380
Anal. Chem. 1985, 5 7 , 380-383
Simultaneous Determination of Zinc and Chromate in Cooling Water by Differential Pulse Polarography Vinod K. Jindal,* Mohammed A. Khan, Ravi M. Bhatnagar, and Satyendra Varma Chemical Research Wing, Projects & Development India Ltd., Sindri, Dhanbad, India-828122 Several chemical formulations are being added to cooling water to control the corrosion, fouling, and scale formation. The most common and popular are zinc-polyphosphate, chromate-zinc-polyphosphate, polyphosphate-zincphosphonate, and polyphosphate-chromatezinc-phosphonate systems (1, 2). Very often stringent environmental and pollution control regulations over plant effluents as well as the economic consideration in such treatments restrict the use of these corrosion inhibitors due to their toxicity and other undesirable effects on the surroundings. This necessitates the optimization and frequent quick monitoring of these inhibitors in cooling water. Generally chromate is estimated volumetrically by potassium dichromate (3),by spectrophotometry ( 4 ) ,and by DC polarography (5). And zinc is estimated by spectrophotometry as zinc dithizonate complex (6) extracted in carbon tetrachloride by solvent extraction. This method for zinc estimation is time consuming and often involves a series of steps. Under these circumstances, there has been a need for some alternative rapid and more reliable method. Hence a more sensitive and versatile method by using differential pulse polarography (DPP) is being reported for the simultaneous determination of zinc and chromate in cooling water where zinc ( 5 ppm), chromate (20 ppm), and polyphosphate (50 ppm) formulation is used as a corrosion inhibitor. This will help in effective control of cooling tower performance. Differential pulse polarography have some advantages over classical DC polarography. The DC polarography technique produces stepwise waves with oscillations, whereas D P P provides an output in the form of peaks and also greatly enhances resolution and sensitivity. A comparison of DP and DC polarographies is made in Figure 1.
EXPERIMENTAL SECTION Instrumentation. Differential pulse polarographic measurements were performed with a Princeton Applied Research Corp., Model 174A, polarographic analyzer equipped with an automated electroanalysis controller, Model 315, and a static mercury drop electrode (SMDE), Model 303. All the currentpotential data were displayed with Houston Instruments omnigraphic X-Y recorder, Model RE 0074. For the spectrophotometric measurements, Beckman spectrophotometer Model DU-2 with 10-mm path length rectangular quartz cells were used. All the pH measurements were made with a Beckman expanded pH meter Model 76. Reagents. All the chemical used were of AnalaR grade, and double-distilled water was used for the preparation of all the solutions and dilutions. The chemical compositions of several process and cooling water samples used in the Sindri modernization project are given in Table I. Procedure. All the differential pulse polarograms were recorded at pH 10.3 in 1M NH3-0.1 M NH&1 supporting electrolyte with 0.005% gelatine as the maxima suppressor. Appropriate quantities of all the chemicals were taken into 100-mL volumetric flasks along with NH3-NH4C1 and gelatine solutions, the final pH adjusted to 10.3 with NH,OH/HCl as and when required, and finally diluted to 100 mL with double-distilled water. About 10 mL of the solution was then taken into the sample cup of the SMDE. Pure nitrogen gas was bubbled through the solutions for 5 min to remove dissolved oxygen prior to recording of the DP polarograms. DP polarograms were recorded at a scan rate of 2 mV s-l and modulation amplitude of 25 mV, with a small-size mercury drop of drop time 1 s. The average drop weight and
Table I
sample no.a
A, B, C, D,
ammonical nitrogen pH ppm
total NO; NO;, PPm
C1-, SO-:, PPm PPm
calcium, PPm
E,
6.6 6.7 6.5 6.5 6.6
75 75 82 74 71
52 50 50 55 50
26 17 17 16 16
40 40 39 40 40
45 50 35 40 40
A, B, C, D, E,
6.5 6.5 6.7 6.5 6.4
79 79 72 84 72
48 50 55 51 48
22 25 26 27 28
39 38 40 42 40
40 35 38 36 40
'A,-E, are the process water samples; A,-E, are the cooling water samples.
surface area of the small mercury drop was 1.2 mg and 9.6 x IO-, cm2,respectively (7). All the polarograms were recorded at 25 h 0.1 "C, and an aqueous saturated Ag/AgCl electrode was used as the reference electrode.
RESULTS AND DISCUSSION The chemical compositions of all experimental sets were made to be almost analogous to that of the cooling water composition by adding polyphosphate and calcium ions also along with supporting electrolyte and gelatine. In 1 M NH,-O.l M NH&l supporting electrolyte and at pH 10.3, the polarogram of chromate consists of three peaks at -0.45, -1.57, and -1.73 V vs. saturated Ag/AgCl reference electrode. These three peaks correspond to the reduction of chromate to Cr3+, Cr2+,and metallic states, respectively (8). The reduction of Zn2+to metallic zinc was a t -1.36 V (Figure 1). Well-defined peaks/waves were obtained in DP and DC polarograms having similar peak and half-wave potentials. Effect of Ca2+Ions and Polyphosphate. Several sets were prepared with 20 ppm chromate, 5 ppm Zn2+,and Ca2+ ion concentration in the range of 0-40 ppm, and D P polarograms were recorded. It was observed that increasing Ca2+ ion concentration greatly decreased the peak current (i,) due to the reduction of Zn2+ions (curve A, Figure 2). But when Ca2+ions were masked by complexing with polyphosphate, this change in i, values was practically negligible above 20 ppm polyphosphate concentration (curve B, Figure 2). It was also observed that in presence of 50 ppm polyphosphate, i, (Zn) is practically constant for Ca2+ion concentration above 20 ppm (curve C, Figure 2). This indicates that some interaction is taking place between zinc and calcium ions, resulting in some electroinactive species. When this electroinactive species is formed, i, will be lower than what was expected in the absence of Ca2+ions (curve A). In strongly alkaline medium, the Zn2+ ion is reported to exist as complex oxyanion zincate (9),Zn(OH),-, and this zincate ion reacts with Ca2+ ion to form electrolytically inactive calcium zincate (IO),Ca[Zn(OH)J2, in the solution. Hence, during all the calibrations and studies, a minimum of 50 pprn polyphosphate was always taken along with the supporting electrolyte, to avoid any interference due to the presence of Ca2+ ions. Calibration for Chromate Ions. A calibration curve for the estimation of chromate ions was prepared by recording several DP polarograms in the chromate ion concentration
0 1984 Amerlcan Chemical Society 0003-2700/85/0357-0380$01.50/0
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
381
Table 11. Comparison of Zinc Estimation by DPP and Spectrophotometry
I
bIV
diff pulse polarography relative error, %
Zn2+taken, ppm
Zn2+found, ppm
1.0 1.0 1.0 1.0 1.0
1.025 1.000 1.000 1.025 1.025
2.5 0.0 0.0 2.5 2.5 +1.5 av
spectrophotometry relative error, %
Zn2+found, ppm 1.20 1.16 1.22 1.08 1.13
+20.0 +16.0 +22.0 +8.0 +13.0
+15.8 av
Cr[PI' - c r [ m '
-bC
-1.0
E w.
ShT
-1'4
-)BV
Aa/h#U
Figure 1. Comparison of DC and DP polarograms of soiutlon containing 3 ppm zinc, 16 ppm chromate, 20 ppm calcium, 50 ppm polyand 0.005% gelatine and at pH 10.3 phosphate, 1 M NH,-O.l M "&I, and scan rate of 5 mV s-'. Figure 3. Cailbration curves for chromate; i , vs. [Cr04'-]
20
40
60
80
DDm
Ca*2/ P o l y p h o s p h a t c
Figure 2. Plots of lNzn1 against (A) [Ca2+]in the absence of polyphosphate, (B) polyphosphate in the presence of 20 ppm Ca2+,and (C) [Ca2+]in the presence of 50 ppm polyphosphate. range of 0-20 ppm. Peak currents (i,), corresponding to all the three waves of chromate, were plotted against chromate concentration (Figure 3). The peak potentials (E,), corresponding to all the three peaks of chromate, were constant for all concentrations. It was observed that for CrnCrm and C r W P waves, the i, vs. concentration plots were throughout linear and passed through the origin. But in the case of the third chromate wave, C r C r o ,the plot though passing through the origin is linear only up to 12 ppm chromate ion concen-
tration. Above 12 ppm chromate ion concentration, the values are lower than expected. It was, therefore, concluded that only first and second chromate ion waves could be used for the estimation of chromate ions. During present studies only the first wave of chromate ion was used for the estimation of chromate ion concentrations. To verify the accuracy of chromate determination by DPP, the values obtained for five replications were also compared with those obtained by spectrophotometry, a t A,, 370 nm, and the percent relative error was calculated. It was observed, the accuracy of D P P (relative error f1.6%) and the spectrophotometric method (relative error f0.5%), to be of almost the same order. Calibration for Zinc Ion. A calibration curve for the estimation of zinc ions, in the range of 0-5 ppm, was prepared in the presence of 20 ppm chromate ions by recording D P polarograms. A plot of i, vs. [Znz+]was linear, passing through the origin. To verify the reproducibility of DPP, values of zinc ion concentrations of five replications were compared with those obtained by spectrophotometry (61,at A, 530 nm (Table 11),and relative error was calculated for both of the methods. The accuracy by DPP, relative error + 1 . 5 % , was very good compared with that of spectrophotometry, relative error +15.8%. These too large values, obtained by spectrophotometry, may be due to the presence of some interfering ions, which are also extracted as dithizonate along with zinc. It has been further observed that the zinc peak current (i,) was affected by the change in chromate ion concentration, as is indicated from the data in column 3 under ip(xoin Table 111. This perhaps is due to the formation of some complex species involving Zn2+and Cr042-ions and some which could not be identified. There are two alternatives to account for this change in the values of peak current due to zinc ions (i,) by the change in the chromate ion concentration, [Cr04*-].
382
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
Table I11
[CrOd2-]
Zn2+
taken, P P ~
2 2
cor
[Zn2+] found,
(x'),
iP(X0,
ipw,
ppm
PA
PA
PPm
av re1 error, %
0
1.400 1.300 1.175 1.050 0.900
1.096 1.114 1.058 0.994 0.900
2.30 2.30 2.25 2.00 1.95 2.16 av
+8.0
2.350 2.250 2.075 1.975 1.700 1.400
1.840 1.842 1.779 1.778 1.610
3.90 3.90 3.75 3.75 3.40
8
2
12
2 2
16 20
3 3 3 3 3 3
0
4 8 12
16 20
1.400
3.00
3.45 av 4 4 4 4 4 4
3.050 2.850 2.700 2.625 2.325 1.950
0
4 8 12 16 20
5 5 5 5 5 5
3.500 3.350 3.150 2.800 2.600 2.300
0
4 8 12
16 20
2.387 2.332 2.315 2.368 2.202 1.950 2.778 2.742 2.700 2.521 2.462 2.300
5.05 4.95 4.90 5.06 4.70 4.10 4.79 av 5.85 5.80 5.70 5.35 5.20 4.90 5.47 av
+15.0
4
+7.8
or
where F = f j [Zn2+] Since ip(r)is the zinc peak height in the presence of x ppm chromate ions, it has been replaced by the term for zinc ion concentration, Le., [Zn2+],in eq 2. The plots of ip(Zn) vs. [Cr042-]are linear (Figure 4 ) and hence prove the validity of eq 1. The slopes of these straight lines, i.e., f , corresponding to different zinc ion concentrations, are as follows: 2
3
4
16
Figure 4. Plots of I p ( x , ) vs. [CrO,*-] In the presence of zinc ion: (A) 2 ppm, (B) 3 ppm, (C) 4 ppm, and (D) 5 ppm.
Equation 1 can also be rewritten as
5
37.5 48.0 52.5 60.0 These values off are different and are directly proportional to zinc ion concentrations and are not constant. But, the ratio 103f
12
+19.8
(a) In one set, maintaining all the experimental conditions, the DP polarogram was recorded and the first Cr042-ion concentration calculated with the chromate calibration curve (Figure 3), and in the second set [Cr042-]is adjusted to 20 ppm by either addition or dilution and in the presence of 20 ppm [CrO:-] and v a i n the DP polarogram was recorded and zinc ion concentration was calculated from the zinc calibration curve made in the presence of 20 ppm chromate ions. (b) The second alternative is to establish a relation between observed zinc peak height and the chromate ion concentration and then apply the necessary correction to calculate actual peak current due to zinc ions in the presence of 20 ppm chromate ions. Suppose ip(x)and,,,i are the zinc peak current in the presence of x and x'ppm of chromate ions, then Ai, = ip(x)- ip(x,)a (x'-x) = f(x'- x) (1)
[Zn2+I,PPm
8
of this f and [Zn2+],Le., F (eq 2'), is constant and can be obtained as the slope of the linear plot o f f vs. [Zn2+]. The value of F was found to be 13.88 X and is independent of zinc ion concentration. Since, under the experimental conditions of the present studies, the calibration curve for zinc ions was made in the presence of 20 ppm chromate ions, eq 2' was used for applying the necessary correction and the value of x corresponds to 20 ppm. All the results are presented in Table 111. It was found that after necessary correction, the relative average error is in the range of 8-20%. Interferences. In addition to calcium ions, as previously discussed, other ions that are likely to be present and interfere with the determination of zinc and chromate ions by DPP are sulfate, chloride, nitrite, and nitrate, and these occur in significant amounts in the cooling and process water. Sulfate and chloride ions do not effect the analytical determination of zinc and chromate ions by DPP. However, the presence of nitrite and nitrate have direct interference in the analysis, because both are reduced a t the same potential, i.e., -1.53 V vs. Ag/AgCl, like the Cr"' ions and resulting in an apparent increase in the Cr"' peak. Also, the resolution between Zn" and CrlI1 waves becomes poor, and this makes the determination of zinc ions difficult and inaccurate. Hence, in process and cooling water before proceeding for the determination of zinc and chromate ions, it becomes essential to reduce nitrite/nitrate ions. Nitrite/nitrate ions were reduced during the present studies by addition of 50 ppm sulfamic acid along with 2-3 mL of HC1 acid as follows:
NH2S03H+ NOz- + H+ NH2S03H+ NO3- + Hf
-
-+
N2 + H20 + HzSO4 (3)
N 2 0 + H20 + H2S04
(4)
Applications. This DPP method has been applied for the simultaneous determination of zinc and chromate ions in process and cooling water samples from fertilizer plants in India. For samples of process water, a 25-mL aliquot was treated with 50 ppm sulfamic acid and 2-3 mL of hydrochloric acid and allowed to stand for about 30 min for the reaction to complete. It was observed that process water normally does not contain any zincjchromate ions. Zincjchromate ions in significant amounts compared to those of cooling water,
383
Anal. Chem. 1985, 57, 383-385 Table
IV Zn'")-Zn@) wave
sample no.
ppm
Zn2+ found, PPm
AP
5 5 5 5 5
5.15 4.90 5.10 4.90 4.80
Zn2+
taken,
BP CP
DP
EP Table
CrOA2taken,
wave
CrOd2found,
PPm
PPm
20 20 20 20 20
19.90 20.25 20.25 20.00 19.50
V
sample no. (cooling
cooling water (cor) chromate,
water)
zinc: ppm
PPm
A, Bc
0.80 1.00
11.00 10.48
0.80
8.00
2.30 2.25
7.52 11.52
C C
Dc
EC a
Cr'v"-Cr'""
[Zn2+]values are corrected by method (b), eq 2'.
therefore, were added (chromate, 20 ppm; zinc, 5 ppm) along with 50 ppm polyphosphate, 1 M NH3-0.1 M NH4Cl, and 0.005% gelatine, and finally the p H and total volume were adjusted to 10.3 and 100 mL, respectively. For the samples of cooling water, the same procedure as that of process water was followed except for the addition of zinc/chromate ions along with the supporting electrolyte. These solutions were then degassed for 5 min with pure nitrogen gas before D P polarograms were recorded by the procedure already discussed. The analytical results are summarized in Tables IV and V. Since 25-mL aliquots of cooling water samples were finally diluted to 100 mL, before the recording of polarograms, the
actual concentration of zinc/chromate will be 4 times the experimental values and the same are recorded in Table V.
CONCLUSIONS The present method is based on the reduction of CrV1CrrI1-Crrr-Cr0 and Znrl-Zno on SMDE in 1 M NH3-0.1 M NH4Cl and 0.005% gelatine supporting electrolyte. Presence of some ions, viz., calcium and nitrate/nitrite, effects the peaks of zinc and Cr"'-Cr", respectively. I t is, therefore, essential to complex calcium ions by adding polyphosphate and to destroy N02-/NOf by adding sulfamic acid along with hydrochloride acid before the actual recording of DP polarograms. The present DP polarographic method for the simultaneous determination of zinc and chromate is comparable in its utility and applicability with spectrophotometric methods. The method has a better accuracy and higher sensitivity and is quick, as both of the ions can be determined in a single scan. Registry No. CrO:-, 13907-45-4;Zn, 7440-66-6; Ca, 7440-70-2; sulfamic acid, 5329-14-6;water, 7732-18-5. LITERATURE CITED (1) Krisher, A. S. Mater. Perform. 1982, 21, 9. (2) Verma, K. M.; Gupta, M. P.; Sinha, B. B.: Rai, J. S.; Oswal, D. R. Chem. Age India 1980, 31, 1137. (3) Vogel, A. I . "Textbook of Quantltatlve Inorganic Chemistry"; Longmans Green and Co.: New York, 1978; p 361. (4) Reference 3, p 738. (5) Bhatnagar, R. M.; Roy, A. K. Technology (Sindri, India) 1966, 3 (3), 131. (6) Sandell, E. B. "Colorimetric Determination of Traces of Metals"; Interscience: New York, 1959; p 949. (7) Peterson, W. M. Int. Lab. 1980, 51. (8) Lingane, J. J.; Kolthoff, I. M. J . Am. Chem. SOC. 1940, 62, 852. (9) Glinka, N. "General Chemistry"; Foreign Language Publlshing House: Moscow, 1960; pp 579-81. (10) Pepenar, Ioan; Pepenar, Elena Rev. Chim. (Bucharest) 1978, 29, 445.
RECEIVED for review August 12, 1983. Resubmitted and accepted July 12, 1984.
Determination of Anionic Impurities In Selected Concentrated Electrolytes by Ion Chromatography James A. Cox* and Nobuyuki Tanaka
Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901 Ion chromatography is recognized as a convenient method for the quantitative determination of mixtures of anions in aqueous samples; however, it cannot be directly applied to matrices composed of concentrated electrolytes. The problems, overloading the relatively low capacity analytical column and increasing the background of the conductivity detector, cannot be overcome by using an anion-exchange concentrator column because of unfavorable partitioning of analyte ions from a concentrated electrolyte matrix. When the matrix can be converted to a nonelectrolyte or to a volatile species by (anion) cation-exchange, the sample can be modified in a manner that will permit quantification of trace levels of anions (cations) by ion chromatography. For example, anions in a 50% NaOH solution can be determined by ion chromatography after the solution is passed through a cation-exchange column in the proton form (1). The direct use of ion exchange has certain limitations for this purpose. If small sample volumes are used. the uDtake or release of solvent by the resin will cause significant error. Adsorption losses of the analyte can also occur. However, with the de-
scribed method the cation-exchange resin is isolated from the sample by a cation-exchange membrane. Therefore, adsorption losses are minimized because of the large sample volume to ion-exchange surface area ratio relative to column or batch methods with resins. The apparatus can accommodate sample volumes of less than 1 mL without introducing error. The described reactions can also be effected by Donnan dialysis; here, sample electrolyte is separated from a solution that contains the reactant ions by the appropriate ion-exchange membrane. When the objective is to determine trace level impurities, this approach is limited by the imperfect exclusion of co-ions by ion-exchange membranes. Hence, the conjugate ion of the reactant will also enter the sample. This source of interference is eliminated by containing the reactant on an ion-exchange resin slurry. The described combination of dual ion-exchange matrix modification with ion chromatography is attractive relative to present methods for determining trace level impurities in concentrated electrolytes because several ions can be quantified simultaneously. For example, sulfate in K,C03 and
0003-2700/85/0357-0383$01.50/00 1984 American Chemical Society
