904 over the range 0--25 ng cobalt(II) in a final volume of 10 ml. The precision was calculated for 12 ng of cobalt(II) per 10 ml of aqueous solution; the relative standard deviation (R.S.D.) was 2.5% (n = 5).

Effect of foreign ions and conclusion. Small amounts of several metal ions such as manganese(II), iron(Ill) and copper(II) showed positive errors, but the coexistence of aluminium(III), zinc(II) and lead (II) was permissible up to about a 10-20:fold excess over cobalt(II). The presence of anions such as fluoride, iodide, thiosulphate and tartrate did not interfere with the determination of cobalt(II). These results are shown in Table 1. Thus, the proposed method is very simple and sensitive in comparison with previous methods [8, 9]. Without the coexistence of cationic surfactants, the preparation of HAP is very easy and its methanol solution is also stable. Accordingly, the proposed method may be used for a simple and sensitive determination of cobalt(II) in pharmaceutical preparations such as drinks containing of vitamin B12 preparations.

References

1. Mori I, Fujita Y, Toyoda M, Kato K, Akagi M (1991) Talanta 38 : 683 - 686 2. P6rez-Bendito D (1984) Analyst (Lond) 109:891 -- 899 3. Salinas F, Berzas-Nevado JJ, Valiente P (1987) Talanta 34: 321 - 324 4. Themelis DG, Vasilkiotis GS (1987) Analyst (Lond) 112:791-795 5. Llobat-Estelles M, Sevillano-Cabeza A, Medina-Escriche J (1986) Analyst (Lond) 111:193-195 6. Medina J, Herandez F, Marin R, Lopez FJ (1986) Analyst (Lond) 111:235-240 7. Otto M, Rentsch J, Werner G (1983) Anal Chim Acta 147: 267 8. Mori I, Fujita Y, Kato K, Toyoda M, Akagi M (1990) Bull Chem Soc Jpn 63 : 3 5 9 - 363 9. Mori I, Fujita Y, Toyoda M, Kato K, Akagi M (1990) Anal Lett 23:2107-2122

Fresenius J Anal Chem (1992) 343:904-905 - © Springer-Verlag 1992

Ascorbic acid as a new reductant in the thiocyanate method for the spectrophotometric determination of rhenium Anu Wahi and L. R. Kakkar

Department of Chemistry, Kurukshetra University, Kurukshetra-132119, Haryana, India

300 mg ascorbic acid, 2 ml 10 mol/1 HC1 and complete the volume with water to 10 ml. Mix gently, heat gradually to 70°C and allow the contents to stand undisturbed for 5 - 6 min at room temperature. Transfer into a 100 ml separatory funnel, dilute to 20 ml with water, extract with an equal volume of isoamyl alcohol (once for 1 rain), filter the organic layer through a Whatman No. 41 filter paper into a 25 ml volumetric flask, make up to the mark with pure solvent and measure the absorption at 430 nm against a blank solution. Determine the rhenium content from a calibration curve prepared similarly.

Received August 3, 1991, revised March 24, 1992

Summary. A spectrophotometric determination of rhenium is based on its reduction with ascorbic acid in the presence of thiocyanate. The orange-yellow complex is extracted into isoamyl alcohol and the absorbance measured at 430 nm. Various foreign ions do not interfere. Molar absorptivity and Sandell's sensitivity are 31162.79 1/mol/cm and 0.0059 gg/cm 2, respectively.

Results and discussion

It has been observed that perrhenate ion, in the presence of thiocyanate, forms a coloured species on reduction with ascorbic acid (solid) under mild acid conditions at 70°C, which on cooling gives a stable dark orange-yellow complex whose absorbance is measured at 430 nm. The effect of various parameters is shown in Table 1.

Effects of diverse ions. Sulphate (200), acetate (100), citrate (50),

Solutions and reagents. KReO4 in water, 1 mg Re/m1. KSCN in water, 20 g in 100 ml. Ascorbic acid, solid, iso-Amyl alcohol, distilled, fraction 111 - 112 ° C.

tartrate (50), phosphate (100) and EDTA (50) do not affect the absorbance of the metal complex; whereas, fluoride (50) and chloride (100) increase it slightly. Oxalate (100), thiourea (50), nitrate (100), H202 (1 ml 30% w/v), decrease it in the same order. The figures shown in brackets indicate the amount of the sodium salt of the anion in mg added initially to the aqueous solution. The cations which do not show any absorption include Ni(II), Zn(II), Zr(IV), Bi(III), As(V), Pb(II), Cu(II), Cd(II), Hg(II), Ba(II), Se(IV), Ce(IV), AI(III), Ca(II), Mg(II), Be(II), Ag(I), 5 mg/20 ml; V(V), 4 mg/20 ml; W(VI), Cr(III), Cr(VI), 3 mg/20 ml; Co(II), U(VI), 2 mg/20 ml; Fe(II), Fe(III), 1 mg/ 20 ml; Rh(III), 0.4mg/20ml; Pd(II), Ru(III), 0.1 rag/20 ml; Pt(IV), Os(VIII), Ti(IV), 0.05 mg/20 ml. Mo, 0.1 mg/20 ml, can be back-washed from the solvent with 10 % ammonium bifluoride; the latter does not affect the absorbance of the Re-complex.

Procedure. Place the solution containing up to 100 gg Re in a

Stoiehiometry of the extracted species. The ratio of rhenium to

The well-known and often used thiocyanate method for the determination of rhenium is based on the formation of a coloured complex, when perrhenate reacts in acid solution with stannous chloride in the presence of thiocyanate [1 - 5]. Several modifications have been suggested from time to time [e.g. 6 - 9]. Our improvement of the method consists in replacing stannous chloride by ascorbic acid and extracting into iso-amyl alcohol. Favourable selectivity and sensitivity have thus been achieved. Experimental

25 ml beaker, add 4 ml 20% potassium thiocyanate solution,

Offprint requests to: L. R. Kakkar

thiocyanate in the extracted species is determined by Job's method of continuous variations [10] using equimolar solutions of perrhenate and thiocyanate (1.613 x 10-3 mol/1) and the ab-

905 Table 1. Effect of various parameters on the absorbance of the Re-thiocyanate complex Temperature a, ° C Absorbance KSCN b, ml Absorbance Ascorbic acid °, nag Absorbance HC1 d, mol/1 Absorbance Equilibration time ~, rain Absorbance

40 0.028 0.0 0.011 0.0 0.530 0.0 0.014 0.5 0.658

45 0.047 0.5 0.190 20 0.590 0.2 0.017 1.0 0.670

50 0.120 1.0 0.400 50 0.615 0.3 0.029 2.0 0.670

55 0.218 2.0 0.595 100 0.640 0:8 0.109 3.0 0.670

60 0.600 3.0 0.635 200 0.665 1.0 0.140 4.0 0.670

65 0.629 3.5 0.655 250 0.668 1.5 0.610 5.0 0.669

68 0.635 3.9 0.670 290 0.670 1.9 0.670

70 0.635 4.0 0.670 300 0.670 2.0 0.670

72 0.635 4.1 0.670 350 0.670 2.1 0.670

75 0.620 4.5 0.665 500 0.670 2.5 0.650

80 0.595 5.0 0.650

85 0.569 6.0 0.625

3.0 0.625

Conditions: Re = 100 txg; KSCN = 3 ml; Ascorbic acid = 300 mg; HC1 = 2 mol/1; colour development time = 5 - 6 min; final aqueous volume = solvent volume = 20 ml; equilibration time = 1 min b Temperature = 70°C, other conditions are the same as in (a) excepting KSCN ° KSCN = 4 ml, other conditions are the same as in (b) excepting ascorbic acid d Ascorbic acid = 300 rag, other conditions are the same as in (c) excepting acid concentration ° HC1 = 2 mol/1, other conditions are the same as in (d) excepting equilibration time a

sorbances are measured at 400, 430, 450 nm. The three curves obtained by plotting mole fractions of rhenium against their respective absorbance values indicate two maxima corresponding to metal: ligand ratios of 1 : 4 and 2: 3 in the extracted species. The mole ratio method [11] also confirms this inference.

Stability and sensitivity of the complex. The absorbance of the extract containing the Re-complex is stable for 2 h. Standard deviation is 0.002. Beer's law is obeyed in the range of 0 to 4 gg Re/ml. The molar absorptivity is 3l 162.79 1/mol/cm, Sandell's sensitivity is 0.0059 gg Re/cm 2. Acknowledgement. Our sincere thanks are due to Prof. S. P. Singh, Chairman, Department of Chemistry for providing laboratory facilities~ Kurukshetra University and the Department of Atomic Energy, Bombay for the financial assistance to one of us (AW).

References 1. Tribalat S (1949) Anal China Acta 3:113 2. Hurd LC, Babler BJ (1936) Ind Eng Chem Anal Ed 8:112 3. Hoffman JI, Lundell G E F 0939) J Res Natl Bur Standards 23 : 497 4. Sandell EB (1959) Colorimetric determination of traces of metals, 3rd edn. Interscience, New York, p 756 5. Geilmann W, Wrigge FW, Weibke F (1932) Z Anorg Chem 208:217 6. Geilmann W, Bode H (1948) Fresenius Z Anal Chem 128:489 7. Ryabchikov DI, Borisova LV (1963) Talanta 10:13 8. Koz'licka M, Wo'jtswicz M, Adamiec I (1970) Chem Anal (Warsaw) 15 : 247 9. Nayak AN, Manjappa S, Ramappa PG, Yathirajan HS (1981) Fresenius Z Anal Chem 309:396 10. Job P (1928) Ann Chim 9:113 11. Yoe JH, Jones HL (1944) Ind Engng Chem Anal Ed 16:111

Fresenius J Anal Chem (I 992) 343 : 9 0 5 - 906 - © Springer-Verlag 1992

Determination of zinc in lubricating oil by polarography of emulsified samples J. Gareia-Ant6n and R. Grima Departamento Ingenieria Quimica y Nuclear, E.T.S.I. Industriales, Universidad Polit6cnica de Valencia, P.O. Box 22012, E-46071 Valencia, Spain Received February 15, 1992

Summary. A new polarographic determination of zinc in lubricating oils is based on the formation of an oil/toluene-water emulsion and bubbling nitrogen through the emulsion. A well developed peak appears at - 1440 mV. The precision is _+3% for a sample with 382 mg Zn/kg.

Correspondence to: J. Garcia-Antdn

Introduction The zinc complexes of organodithiophosphoric acid or zinc organodithiocarbamates are used as zinc additives in the commercial formulations of lubricating oils. They combine antioxidant properties with an ability to inhibit corrosion and to function as antiwear agents [1] in motor oil applications. The activity of zinc additives in the protection of rubbing metal surfaces depends on the concentration of the additive in the lubricating oil [2]. The standard method IP 117/82 [3] for the determination of zinc in unused lubricating oil requires the prior ashing of the organic matter at 550°C and acid digestion of the residue, the re-solution of the zinc and later precipitacion as zinc oxinate. All these operations are very tedious and time consuming. The standard method ASTM D-1549 [4] also follows the same way for the sample destruction and re-solution of the zinc. In the A S T M method the zinc determination is carried out by means of polarography in ammonium chloride-ammonia medium. An alternative method is polarographic technique using the formation of an oil-water emulsion. The technique consists of