. Bv more recent ~olaroeraohictechniaues. such as different(al pulse polarograph; (DPP), the signal arising from each individual analyte is in the form of a peak, rather than the wave usually associated with d.c. polarography. The height of the peak is directly proportional to concentration. The peak current (i,) is generally ohtained by a graphical Drocedure, based on the measurement of the height of the peak from the hase tangent. This method, however, especially a t low concentrations, often remains subjective and can be remarkahle source of errors.' A more reliable nrocedure for measuring the peak current is t o record a pol&ogram of the blank solution, if available, and suhsequently subtract i t from the record of the test solution. However, problems can still arise in D P P analysis when the peaks of two or more components are not well separated and appear to merge, as, for example, in the polarographic analysis of cadmium and indium in 0.1 M HC1, as shown in Figure 1A and 1B. In this circumstance the i, of a given component can be heavily influenced by the contribution of the other components. Thus the value of i, for the species sought cannot be obtained merely by subtraction of the background current from the polarogram of the test solution. Since this is a common problem in practical analytical work, it seemed interesting t o examine the possibility of applying to polarographic techniques a general approach such as the one successfully applied t o other techniques such as UV and visible adsorption spectrophot~metry.~ As polarographic current is an additive property of the electroactive species a t the applied potential, if linear proportionality between current and concentration of the components a t various potentials exists, in principle the concentration of all the components is attainable. For this purpose, n measurements of current a t n different a o ~ l i e dDotentiale are necessary for determining the concentration of n components in the solution. Bv suitable calibration the current's values for each cornpone& a t the different potentials have to be determined. The resolution of a system of n linear equations with n unknowns gives the sought concentrations. Practically, the proposed method is based on the recording of the pure supporting electrolyte (hase line), of the solutions of the separate components a t suitahle and known concentration and of the unknown solution or solutions. The method requires a very accurate measurement of the current and a reproducible measurement of the applied potentials, so that the currents of the oure comnonents and of the mixture are effectively referrLd to the same potentials. Conventional DPP polarograms, formed by step-shaped peaks, do not permit an exact measurement of the current a t everv ~ o t e n t i aand. l owine to the excessivelv reduced scale of the idtentid, make its determination uncertain. In order to
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Bologna, Italy
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704
Journal of Chemical Education
Figure l.A. (a) In 0.8 pg/mL in 0.1 M HCI. (b) Cd 1.0 pg/mL in 0.1 M HCI. Dropping time 2 s. sweep rate 2 mV/s. damping 30 ms. recorder potential Scale 100 mV/cm. 0. In 0.4 pg/mL + Cd 0.5 pg/mL in 0.1 M HCI. Same instrumental conditions as A. overcome these difficulties, polarograms can he smoothed by hieh d a m ~ i n and e a rather reduced d r o ~ ~ i time n e (1 s). and the poten&lvscale of the recorder can hikuiarged to 10'mvl cm. All polarograms can he successively recorded, one upon the other, on eraph In this way, reproducihili- . paper. . . . greater . ty and accuracy can he achieved.
Preliminary Measurements and General Procedure In order to check the linear relation hetween concentration and current at various potentials, at high damping and 1 s dropping time, the following experiments can he carried out. A d.p.p. polarogram of the reference solution (0.1 M HCI) was recorded on graph paper (instrumental conditions: impulse height-50 mV, dropping time 1s, damping 3 s, recording potential scale 10 mVIcm). Superimposed upon this base line, polarograms of indium and cadmium at various concentrations in the range 0-1 pg/mL in 0.1 M HCI were recorded. Currents of both metals from the hase line st -0.557 V and - 0 . 3 7 V uere meanuwd at each concentration. Lmenrity was verified perfectly nsuell at potenrials tar frum the peak values. The method was checked hy analyzing some mixtures uf substances whose peak potentials are very near each other and could not be determined by conventional d.p.p. without masking or separation procedures. Analysis of two components can be carried out by the following procedure. Cadmium and indium mixtures are presented as an examole. In weighed, 25-mL volumetric flasks. introduce suitnhlp quantities of 100 rglmL Cd ur In standard sulutian. After checking by weighing the introduced quantity, add 0.1 M HCI to the mark, and
' Bond. A.M.: Grabaric. 6. S. Anal. Chem. 1979. 51.337-341
Kolthoff, I. M.; Elving, P. J., Eds. Treatise o f ~ n ~ l ~ i & ~ h s r n i s t r y 2nd ed. Interscience: New York; Part I, Vol. 7, p 96.
able 1. Solutlonr Contalnlng Indium and Cadmlum In
sol.
Cd pglrnL
pg/mL
O8 1 2 3
0.800b 0.607 0.402
4
0.204
5
-
-
0.205 0.806 0.610 0.793"
I (mm) -0.597 V
I (mm)
-0.557
V
19.5 200.5 167.0 160.5 96.0 58.5
19.0 87.5 09.5 175.0 121.0 128.5
'HCL 0.1 M. I" or Cd sfandad
Flgure3. (a) Pb 1pg/mL. (b)T1 2 pg/mL. (c)In 0.4 pg/mL. (d) Pb 1 pg/mL + TI 1pg/mL + In 0.4 pg/mL. lnshumental conditions see Figure 2. Table 3. Analyllcal results
+
Table 2. Analytical Results ~n Cglm~)
solutlon~
prewnt
found
present
2
0.606
0.607
3
0.402
0.205 0.808
4
0.204
0.402 0.204
0.205 0.806 0.610
~~~~
~~
0.609
~
~~,,.~
~
~
~
~
In bg/mL)
present
found
prewrnt
found
present
4 5
0.977 2.007 0.405
0.999 1.997 0.398
1.991 2.055 2.063
1.983 2.004 2.003
0.395 0.207 1.002
0.398 0.200 1.001
where the coefficients A1, B, are the currents (in millimeters) of In a t the concentration of 1rg/mL, a t the potential -0.557 V and -0.597 V, respectively, Ap and Bz are the corresponding currents of Cd. These values were calculated from the polarograms obtained with the solutions 1 and 5 containing the single components (Table 1). One obtains:
cd ( ~ r g ~ m ~ )
found
weigh the flasksagain with a precision of 10.1 mg. In this way the oreoared. solutions of the oure comnonents are . . i n the same wav. oreoare solutions of mixtures bv introdueine" in ~, other vt~l~rnetr~e flasks suitable quantities of hoth componentr and 0.1 M HCI ro the mark and by weighing aftpr every addrtim (sea Table 1). Introduce into the polarographir cell U . l M HCI solution. Deaerate for 10 min, and record the background current on graph paper at the indicated instrumental conditions (impulse height -50 mV. droo time 1 s. darnoine 3 a).. Reoeat . the meratian with the nre&redsolutions A d reeori successiveIvone uo& the other. Read on thegraph paper thecurrentsat the E,potentials(ln = -lJ 557 V , Cd = -0.:,79 V), and subtract from each reading the value of the hackground current. A lens can he useful for accurate reading. Solution concentrationand resultingcurrentresponses, corrected for the background, are given in Table 1.The use of these measurements in multiple analysis is reported in the following section. ~~~
11(IrgImL)
found
6
Figure2 (a) 10 0.8pgImL. (b)Co I.OpglmL, (c) In 0.4 pglmL Cd 0 8eglmL ~roppingtime 1 s, rwwp rate 2 mV/s,damping 3 s. remdar potentla1 scale
F% (admL)
sol.
~~
~
Examples ol Appllcatlon and Dlscusslon Analysis of Solutions Containing Cadmium and Indium
Peaks of indium and cadmium obtained by conventional D P P are reported in Figure 1A. Figure 1B shows a polarogram obtained with a solution containing both metals. The same solution, recorded with the instrumental conditions suitable for this type of analysis, gives D P P polarograms as shown in Figure 2. The concentration of In and Cd in the solutions can be calculated by the system
C a n d D are the currents measured with thesolution 2,3, and 4 a t the same potentials. The solutions of system 1give the results shown in Table 2. Analysis of Solurions Containing Lead, Thallium, and Indium
As an example of simultaneous analysis with three components, a mixture of lead, thallium, and indium was chosen, whoseE,'s are, respectively, -0.385 V, -0.455 V, and -0.557
v..
Some polarograms are shown in Figure 3. From the analysis of three mixtures the results shown in Tahle 3 were obtained. Quoted examples show that the procedure is rather simple and yields excellent results. Limits and reliability can be compared with those ohtainable by analyzing mixtures with spectrophotometry by measuring n absorhancy values a t n wavelengths for determining n components. Precision of the results can be hiehlv .. . reduced when a component is present at too low a concentrafion in comparison with the others. The limit of determination should be defined every time because, obviously, i t strongly depends on the s h a m of the ~olaromams and on the demee . of overlapping. The choice of the potentials for measuring the currents is important, hut it is not critical. Often the values of the E, of the analytes are suitable, hut, generally, potentials where the currents of the components show the greatest differences are the preferable ones. Volume 67
Number 8 August 1990
705
