Determination of microgram amounts of cyanide by the induced iodine-azide reaction with the use of sodium tetrathionate

Analyttca Chumca Acta, 263 (1992) 155-158

155

Elsevler Science Pubhshers B V , Amsterdam

Determination of microgram amounts of cyanide by the induced iodine-azide reaction with the use of sodium tetrathionate Jan Kurzawa *, Krystyna Janowlcz and i%gmew Kurzawa Instrrute of Chemtstry, Techmcal Umuersrty,Poznan (Poland)

(Recewed 16th October 1992)

AbStlXCt

The lodme-azlde reactIon Induced by sulphutt -II) compounds was apphed to the determmatlon of cyamde ions m aqueous solution The method ISbased on the converSion of cyamde by an excess of tetratluonate mto tluosulphate and thmcyanate followed by the determmahon of the products by the mdme-azlde reaction The method allows the determmation of 0 05 pg of cyanide m a 5-ml sample

Keywords Wrlmetry, Cyamde, Iodine-azlde reactlon

The lodme-azlde reaction, mduced only by compounds which contam sulphur m the -II oxldatlon state, has found wide application for the determmatlon of these compounds usmg several analytical techmques The reactton has also been employed for the mdlrect determmatlon of metal Ions [l-3] and of chlorine 141 The hterature on the analytical apphcatlons of the mdmeazrde reactlon has been revlewed [5] This paper describes the determmatlon of cyanide Ions by means of the lodme-azlde reactlon The method IS based on the work of Koh et al [6], who devlsed a method for the determmatlon of polytlnonates using the followmg cyanolysis reaction

Both thlosulphate and thlocyanate are mductors of the lodme-azlde reactlon and react rapidly, the reaction bemg vu-tually completed m 30 s, but polythlonates mduce the reactlon only after heterogeneous cleavage of the -S-S- bond by azlde Ions and consequently their mducmg actlvlty after 30 s 1s low This phenomenon allows the application of the cyanolysls reactlon for the determmatlon of free cyanides An excess of tetrathlonate m relation to cyamde 1s used

S,O;- + 3CN-+ H,O +

A stock solution of todme (0 01 M) contammg 4 g I-’ of potassmm Iodide was standanzed by tltratlon wrth 0 01 M sodmm arsemte solution A stock solution of sodmm azlde contamed 200 g NaN, I-’ A 0 1 M stock solution of potassuun cyanide m 0 1 M sodmm hydroxide was standardized by titration wth silver nitrate Less concen-

S,O;- + SO:- + 2HCN + SCN-

(1)

Apphcatlon of an excess of cyanide m relation to tetrathlonate and measurement of the thmcyanate formed m the reaction pernuts the determmatlon of tetrathlonate

EXPERIMENTAL.

Reagents

0003-2670/92/$05 00 0 1992 - Elsewer Science Pubhshers B V All nghts rewved

156

trated solutions were prepared by ddutlon A 0 01 M stock solution of sodium thlosulphate and of potassium thlocyanate were used Carbonate buffer solution (pH 9 2) was prepared by rmxlng 0 1 M sodium carbonate and 0 1 M sodium hydrogencarbonate (1 + 9) A 0 01 M standard tetrathlonate solution was prepared dally by stolchlometrlc oxldatlon of a 0 2 M thlosulphate solutlon with lodme solution and appropriate dllutlon Use of solid potassmm or sodium tetrathlonate for the preparation of a standard solution 1s not recommended because these compounds are not stable enough and always contam a small amount of elemental sulphur and variable water contents Procedure

The results obtained m the mvestlgatlon of the cyanolysls reaction, for the influence of the pH on its rate and for the ldentlflcatron and detennation of the reaction products permit the followmg procedure to be recommended for the routme determmatlon of cyanide at concentration from 01 to36mgl-’ Into a B-ml vessel, 5 ml of water (blank) or 5 ml of sample contammg 0 5-18 pg of cyanide ions are introduced, followed by 0 1 ml of 0 01 M tetrathlonate solution and 1 ml of carbonate buffer The cyanolysrs reaction starts at the moment of addition of the buffer solution and It should be stopped after 1 h by addltlon of 10 ml of 0 1 M HCI The Induced reactlon 1s carried out m the same vessel without dllutlon of the sample A 0 5-ml volume of sodium azlde solution (0 1 g NaN,) and 0 1 ml of 0 1 M HCl are added to the reaction mncture, giving a pH of 6 7 The contents of the vessel are stlrred vigorously with a magnetic stirrer, 1 ml of 0 01 M lodme solution IS added and the time 1s measured from this moment After 30 s the excess of unreacted lodme 1s back-titrated with 0 01 M sodium arsemte solutlon usmg starch as indicator and the amount of lodme consumed m the reactlon IS measured The difference m lodme consumption between the blank test and the sample is due to thlosulphate and thlocyanate produced m the cyanolysls reaction and is linearly related to the amount of cyamde m the sample

J Kurzawa et al /Anal Chm Acta 263 (1992) 155-158 RESULTS AND DISCUSSION

Under the above condltlons the dependence of Iodine consumption on the amount of cyamde m the sample 1s lmear with a slope of 39 698 (pg CW vs ~1 12) and a correlation coeffictent r > 0 999 It was demonstrated expernnentally that thlosulphate and thmcyanate are among the products of the cyanolysls and are produced m a molar ratio of 1 1, confirming the stolchlometry of reactlon 1 Advantage was taken of the phenomenon of the selective blockmg of mducmg properties of thmcyanate by iodide ions Another possible equation of the reactlon, as mentloned m the literature [71 2NaCN + Na,S,O, + 2NaOH + 2NaSCN + Na,SO, + Na,SO, + H,O IS excluded because it does not account for the thlosulphate ions which are produced m the reaction Hydroxide Ions do not participate m the reactlon and they only shift the eqmhbrmm of the protolytic reaction CN-+H,O+ HCN + OH- [ K,(CN-)

= 2 5 x 1O-5]

towards the formation of CN- and only these ions, not HCN, take part m the cyanolys~s Thus was also confirmed by the results of an mvestlgatlon of the influence of pH on the cyanolysls The reaction proceeds very slowly m neutral solution and even after several hours the progress of the reaction IS hardly noticeable Alkahmzatlon of the reaction medium increases the reaction rate proportlonally to the CN- concentratron However, too high a pH causes the alkalme decomposition of tetrathtonate mto products that mduce the lodme-azlde reactlon The optimum was found to be pH 9 2, at which the lodme consumptlon by a blank sample (only tetrathlonate) IS low At this pH the cyanolysls IS virtually completed after 1 h The determined amount of tluocyanate and thlosulphate formed by a known amount of cyanide 1s greater than that calculated from Eqn 1 This 1s due to the reactlon of the excess of

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Chrm Acta 263 (1992) 155-158

tetrathlonate w&h HCN, which IS generated as a product of the cyanolysls (and then transformed mto CN-), and hence amphficatlon IS observed The reproducrbdity of lodme consumption m the lodme-azlde reaction Induced by products of the cyanolysis reactlon is 5 ~1 of 0 01 M lodme solution (13 pg of lodme) over the whole range of the determmatlon carried out according to the proposed procedure This amount of Iodine corresponds to 0 13 pg of CN- The accuracy of the determination 1s constant and the percentage error of the determination decreases wth mcreasmg amount of cyanide m the sample The standard deviation calculated from five independent determmatlons is 0 1 pg for 2 5 pg of CN- and 0 2 pg for 15 pg of CN- m a 5-ml sample Thlocyanates, thlosulphates and sulphldes interfere m the determination of cyamdes by the proposed method However, cyanide can be determined m the presence of 2 mg 1-l of thlosulphate or 4 mg I-’ of thlocyanate by subtraction of the iodine consumed m the lodme-azlde reaction Induced by the sample alone (without carrying out the cyanolysls) from the lodme consumed m the lodme-azlde reaction induced by products of the cyanolysls reaction In both titrations the same volume of sample and the same reaction condltlons should be used Cyanides should be sepa. rated by dlstlllatlon from solutions containing more than 2 mg l- ’ of thlosulphates Interference from thlocyanates up to 20 mg 1-l can be overcome by the mtroductlon of 0 1 ml of 10% potas-

slum lodlde solution before the lodme solution 1s added Potassium lodlde selectively blocks the mducmg properties of thmcyanates In that event a separate cahbratlon graph must be constructed because potassium lodlde also blocks thmcyanates produced m the cyanolysls reaction It decreases the sensltlvlty but extends the range of the cyanide determination to 5 mg 1-l The interference from sulphldeq presents a malor problem Sulphldes induce the lodmeazlde reaction and also react Hrlth tetrathlonate according to the equation [S]

s,o;-

+ s2-+ 2s,o32- + s

Because the products of this reactlon induce the lodme-azlde reaction, the procedure applied for thlocyanate or thlosulphate cannot be used Another disadvantage is that sulphldes co-disk1 with cyanides and cannot be separated m this way In order to overcome this mterference, the preapltatlon of sulphldes was employed Apphcatlon of a suspension of zmc carbonate hydroxide was unsuccessful owmg to losses of cyamde caused by the formation of a zinc-cyamde complex Lead salts, which are recommended for sulphlde removal [g-11], gave satisfactory results up to a sulphlde concentration of 5 mg 1-l Sulphldes were removed by shakmg 10 ml of the sample with about 200 mg of lead carbonate followed by immediate filtration of the copreclpltated sulphlde The cyamdes were then determined m 5 ml of the filtrate The recovery of cyanide de-

TABLE 1 Determmatlon of cyanide In the presence of Interfering ions a Sample contammg

Consumption of lodme (~1) by Sample

CNCN-+ S*- (2 mg 1-l) CN-+ S*- (20 mg 1-l) CN-+ SCN- (4 mg 1-l) CN-+ S,O:- (2 mg I-‘) CN-+Olm110%Kl CN-+ SCN- (10 mg I-‘) + 0 1 ml 10% KI

458 535

Blank (only S40i-)

Sample after cyanolysis

Products of cyanolysis

40 42 40 38 40 44 42

238 233 227 692 769 183 183

198 191 b 187 c 196 194 139 d 141 d

a The results are presented as ~1 of 0 01 M iodine solution consumed m the iodine-azlde reactlon by 5 ml of the sample contammg 1 mg I-’ of CN- b SuIph 1des removed by shakmg with lead carbonate ’ Sulphldes removed by tltratlon with lead acetate d Thlocyanates produced m the cyanolysls reaction are blocked

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creases at higher sulphlde concentration and drops to zero m the presence of 50 mg 1-l of sulphlde Tbls effect cannot be explained on the basis of the solublhty of products and the stablhty constants, which are pK,,(PbS) = 27 5, pK,(PbCO,) = 13 5 and pK(Pb(CN),]‘-= 10 3 The same phenomenon was observed by Dnkas and Routley [12], wlio suggested that cyamde 1s lost by complexatlon or by occlusion m the preclpltated lead sulplude Their method of addltlon of a stolchlometrlc amount of lead acetate solution (10 g 1-l) with a redox electrode used for momtormg the oxldatlon potential was successfully applied m the presence of more than 5 mg I-’ of sulphlde The recovery of cyanide from samples containing 0 5 mg 1-l of cyanide ions and 10, 30 and 50 mg I-’ of sulphide was wthm the range 91-96% Interference caused by sulphlte, which reacts with tetrathlonate to produce thlosulphate, can be avoided simply by addition of the stolchlometrlc amount of lodme solution to the sample The results of cyanide determination m the presence of interfering ions are shown m Table 1

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Chrm Acta 263 (1992) 155-158

REFERENCES 1 2 Kurzawa and E Kubaszewslu, Chem Anal (Warsaw), 19 (1974) 483 2 J Kurzawa, Z Kurzawa and Z Svnt, Chem Anal (Warsaw), 21 (1976) 791 H Matusiemcz and Z Kurzawa, Chem Anal (Warsaw), 21 (1976) 1035 J Kurzawa, Z Kurzawa and K. Janowxz, Anal Chun Acta, 252 (1991) 127 G Rarms Ramos, M C Garcia Alvarez-Coque and R M Vdlanueva Camanas, Analyst, 1110986) 1001 T Koh, N Salto and I Iwasakl, Anal Chum Acta, 61 (1972) 449 E A Plenegud and E W Glermet, Chemlczna Anahza Powtetrza w Zakladach Przemyslowych, WNT, Warsaw, 1967, p 323 (translated from Russian) 8 T Koh, Y Mmra and M Suzulu, Anal SC], 4 (1988) 267 9 Standard Methods for the Exarmnatlon of Water and Wastewater, Amencan Pubhc Health Association, New York, 17th edn , 1985, p 327 10 Water Analysis Handbook, Hach, Loveland, CO, 1989, pp 255,256 11 The Testing of Water, E Merck, Darmstadt, 9th edn , p 77 12 M Drdcas and B I Routley, Analyst, 113 (1988) 1273