silver sulphide wire electrode

Analytica Chimica Acta, 196 (1987) 221-227 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

DETERMINATION OF LOW LEVELS OF CYANIDE SILVER/SILVER SULPHIDE WIRE ELECTRODE

VLADISLAVA

WITH A

M. JOVANOVIC:*

ZCTM Institute for Electrochemistry, Belgrade (Yugoslavia)

University

of Belgrade, P.O. Box 815, YU-11000

MILAN SAK-BOSNAR Pedagogical

Faculty,

University

of Osijek,

YU-54000

Osijek (Yugoslavia)

MOMIR S. JOVANOVIC Department of Analytical Chemistry, of Belgrade, P.O. Box 494, YU-11000

Faculty of Technology Belgrade (Yugoslavia)

and Metallurgy,

University

(Received 27th August 1986)

SUMMARY A silver/silver sulphide electrode is prepared quickly by holding a cleaned silver wire in vapours from molten sulphur. In 1000-10 mg 1-l cyanide solutions, the electrode exhibits a linear E/log Cm function which becomes slightly sinusoidal for lo-O.1 mg 1-l cyanide. The average slope is slightly super-Nernstian (120 mV/decade concentration). The applicability of the electrode is demonstrated for the determinations of microgram quantities of water-soluble cyanide from the Prussian blue pigments which are constituents of externally applied cosmetics. The home-made electrode provides results agreeing with those obtained with commerically available electrodes.

The cyanide response of ion-selective electrodes is based on cyanide corrosion of the sensing phase of the electrode. As explained by Pungor and coworkers [l-3] and Veseljr et al. [4] , the electroactive phase creating the cyanide response can be either a silver iodide or silver iodide/silver sulphide mixture, or pure silver sulphide. In any of these cases, dicyanoargentate(1) is formed, releasing either iodide or sulphide ions from the solid phase and creating a theoretical slope of either 59 or 118 mV. Thus, the ion-selective electrode responds to the relevant primary ion, the activity of which is governed by the activity of the measured cyanide. It is clear that the corrosion of the sensing phase shortens the lifetime of the electrode and its durability depends on the kind of silver salt applied as sensor. Thus, the silver iodide-based electrode should not be used in cyanide solutions with concentrations exceeding 10e2 mol 1-l. In fact, such electrodes, and not the silver sulphide-based ones, are usually termed “cyanide-sensitive” by manufacturers of ion-selective electrodes. Hence, it is not surprising that for the potentiometry of cyanide, some authors have investigated the applicability of membranes made of silver sulphide, which has a much lower 0003-2670/87/$03.50

o 1987 Elsevier Science Publishers B.V.

222

solubility product than silver iodide. The most frequent requirement for the maximum cyanide concentration lies at the 0.1 mg I-’ or even lower level. But direct potentiometry of sub-mgl-’ cyanide solutions based on calibration graphs is not precise enough, because of the difficulty of preparing stable standard solutions. These aspects led Frant et al. [5] and Clusters et al. [6] to evaluate the multiple standard addition method with dicyanoargentate solutions for the determination of mg 1-l to pg 1-l cyanide levels, using the Orion 94-16A silver sulphide membrane. This procedure has been recommended as a standard method [7]. However, Penland and Fischer [8] described a single standard addition method for the determination of these cyanide levels; they added different standard potassium cyanide solutions to the unknown test solution, applying the Orion 94-06A silver iodide/silver sulphide pellet membrane. This procedure, which is much simpler than the dicyanoargentate additions, has also been accepted as a standard method [ 91. The choice of method for determining trace amounts of cyanide by means of ion-selective electrodes is thus confused with regard to both procedure and electrode. Silver sulphide should have priority because it is much more resistant to cyanide corrosion and because it can easily be prepared and reconditioned. The procedure used here for the preparation of a second-kind electrode made of a silver strip coated with silver sulphide, is based on early work by Hijnigschmid and Sachtleben [lo]. In order to assay the atomic mass of sulphur, these authors synthesized very pure silver sulphide by keeping silver pellets in vapour from molten sulphur, all in an atmosphere of nitrogen. Other methods of preparing metal/metal sulphide wire electrodes are possible with gaseous hydrogen sulphide [ 111, or buffered sodium sulphide solutions [12], but the first method seemed to be the most promising. The silver/silver sulphide wire electrodes were produced here by holding silver strips in the vapour from molten sulphur under ambient conditions. The electrodes were tested in three steps: (1) evaluation of the lowest cyanide concentration which could be titrated with silver ion; (2) determination of mg 1-l and c(g 1-l concentrations of cyanide in pure solutions, by using Penland and Fischer’s standard potassium cyanide addition method [ 81; and (3) application of the electrode for the determination of low levels of water-soluble cyanide in iron(II1) ammonium hexacyanoferrate(II), a type of Prussian blue which is used as the chief component of some cosmetic preparations. For this application, the standard extraction procedure described by Thieman et al. [13] was used; in determining the extracted cyanide, Penland and Fischer’s standard addition technique [ 81 was preferred. EXPERIMENTAL

Instrumentation and solutions A MA5705 Iska (YU) modelpH/mV meter and an Orion 801A “Ionalyzer” were used for potentiometric readings. Titrations were done with a Metrohm E-415 digital piston burette.

223

Three ion-selective electrodes were used: an Orion 94-16A and a Radiometer F1212S electrode, both of which have crystalline silver sulphide pellet membranes, and a home-made silver/silver sulphide wire electrode. The reference was a commerical saturated calomel electrode, with a salt bridge (PTFE tube) filled with potassium nitrate in agar-agar gel. In order to avoid adsorption of cyanide on the walls of glass vessels, PTFE beakers were used throughout. Solutions Potassium cyanide solutions were prepared in the range from 1000 mg 1-l CN- (3.8 X lo-* mol 1-l) down to 10 pug1-l CN- (3.8 X lo-’ mol 1-l). To ensure that all the cyanide was present as free CN-, and to maintain uniform ionic strength, solid potassium cyanide for the 1000 mg 1-l solution was dissolved in 0.05 mol 1-l sodium hydroxide, the pH being ca. 11.3. All the other cyanide solutions were made by successive ten-fold dilutions with the same hydroxide solution. Solutions with concentrations 99.9% pure; ca. 0.3 mm thick, 2-3 mm wide and ca. 12 cm long) were first placed in a very low Bunsen flame until they became just redhot; the silver must not melt. This pre-treatment provides a tiny-grain structure of the metal surface. The metal was then polished with a filter paper. Simultaneously, a few grams of pure, sublimed sulphur were melted (without the appearance of a flame) in a porcelain crucible. Immediately after all the sulphur had melted, the prepared silver strip was held in the yellow vapour (125-130°C) until there was no further change in the colour of the sulphide layer (l-2 min). The fine porosity of the silver and its slow conversion to sulphide are essential in obtaining a durable film of silver sulphide. Finally, the sulphide layer, which is black in appearance, was rubbed with filter paper in order to produce a shiny surface. A silver sulphide film obtained in this way can resist continuous corrosion from 1000 mg 1-l cyanide solution for at least 24 h. In order to achieve reproducible results, the silver/silver sulphide electrode must always be immersed to the same depth in the test solution (1 or 2 cm from the tip). After prolonged usage, especially in concentrated cyanide solutions, the sulphide layer will be partially removed; damage can be seen as bright spots of metallic silver. In order to revive the same electrode, it can be heated part by part in a very low Bunsen flame until the black layer becomes red-hot and disappears completely. After the strip has been rubbed with filter paper, it can be recoated as described above.

225 TABLE 1 Cyanide recovery tegt by the standard addition method Cyanide taken (rg) No. of tests Recovery (%)a Orion 94-16A/SCE Ag/Ag,S/SCE

300 3

200 3

100 5

99.7 (0.2) 101.0 (0.3)

97.5 (0.2) 103.0 (0.4)

107.0 (0.4) 108.0 (0.4)

50.0 10

30.0 10

10.0 10

5.0 10

101.2 (0.9) 106.8 (1.0)

101.0 (4.5) 103.0 (5.5)

110.3 (9.9) 120.0 (10.3)

108.0 (10.2) 108.0 (10.7)

aThe standard deviation from the mean is given in parentheses.

Procedure for soluble cyanide in the hexacyanoferra te(II) cosmetic preparation The extraction step [13] was slightly modified. The soluble cyanide was extracted with ca. 20 ml of distilled water by stirring magnetically in a PTFE beaker for at least 30 min. The filtrate was collected in a 50-ml volumetric flask (pre-rinsed with 0.1 mg 1” cyanide solution). Before final dilution, ca. 100 mg of sodium hydroxide was added to adjust the pH and ionic strength. This solution was transferred to a PTFE beaker, and the subsequent procedure was as described above for the standard addition method. The results obtained are shown in Table 2. In order to check the procedure, extra cyanide was added after the filtration of the extract. This produced an acceptable cyanide recovery after subtraction of the amount added (Table 2). RESULTS

AND DISCUSSION

Figure 1 clearly shows that the differences in behaviour of cyanide-sensitive electrodes depend strongly on the manner of their production even if they are of similar morphology. Thus, the commercial electrodes, both of silver sulphide pellet membrane construction, exhibited a sub-Nernstian cyanide response and differed in the (non)linearity of their E/log CcN functions. In contrast, the home-made silver/silver sulphide wire electrode, exhibited a super-Nernstian response at low concentrations before fading at 0.01 mg 1-l levels (Fig. 1). A probable explanation of these deviations from theoretical linearity was indicated by Veseljr et al. [4] who proved that there is a sinusoidal dependence of potential on cyanide concentration and ascribed this to the different cyanoargentate complexes formed during the corrosion processes of the sensing phase. From the analytical point of view, it is clear that the dilution technique for standards in the micromolar region is not absolutely reliable. The instability of such solutions and the tendency of cyanide to adsorb on glass create obvious problems. Calibration of the electrode to establish the E/log CcN

226 TABLE 2 Results for cyanide in the hexacyanoferrate( II) cosmetic preparation Cell

Cyanide level found (mg kg“)

&/Ag,S/SCE Ag/Ag,S/SCEu Orion 94-16A/SCE

sa

Lowest

Highest

Mean

1.9 2.0 2.1

3.1 2.1 2.5

2.41 2.07 2.34

0.1142(10) 0.0334(3) 0.1212(3)

%tandard deviation from the mean with number of tests in parentheses. bResults obtained after addition of 3pg of cyanide to the filtrate.

-3o( I-3oc I-

z w -5oc )-5oc )-

-7oc I-7oc I-

0.01

0.1

I

IO

Cyanide

(mg I-‘)

100

1000

I

0.01

I 0.1

I Cyantde

I

IO (mg 1~‘1

I

loo

1000

Fig. 1. Calibration graphs of different ion-selective electrodes in cyanide solutions: (1) Radiometer F1212S; (2) Orion 94-16A; (3) home-made Ag/Ag,S; (4) theoretical 118 mV slope. Fig. 2. Calibration graphs of the home-made Ag/Ag,S electrode after exposure to cyanide corrosion: (1) freshly prepared electrode; (2) after 24 h exposure to 1000 mg 1-l cyanide; (3) next day, after conditioning in air; (4) theoretical 118 mV slope.

227

slope is essential for computation of the results by the standard addition method [8, 91. Though the silver/silver sulphide wire electrode showed a very high resistance to cyanide corrosion, as shown by the calibration graphs in Fig. 2, daily recalibration was necessary because of the preparation of fresh solutions. But in all the tests conducted the simple silver/silver sulphide wire &&rode proved to be as suitable as the commercial pellet membranes. Recovery tests from pure cyanide solutions were made either by adding silver nitrate (titrimetric procedure), or by adding potassium cyanide (standard addition method). Obviously, there was a silver response of the silver sulphide sensing phase in the former case, and at the lo-’ mol 1-l cyanide level the end-point potential jump was barely recognizable. With the standard addition methods [5, 6, 81, only the cyanide response need be considered; though the reliability of the standard addition method may be less satisfactory for 100 pg of cyanide, this technique provides the only possibility of estimating GO.1 mg 1-l cyanide. For the determination of cyanide in the cosmetic preparation (Table 2), the home-made and commercial silver sulphide electrodes provided similar results. Addition of 3.00 ml of 1 mg 1-l cyanide solution to the filtrate from the original cyanide extract confirmed the reliability of the results; the differences correspond to only 0.3 pg of cyanide. It can be concluded that the proposed silver/silver sulphide wire electrode, which is simple to prepare and renew and which is resistant to cyanide corrosion, is useful for the direct potentiometry of very low concentrations of cyanide. REFERENCES 1 B. GyBrgy, L. Andre, L. Stehli and E. Pungor, Anal. Chim. Acta, 46 (1969) 318. 2 K. Toth and E. Pungor, Anal. Chim. Acta, 51 (1970) 221. 3 E. Pungor, M. Gratzl, L. Pblos, K. Toth, M. F. Ebel, H. Ebel, G. Zuba and J. Wernisch, Anal. Chim. Acta, 156 (1984) 9. 4 J. Veselg, 0. J. Jensen and B. Nicolaisen, Anal. Chim. Acta, 62 (1972) 1. 5 M. S. Frant, J. W. Ross Jr. and H. Riseman, Anal. Chem., 44 (1972) 2227. 6 H. Clusters, F. Adams and F. Verbeek, Anal. Chim. Acta, 83 (1976) 27. 7 D. Midgley and K. Torrance, Potentiometric Water Analyses, Wiley, New York, 1979, p. 306. 8 J. L. Penland and G. Fischer, MetalloberfBiche, 26 (1972) 391. 9 Standard Methods for the Examination of Water and Wastewater, 14th edn., American Public Health Association, Washington, 1976, p. 372. 10 0. Hanigschmid and R. Sachtleben, Z. Anorg. Allg. Chem., 195 (1931) 207. 11 A. V. Vishnyakov, A. F. Zhukov, T. A. Lyubchak, Yu. I. Ursov and A. V. Gordievskii, Zh. Anal. Khim., 32 (1977) 840. 12 Nj. RadiE, K. J. Mulligan and H. B. Mark Jr., Anal. Chem., 56 (1984) 298. 13 H. W. Thieman, H. W. Ziegler and W. H. Oakes, Drug Cosmet. Ind., 125 (1979) 78.