Polarography of the alkaline-earth metals—II

Talanta. Vol. 32, No 6, pp. 479-482, 1985 Printed in Great Britam. All rights reserved

POLAROGRAPHY THE ADSORPTION

CopyrIght

OF THE ALKALINE-EARTH

0039-9140/85 $3.00 + 0.00 Q 1985 Pergamon Press Ltd

METALS-II

WAVE FOR THE MAGNESIUM-ERIOCHROME BLACK T COMPLEX

AN JINGRU, ZHOU JINKUI and WEN XIAODAN Chemistry Department, Fuzhou University, Fuzhou, Fujian, China (Receiued

11 September

1984. Revised

I December

1984. Accepted

11 January

1985)

Summary-Magnesium-Eriochrome Black Tin ethylenediamine medium gives a polarographic adsorptive wave at -0.7 V (vs. an Ag/Hg electrode). It gives a limit of detection for magnesium of 2 x 10-8M. It is made the basis of a method for determmation of magnesium in serum, water and various salts, wrthout any pretreatment.

The method

is very rapid,

sensitive

Magnesium ions are very difficult to reduce at the dropping mercury electrode in aqueous solution, but give a polarographic wave at -2.2 V (us. SCE) in tetramethylammonium chloride supporting electrolyte solution.’ Richardson2 determined magnesium in calcium carbonate by polarographically reducing the magnesium complex of Solochrome Violet R. S. at pH 11 in a piperidine buffer. In his method, calcium and iron were removed by precipitation and extraction. The detection limit for magnesium was lo-“M. Luo and Zhao’ determined magnesium in hair by using the polarographic adsorption wave of the magnesium meso-tetra (4-trimethylammoniumphenyl)porphine complex in a methylamine buffer solution. In their method, the solution had to be heated at 100” for 50 min to form the complex. The detection limit was 10-‘&f. In this report, we describe an adsorption wave for the magnesium-Eriochrome Black T complex in aqueous ethylenediamine medium. It can be used to determine magnesium in serum, water and chemical reagents, without any pretreatment. Magnesium in serum is usually determined by flame photometry, atomic-absorption spectroscopy or spectrophotometry after appropriate sample pretreatment. Because of its sensitivity and good selectivity, our method can be used to determine magnesium in serum rapidly and conveniently.

and convenient

for sermn

analysrs.

Eriochrome Black T (EBT) solutton, 5.0 x 10-4M. Prepared in water as solvent. The solution was stable for two days. Ethvlenedramine, 10% aqueous solutron. Demineralized water was used in all experiments. All solutions were stored in polyethylene bottles, and contact between the solutions and glass vessels was avorded as much as possible. Procedure Transfer the magnesium solution into a 25-ml quartz standard flask, add 5ml of 10% ethylenediamine solution and 3 ml of 5 x 10m4M EBT and dilute to the mark with water. Pour all the solution into the PTFE polarographic cell and record the dertvative polarogram, starting the potential scan at -0.4 V. The temperature should be kept below 2CY’.

RESULTS Oscillogruphic

polarograms

In 2% ethylenediamine solution, EBT gave a reduction wave P,, with a maximum at -0.53 V, and a derivative-peak potential of -0.50 V (see Fig. la,b). When magnesium ions were added, P, decreased and a new maximum P? appeared at a more negative potential, -0.72 V, and the derivative-peak potential was -0.70 V. The derivative mode is preferred because

of its better

resolution.

Dependence of peak current on the ethvlenediamine concentration The magnesium-EBT complex gives no wave in either acidic or ammoniacal medium, but does in the presence of ethylenediamine. The dependence of peak current on the concentration is shown in Fig. 2. The peak current reaches a maximum at 1% ethylenediamine and then remains constant, so 2% was chosen as the working concentration.

EXPERIMENTAL Apparatus A single-sweep polarograph with three electrodes and a type 82-l polarograph (made in Fuzhou University), with an Ag/Hg reference electrode. A PTFE polarographic cell was used. Reagents Standard magnesium solution. Magnesium oxide was tgnited at 800” and 1.658 g of it was dissolved in 30 ml of 2M hydrochloric acid and the solutton was diluted to 1000 ml. This 1000-pgg/ml stock solution was further drluted as required.

Dependence of peak current on the EBT concentraion Magnesium does not give any reduction wave in ethylenediamine solution in the absence of EBT, but the Mg-EBT complex gives a polarographic wave at 479

AN JINGRU et al.

480

Fig. 1. Bngle-sweep polarograms: (a) 2% en + 6 x 1W’M EBT, normal wave; (b) derivative wave; fc) a + 8 x 10m61MMg*+, normal wave; (d) derivative wave.

-0.7OV. As sItawn in Fig. 3 the peak current remains maximal and constant for the EBT range

1.5 x IO-5-S x IOm51trf:5 x 10-5M EBT was chosen as the working ~on&entration.

Figure 4 shows that the peak current was independent of temperature between 4” and 20”, so it is only necessary to keep the temperature below 20”.

Under the optimum conditions chosen, the peak current is proportional to the magnesium concen-

EBT

x ?0-5iM)

Fig. 3. Dependence of peakcurrent on the concentration of EBT: 2% ethylenediamme + 5 x 10A6M Mg*+.

7.5

2

_ ._.-.

6.0

. /*

I

a.45 N

‘9 * 5

--.m”w.-.-

2

3.0

*

i

*5-// * I o”*.O

0.8

I

I

16

2.4 0

Ethylenedlamine

$0

20

40

30

(%)

Fig. 2. Dependence of peak current on the concentration of e~ylenediamine: 5 x IO-&M Mg*+ -+ 6 x IOm5itf EBT.

Tt*c

1

Fig 4. EfTeectof temperature on the peak current.

Polarography of Mg-EBT complex 43

\ :

Relation between peak current and rest-time

r

37

l-*,

; 35

.'

"/ I./*

330-

1.0 -E

(VI

Fig. 5. Electrocapillary curves: (A) 2% ethylenediamine, (B) A+ 6 x IO-‘M EBT, (C) A + 1.8 x 10-4M EBT, (D) A + 1.5 x 10-6M Mg2+, (E) B + 1.5 x 10-6M Mg2+.

tration in the range 5 x 10-8-9 x 10m6M, and the limit of detection is 2 x 10m8M. Effect of foreign substances The effect of foreign substances was studied with 2 pg of magnesium in 25 ml of solution. The amounts of other ions which did not interfere (error < 10%) are as follows: large amounts of K, Na, 5 mg of Ca,

Ba, Sr; 2 mg of Ge; 1 mg of Bi(II1); 0.6 mg of W(VI), V(V); 0.4 mg of Mo(VI), Ag, Al; 0.3 mg of Zn; 0.2 mg of Be, Tl(III), As(II1); 0.1 mg of Sn(II), SC; 60 p g of In; 40 pg of Sb, Gd, Cr(V1); 30 pg of Pb, Cu(I1); 20 pg of Ga, Ti(IV); 6 fig of Mn(I1); 4 pg of Ni, Co(I1); 1 pg of Fe(II1). Fe(II1) interfered by giving a peak at -0.67 V, but the interference of 200 pg could be eliminated by adding sodium sulphite (1 g per 25 ml). Sodium cyanide present at 0.2% concentration masked up to 80 pg of Fe(III), 100 pg of Co(I1) or Ni, and 20 pg of Mn(I1). Small amounts of surfactants did not interfere, e.g., 1.O ml of 0.02% polyoxyethylene alkylphenol, 0.5 ml of 0.02% cetylpyridinium chloride, or 2.0 ml of 0.02% sodium dodecyl benzenesulphonate. MECHANISM

Electrocapillary curves

Eriochrome Black T is a hydroxyazo dyestuff, and would be expected to be easily adsorbed at the mercury electrode. As shown in Fig. 5, the electrocapillary curves were considerably influenced by the presence of EBT, which was adsorbed very strongly at -0.50 V, resulting in the surface tension being reduced and the adsorption becoming stronger with increasing EBT concentration. When magnesium ions were added they reacted with the EBT to form a red complex which was adsorbed at the electrode to give a reduction peak at -0.7 V. Curve A was the same as D, B the same as E, so we conclude that the adsorption depended mainly on the ligand EBT in the complex, and that the magnesium itself had little effect on the adsorption.

481

The dependence of the maximum peak current on the rest-time was investigated with a hanging mercury drop electrode. For aqueous ethylenediamine solutions the peak currents for both reagents and complex increased with prolongation of the rest-time, which allowed much more reagent and complex to be adsorbed at the electrode. The conclusion from this experiment agreed with that from the electrocapillary curves. Relationship between peak current and voltage scanrate

The peak current iPfor the reagent and the complex varied with scan-rate I’. The plot of ip vs. I”/’ is shown in Fig. 6 for scan-rates from 125 to 350 mV/sec: the plot is not linear, as it should be for a diffusion-controlled process, but is distinctly curved. Such deviations from the Randles-Se&k equation have been explained by Anson. It is evident from the experiments described that both reagent and complex are strongly adsorbed on surface of the electrode. This has the effect of giving a significant increase in sensitivity. CycIic voltammetry

The cyclic voltammetry of the system was investigated with the Type 82-l polarograph. Cyclic voltammetric curves for aqueous ethylenediamine solutions are shown in Fig. 7. EBT gives a cathodic peak at -0.5 V, owing to its reduction. If magnesium ions are present, a second peak appears at -0.7 V, due to reduction of the ligand in the complex. No anodic peak was observed, from which we conclude that the reduction of EBT and of the Mg-EBT complex is irreversible. Composition of the electroactive complex

The composition of the complex was examined by the linear’ and continuous-variations methods and found to be 1: 1, as expected. The conditional stability constant (2% ethylenediamine medium, pH = 11.8) was found to be 1.1 x 10’.

Fig. 6. Relationship between iP and V: (I) 2% ethylenediamine + 6 x 10-5M EBT, (II) I + 7.5 x 10-5M Ma’+.

482

AN JINGRU et al

-E(V

-E(V)

I

Fig. 7. Cychc voltammetric curves. Table 1. Determination of Mg in water and serum

APPLICATIONS

Determination

of magnesium

in water ad

serum

Ma found bv thus method

Transfer 2.5 ml of water into a 25-ml standard flask, then add 5 ml of 10% ethylenediamine and 3 ml of 5 x 10e4M EBT and dilute to the mark with water. Measure the peak current at -0.7 V. Transfer 0.1 ml of serum, without any pretreatment.

into a 25-ml standard

flask, and continue

Sample Water Serum A Serum B Serum C

PPm

Pg 2.0 2.26 1.83 2.24

0.8 22.6 18.3 22.4

Mg found by AAS a

2.0 2.21 1.72 2.18

PPm

0.8 22.1 17.2 21.8

Table 2. Analysis of various salts for Ma Sample Potassium nitrate, 200 mg Potassmm chloride, 200 mg Barium nitrate, 0.8 mg Stronnmm mtrate, 0.6 mg *Calcmm chloride, 0.3 mg

Mg added, Mg found, ng K? 0 0.2 0 0.2 0 0.3 0 0.2 0 I.0

0.360 0.564 0.408 0.600 0.204 0.498 0.276 0.474 1.25 2.25

Recovery, %

Content, %

102

0.00018

96

0.000204

98

0.025

99

0.046

100

0.42

*For this sample, a chemically pure grade, the standard deviation (11 determinations) was 0.01 I pg, coefficient of variation 0.9%. The other samples were analytrcal-reagent grade. All the results are the means of 3 determinations.

as for a water sample. Determine the magnesium by the standard-additions method. The results shown in Table 1 agreed satisfactorily with those obtained by atomic-absorption spectroscopy. Determination

of magnesium

in salts

Transfer a weighed amount of salt to a 25-ml standard flask, dissolve it in water, then add the ethylenediamine and EBT solution as before and dilute to the mark with water. Record the derivative polarogram and measure as before.

REFERENCES 1. T. A. Kryukova, S. I. Sinyakova and T. B. Arefeva, Polarographic Analysis (in Russian), p. 685. National

Chemical Publishers, Moscow, 1959. 2. M. L. Richardson, Talanta, 1965, 12, 1009. 3. Luo Denbai and Zhao Zaofan, J. Wuhan Unio. (Natural Sci. Ed.), 1982, 100.

4. F. Anson, Electrochemistry and Electroanalytical Chemistry, Huang Weiceng (ed.). (in Chinese), p. 6. Beijing Univ. Press, 1983. 5. E. Asmus, Z. Anal. Chem., 1960, 178, 104.