Analyst, May 1994, Vol. 119 1099

Simultaneous Determination of Zinc and Nickel With 2-(2-Pyridylmethyleneamino)phenol by First-derivative Spectrophotometry

Zorana GrabariC, Zvjezdana LazareviC and Natalija Koprivanac Laboratory of Polymer Engineering and Organic Chemical Technology, Faculty of Chemical Engineering and Technology, University of Zagreb, 41 000 Zagreb, Croatia

A first-derivative spectrophotometric method using a zero-craming technique of measurement is described for the simultaneous determination of zinc and nickel in mixtures. The procedure is based on the reaction of zinc and nickel with 2-(2-pyridylmethyleneamino)phenol in buffered 10% vlv methanol-water solutions at pH 8. The determination of nickel and zinc in mixtures in the concentration ranges 0.3-3.0 and 1.M.O pg ml-1, respectively, shows good linearity of the calibration graphs. The proposed method was applied to the determination of nickel and zinc in a real bronze sample. The main interferences present in copper alloy were also examined. Keywords: First-derivative spectrophotometry; simultaneous determination; zinc determination; nickel determination; 2- (2-pyridylmethy1eneamino)phenol; bronze

Introduction Derivative spectrophotometry has the advantage of higher selectivity than zero-order spectrophotometry . Fundamental theoretical and practical work by O’Haver and Green1.2 and Fell and co-workers3-7 showed that this technique can lead to the faster and more accurate determination of multiple- component mixtures that previously would have required time-consuming separation techniques. The increased selec- tivity in derivative spectrophotometry results because bands that are overlapped in zero-order absorption spectra can be separated in the derivative mode.

The solution properties, such as extraction in organic solvents, and stability constants of nickel and zinc with 2-(2- pyridylmethy1eneamino)phenol (PMAP) were reported ear- 1ier.g-11 The extraction of Ni-PMAP and Zn-PMAP com- plexes in organic solvents8 indicated that the complexes are readily extracted in chloroform, 1 ,Zdichloroethane and 0- dichlorobenzene, suggesting that uncharged 1 : 2 metal-to- ligand complexes are formed. Stability constants obtained by potentiometric titration have been reported in water9 and in 1 + 1 dioxane-water mixtures.10 As PMAP appeared to be a very sensitive reagent for the determination of nicke1,ll the solution properties of zinc were investigated in detail. The application of first-derivative spectrophotometry to the simul- taneous determination of nickel and zinc using the zero- crossing method1.2 is described. The amounts of nickel and zinc in bronzes were determined with good reproducibility. A single sample can be used and no prior separation, such as ion exchange,12,13 necessary, for example, in zero-order spectro- photometry, is required.

Experimental Apparatus A Varian Model DMS-80 ultraviolet-visible recording spec-

trophotometer and 1 cm cells were used for the zero-order and first-derivative spectrophotometric measurements.

For complexometric titrations a Metrohm Model E-436 potentiograph was used.

The pH of the solutions was measured using a microproces- sor-controlled Iskra Model MA 5740 pH meter with a glass- saturated calomel combination electrode.

Data evaluations for stability constants were performed with an AT 286 personal computer.

Reagents All experiments were performed with analytical-reagent grade chemicals and pure solvents. Redistilled water was used in all experiments.

PMAP was synthesized as reported earlier.9 The purity was checked by C, H, N analysis, melting-point determination and infrared spectrometry.

A zinc(I1) ion stock standard solution was prepared from ZnS04-7H20 (Merck) and standardized by potentiometric titration with ethylenediaminetetraacetic acid (EDTA).

A nickel(I1) ion stock standard solution was prepared from Ni(NO3)*-6H2O (Merck). The exact nickel content was determined by potentiometric titration with EDTA.

For pH adjustment, Titrival standard buffer solutions (Kemika) were used.

Procedure

Suitable aliquots of stock standard solution containing between 5.0 and 30.0 pg of zinc and between 1.5 and 15.0 pg of nickel were mixed in a 5 ml calibrated flask with 0.5 ml of methanol to ensure a final methanol volume fraction of lo%, 1 ml of 10 mmol 1-l PMAP and 2 ml of buffer solution (pH 8). The final PMAP concentration was 2 mmol I-’. The mixture was then diluted to volume with redistilled water. The absorption and first-derivative absorption spectra were recor- ded against a reagent blank at a scan speed of 100 nm min-l , a chart speed of 50 mm min-1, a slit-width of 2 nm and a scan range from 550 to 350 nm. The nickel content was determined from the height (h l ) of the first-derivative signal (dAldh) at the zero-crossing point of zinc (A = 430 nm) and comparison of the value with the corresponding calibration graph. The zinc content was determined by measuring the height (h2) of the first-derivative signal (dAldh) at the zero-crossing point for nickel (A = 448 nm) and comparison of the value with the appropriate calibration graph.

Synthetic samples of copper alloy were prepared by mixing the solutions of the constituents at concentration levels according to the DIN14 composition prescription for this type of alloy.

Sample pre-treatment for determination of nickel and zinc in real samples of bronzes, obtained by courtesy of Strojar

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1100 Analyst, May 1994, Vol. 119

bronze casting company, was performed according to the previously described procedure. 15

Results and Discussion The solution properties of Ni-PMAP complexes using the same experimental conditions as in this work have been reported earlier." In this paper only the solution properties of Zn-PMAP complexes are reported in methanol-water solu- tions at constant temperature (25 k 1 "C) and at constant ionic strength (I = 0.5 mol 1-l), adjusted with KN03. A mixed 10% v/v methanol-water medium was used owing to the low solubility of the PMAP ligand in water.

The results presented in Fig. 1 show that the absorption spectra of the Zn-PMAP complexes are strongly influenced by pH. At pH 4 there is no absorption maximum in the visible region of the spectrum. With increase in the pH of the solution an absorption band at 430 nm appears, indicating that Zn- PMAP complexes are formed, i .e., a bathochromic shift (AA - 80 nm) of the absorption maxima, compared with the free ligand, occurs.11 This band increases up to pH 12.

In Fig. 2 the time stabilities of the formed Zn-PMAP complexes are shown. At pH 6 the absorbance does not change with time within 1 h. At pH 8 a slight increase in the absorbance with time (10% during 1 h) is observed but if the

1.5

1 .o 8 z

0.5

0

pH 12

350 400 450 500 550 Unm

Fig. 1 Absorption spectra of Zn-PMAP complexes at different pH. [PMAP] = 2 mmol l-1; bZn = 2 pg ml-I; c = 25 min for pH = 4,6 and 8; t = 60 min for pH = 10 and 12; methanol-water = 10% v/v

pH 12

T inL P(

1 I I I j 0 20 40 60

lmin

Fig. 2 Time stability of Zn-PMAP complexes at different pH. [PMAP] = 2 mmol l-l; yzn = 2 pg ml-I; methanol-water = 10% v/v

absorbance measurement is made after equal time incre- ments, better sensitivity could be obtained than that obtained at pH 6. At pH 10 and 12 there is a significant change in the measured absorbance of the Zn-PMAP complexes with time and a constant absorbance is achieved after 1 h. Therefore, all quantitative zinc determinations were performed at pH 8, allowing the solution to stand for 25 min prior to the absorbance measurement.

The composition and stability of zinc complexes with PMAP were determined by a computerized version of the mole ratio method,16 which is based on a successive approximation calculation using the general equation valid for the method with the assumption of the existence of a single predominant complex species. The results obtained from the experimental curves in Fig. 3 for different pH values gave the best fit (the smallest standard error) using the assumption of a 1 : 2 metal- to-ligand ratio. This M b stoichiometry is in accordance with a tridentate PMAP ligand (coordination sites: N atom in pyridine, N atom in the azomethine group and 0 atom in the phenolic OH group) and with the results obtained ear-

The logarithmic values of the stability constants of the predominant complex species at different pH values are given in Table 1. From the stability constants obtained in this work, and from some results in the literature obtained by poten- tiometric titration in water9 and dioxane-water (50% v/v),10 it can be concluded that the stability constants are influenced by the solvent used and by the pH of the solution. A similar influence has been reported for Ni-PMAP complexes, which were studied in earlier work.'' From those studies the optimum conditions for the formation of the Ni-PMAP complexes and the procedure for the determination of nickel were established.

lier. 10,11,17

1.5 c

1 .o 9)

2

5: G ' 0.5

0 10 20 [PMAPy(Zn]

Fig. 3 Mole ratio curves for determination of the composition and stability constants of Zn-PMAP predominant complex species at different pH. [Zn] = 0.02 mmoll-l; I = 0.5 mol I-' KNO,; f = 60 min; methanol-water = 10% v/v

Table 1 Composition and stability constants of the predominant Zn- PMAP complex species determined using the mole ratio method at different pH values. Experimental data as in Fig. 3

PH Zn : PMAP mole ratio Log p2 5 standard error 6 1:2 4.93 * 0.02 8 1:2 6.01 5 0.04

10 1 : 2 7.28 k 0.06 12 1:2 8.40 5 0.08

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Analyst, May 1994, Vol. 11 9 1101

Fig. 4 shows the absorption spectra of Ni-PMAP, Zn- PMAP and Ni-Zn-PMAP complexes at pH 8 with broad absorption maxima.

Owing to the broad absorption maxima and the consider- able overlap of the spectra of Ni-PMAP and Zn-PMAP complexes, the determination of both metals was performed by first-derivative zero-crossing spectrophotometry . Fig. 5 shows the first-derivative spectra of the zero-order spectra given in Fig. 4. In this technique, one chooses a suitable wavelength at which dAldh, measured as hl, proportional to the concentration of one metal is measured and where the other metal does not interfere (zero-crossing in first-derivative spectrum of the first metal ion) and vice versa for the other interfering metal, measured as hz.

The absorbance maximum of the zero-order spectrum of the Ni-PMAP complex found as the zero-crossing point in the first-derivative spectrum of the same complex is located at 448 nm, whereas that for the Zn-PMAP complex is located at 430 nm. Reading dAldh for the mixture of both metal ions at 448 nm, interference by nickel is avoided and the zinc concentra- tion can be determined. Reading dAldh for the same mixture at 430 nm, interference by zinc is avoided and the nickel concentration can be determined. The first-derivative spectra were obtained by the electronic derivation circuit in the instrument and recorded using an x-y recorder. A Genitizer Model GT 1212B (Genius) digitizing table was subsequently used to digitize the spectra at every 5 nm and input them into

the computer for further data handling. The heights hl and hz were obtained by extrapolation using polynomial least- squares fitting.

Fig. 6 shows the first-derivative spectra for different Ni concentrations with 0.4 pg ml-l of Zn, and Fig. 7 shows the same for Zn with 0.3 pg ml-1 of Ni. Calibration diagrams for Ni and Zn determinations with different interfering metal ion concentrations are shown in Figs. 8 and 9, respectively. Coefficients of the linear regression equations and corre- sponding standard errors together with correlation coef- ficients are given in Table 2. The linearity of the calibration graphs and the adherence to Beer's law is validated by the high value for the correlation coefficients of the regression equations. The results obtained indicate that a mass concent- ration of zinc up to 1.8 pg ml-l does not interfere with the determination of nickel, and up to 3.0 pg ml-l of nickel does not interfere with the determination of zinc. Using the proposed method, nickel can be determined in the presence of an approximately 6-fold mass concentration excess of zinc. In turn, zinc can be determined in the presence of a 3-fold mass concentration excess of nickel. Calibration diagrams were recorded with six different concentrations and the average values of three independent over-all experiments were taken for statistical analysis.

i 0.025

2.0

1.5

al c 2 1.0

8 9 0.5

Zn+Ni

0 350 400 450 500 550

AJnm

Fig. 4 Absorption spectra of Ni-PMAP, Zn-PMAP and Ni-Zn- PMAP complexes. [PMAP] = 2 mmoll-l; 6 ~ i = 3 pg ml-l; Yzn = 2 pg m1-I; t = 25 min; pH = 8; methanol-water = 10% v/v

Zn+Ni

-0.04 I I I I I 350 400 450 500 550

Unm

Fig. 5 First-derivative spectra of Ni-PMAP, Zn-PMAP and Ni-Zn- PMAP complexes. [PMAP] = 2 mmol 1-I; YN, = 3 pg ml-'; Yzn = 2 pg ml-1; t = 25 min; pH = 8; methanol-water = 10% v/v

-0.025 I 1 I I I 350 400 450 500 550

AJnm

Fig. 6 Ni determination by first-derivative spectrophotometry using the zero-crossing method and PMAP ligand in the presence of interfering Zn. yzn = 0.4 pg ml-l; [PMAP] = 2 mmol I- l ; pH = 8; t = 25 min; methanol-water = 10% v/v. Curve: 1, 0.3; 2, 0.6; 3, 0.9; 4, 1.5; 5,2.1; and 6,3.0 pg ml-l of Ni

-0.04 ' I I I 350 400 450 500 550

Wnm

Fig. 7 Zn determination by first-derivative spectrophotometry using the zero-crossing method and PMAP ligand in the presence of interfering Ni. yNi = 0.3 pg ml-I; [PMAP] = 2 mmoll-l; pH = 8; t = 25 min; methanol-water = 10% v/v. Curve: 1, 1.0; 2. 2.0; 3, 3.0; 4, 4.0; 5, 5.0; and 6, 6.0 pg ml-l of Zn

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1102 Analyst, May 1994, Vol. 119

The proposed method was applied to the determination of nickel and zinc in copper alloys, and therefore the most common cationic interferences present in these alloys were investigated in the concentration range relevant for this type of matrix. Some anionic interferences as constituents in solutions for sample treatment and preparation were also investigated in the determination of Ni and Zn with PMAP; Fell1, A P , ZrlV and Be" do not interfere, whereas Mn", Coil

0.010

.- E 5 0.005

Y 3

0

Fig. 8 Calibration diagrams for Ni determination using different interfering Zn concentrations. yzn = 0 , O ; 0,0.4; A , 0.6; and 0, 1.8 pg ml-I. Experimental conditions are the same as in Fig. 5

0.02

Fig. 9 Calibration diagrams for Zn determination using different interfering Ni concentrations. yNi = 0 , O ; Q0.3; and A, 3.0 pg m1-I. Experimental conditions are the same as in Fig. 6

Table 2 Regression lines dA/dh = A + B y for the determination of nickel (0.3-3.0 pg ml-I) and zinc (1.0-6.0 pg ml-I) in mixtures by first-derivative spectrophotometry. Experimental data as in Figs. 8 and 9, respectively. Parameters: A = intercept on ordinate; B = slope; r = correlation coefficient; y = mass concentration (pg ml-I)

yzn/pg ml- A f standard error B f standard error r Determination of Ni with Zn as interferent- - (-1.5 f 2.8) X (2.51 f 0.01) X 0.9999 0.4 (-1.6 f 0.8) x (2.53 ? 0.03) X 0.9997

1.8 (-1.4 f 0.9) x 10-4 (2.58 f 0.03) X 0.9997 0.6 (-1.1 1.1) x 10-4 (2.57 f 0.04) x 10-3 0.9998

yNi/pg ml-l A f standard error B f standard error r Determination of Zn with Ni as interferent- - (6.8 2 1.3) X 10-4 (2.84 k 0.02) X 0.9996 0.3 (1.1 f 0.3) x 10-3 (2.76 k 0.05) X 0.9991 3.0 (7.0 k 1.5) x 10-4 (2.84 f 0.03) X 0.9996

Table 3 Simultaneous determination of zinc and nickel in synthetic copper alloy samples. y = Mass concentration (pg ml-l)

Added Found*

yzn/pg ml-1 YNi/pg m1-I yzn/pg ml-l yNi/pg ml-I 2.25 1 S O 2.23 _+ 0.03 1.52 f 0.04 3.00 1.20 3.01 f 0.06 1.19 f 0.02 3.00 3.00 2.99 f 0.04 3.06 f 0.05

determinations. * Average values f standard deviations of five independent

and CrlI1 interfere in the concentration ranges present in most of the copper alloys. Chloride, sulfate, nitrate and citrate anions do not interfere.

To investigate the validity of the proposed method for the simultaneous determination of nickel and zinc in copper alloys, a synthetic sample of a mixture of these two metal ions was prepared with known Ni and Zn additions and the mass concentrations of the added Ni and Zn ions were determined as the average of five independent analyses (sample prepara- tion and measurement). The results obtained are given in Table 3. Good reliability of the determination was proved.

The proposed method was tested on a real sample of tin bronze (CuSn7Pb).*4 The mass fractions obtained were Ni = 1.89 k 0.04% m/m and Zn = 3.53 k 0.06% d m , compared with Ni = 1.91 k 0.02% m/m and Zn = 3.50 -+ 0.04% m/m by atomic absorption spectrometry (mean k standard deviation of five independent determinations in each instance). The good agreement clearly demonstrates the utility of the proposed method for the simultaneous determination of zinc and nickel. The precision of the method is acceptable and the method has the advantages of speed, simplicity and good sensitivity.

The authors gratefully acknowledge the financial support of the Croatian Ministry of Science for Project No. 2-13-047.

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References O'Haver, T. C., and Green, H. L., Anal. Chem., 1976,48,312. O'Haver, T. C., Anal. Proc., 1982,25, 1548. Fell, A. F., Proc. Anal. Div. Chem. SOC., 1978, 15,260. Fell, A. F., J . Pharm. Pharmacol., 1979,31,23P. Fell, A. F., Anal. Proc., 1980, 17, 512. Fell, A. F., and Smith, G., Anal. Proc., 1982, 19, 28. Fell, A. F., TrAC, Trends Anal. Chem., 1983,2,63. Capitan, F., Salinas, F., and Capitan-Vallvey, L. F., Talanta, 1978,25, 59. Otomo, M., and Kodama, K., Bull. Chem. SOC. Jpn., 1973,46, 2421. Geary, W. J., Nickless, G., and Pollard, F. H., Anal. Chim. Acta, 1962,27,71. GrabariC, Z., Grabarid, B. S., Koprivanac, N., and ESkinja, I., Chem. Pap., 1993,47,282. Ogrizek, M., BokiC, Lj., GrabariC, Z., and ESkinja, I., Kem. Ind., 1983,32, 163. Victor, A. H., Anal. Chim. Acta, 1986, 183, 155. DIN Taschenbuch 53, Metallische Gusswerkstoffe, Beuth, Berlin, 1974, DIN 1705. ESkinja, I., Grabarid, Z., and GrabariC, B. S., Mikrochim. Acta, 1985, 11, 443. Grabarid, Z., ESkinja, I., Koprivanac, N., and MeSinoviC, A., Microchem. J . , 1992,46,360. Geary, W. J., Nickless, G., and Pollard, F. H., Anal. Chim. Acta, 1962,26, 575.

Paper 3/01 765 F Received March 29, 1993

Accepted September 7, 1993

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