Gravimetric determination of tin with sodium cyclotetramethylenedithiocarbamate and its applications in metal analysis

Talanta 46 (1998) 1237 – 1243

Gravimetric determination of tin with sodium cyclotetramethylenedithiocarbamate and its applications in metal analysis Zhou Nan * Shanghai Research Institute of Materials, 99 Handan Lu, Shanghai 200437, People’s Republic of China Received 27 June 1997; received in revised form 10 September 1997; accepted 11 September 1997

Abstract A gravimetric determination of tin is proposed with sodium cyclotetramethylenedithiocarbamate as precipitant at pH 5.0–5.5. By optimizing the reaction conditions the theoretical conversion factor 0.1687 can be used for determining ]5% of Sn. Its standard deviation (n= 10) at the level of 20 mg of Sn was found to be 0.08 mg. Effects of diverse cationic species can be eliminated by a clear-cut group precipitation at pH 9 with DDTC in the presence of tartrate and subsequent masking with EDTA. Provision is also made for removal of individual species in occasional cases. Hence, the proposed method is flexible and versatile and was successfully applied to the macro-determination of Sn in diverse alloys. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Gravimetric determination; Tin; Metal analysis

1. Introduction Oxidimetry of Sn(II) with KIO3 [1] is generally used for the macro-determination of tin, an important component of industrial alloys. This method is empirical, needs thorough exclusion of air, the titre of the titrant should be standardized each time [2] and its values may be inconsistent if different CRM’s are used for standardization. The chelatometric titration of Sn is poorly selective [3]. More so are its gravimetric methods hitherto published which are time-consuming, hence, rarely

used. It reacts with N-benzoyl-N-hydroxylamine only in an ice bath for 4 h [4]. The precipitates formed with benzoate [5], phenylarsonic acid or N-nitroso-N-phenylhydroxylamine or as metastannic acid should be ignited to SnO2. Owing to the disproportionation of Sn(II) its electrodeposition is not quantitative. Indeed its macro-determination remains a problem and deserves further study. Sodium cyclotetramethylenedithiocarbamate (TDTC) is proposed in this paper for precipitating Sn(IV). High selectivity can be achieved by incorporating a preliminary clear-cut group precipitation at pH 9 in the presence of tartrate with DDTC and a subsequent masking with EDTA.

* Fax: + 86 21 5420554. 0039-9140/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0039-9140(97)00410-4

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2. Experimental

2.1. Reagents Deionized water was used for the preparation of all reagents and for dilutions. Analyticalreagent grade chemicals, supplied by Shanghai Reagent Chemicals, were used, unless otherwise specified. TDTC solution, 10% was prepared freshly from a product of Heyl, Germany. It was filtered through a 0.45 mm membrane before use. DDTC (sodium diethyldithiocarbamate) solution, 4% was prepared freshly and filtered before use. Tartaric acid solution, 0.5 M. HCl (3 M) and concentrated H2SO4 (9 M). Ammonia solution, 25% NaOH solution, 5 M TiCl3 solution, ca. 15% in 3 M HCl. Al(III) solution, 0.1 M in 0.15 M NaOH by alkaline dissolution of ] 99.9% pure Al metal. EDTA solution, 0.03 M. Buffer solution, pH of 5.2; containing 10 M of sodium acetate and 2 M of HCl. m-Nitrophenol solution, 0.3%. Screened indicator of pH 5.1, containing 0.08% of methyl red and 0.06% of bromocresol green in ethanol.

2.2. Wash solution A To 100 ml of water add 2 ml of HCl conc., one drop of m-Nitrophenol solution and dropwise ammonia solution till the indicator colors yellow.

2.3. Wash solution B To 100 ml of water add 2 ml of buffer solution and 1 ml of TDTC solution.

Nitrophenol solution (note 5), neutralize with NaOH solution to pH 6–7, then with ammonia solution till yellow color appears (ca. pH 9) and dilute to 80–100 ml. Add DDTC solution dropwise with vigorous stirring till the precipitate forms slowly. Add 5 ml of Al solution (note 6), adjust the pH with HCl if necessary and continue the addition of DDTC till no further precipitate appears, then one or two drops more. Digest the precipitate in a water bath thermostated at 60°C for 5 min. Cool and filter off the precipitate through a 0.45 mm membrane and wash the precipitate and the original flask three or four times with wash solution A (note 7). To a 250 ml beaker containing 1 ml of 9 M H2SO4 and 5 ml of HCl conc., pour in quantitatively by portionwise the combined filtrate and washings while stirring vigorously to make a clear solution. Boil to decompose the residual DDTC and concentrate the volume of the analyte solution to 60–70 ml. Add, while hot, 5–10 ml of EDTA solution (note 8), adjust pH to 4 with ammonia solution, then add one or two drops of the screened indicator solution and sufficient buffer solution till the indicator just turns green. Finally, add 10–30 ml (note 9) of TDTC solution dropwise with constant stirring. Digest the precipitate in a water bath thermostated at 70°C for 15 min. Cool to room temperature, filter the precipitate quantitatively to an IG 4 filter crucible dried and weighed to constant mass at 95°C and wash the glass crucible and the original beaker three or four times with wash solution B. Dry the crucible at 95°C to constant mass and calculate the content of Sn in the sample as follows (note 10):

2.4. General procedure Sn%= Take 200 mg of the sample containing ] 5% of Sn, weighing to the nearest 0.1 mg. Transfer it to a 250 ml conical flask and decompose with HCl alone or together with a few drops of HNO3 (note 1). Cap the flask loosely and warm gently at 50–60°C till decomposition is complete (note 2). Dilute to 30 ml with water, add TiCl3 solution in excess, cap the flask as before and boil gently for 10 min (note 3). Add an appropriate amount of tartaric acid (note 4), one or two drops of m-

(m2 − m1)× 0.1687 ×100 G

(1)

where m1 is the mass of the crucible itself, mg; m2 is the mass of the crucible and precipitate, mg; G is the mass of the sample or its aliquot taken for the determination, mg; 0.1687 is the conversion factor of the precipitate to Sn (vide infra).

2.4.1. Note 1 For Ti alloys add 10 ml of HCl conc. alone; for Al-Sn alloys add 8 ml of HCl(1+ 3) first, then 8

Z. Nan / Talanta 46 (1998) 1237–1243

ml each of water and HCl conc. only after vigorous reaction subsides. For Cu, Pb, Sn and fusible alloys add 10 ml of HCl conc. and a few drops of HNO3 (just sufficient but no more) [6].

2.4.2. Note 2 If there is any insoluble matter left, filter it off through a medium-texture filter paper and wash the paper thoroughly with HCl. 2.4.3. Note 3 If As is present, transfer the analyte solution to a distillation flask in a smaller size, and distil till the temperature of the vapor in the flask reaches 108°C under standard pressure. 2.4.4. Note 4 Its optimum amount is 9 ml for Pb, Sn and fusible alloys; 13 ml for Cu alloys; 17 ml for Ti alloys; 30 ml for Al – Sn alloys. Sometimes heating is necessary, e.g. to complex Cr(III). If the sample contains ]35% of Sn; or Mo or V or both, dilute its solution to a definite volume. For the determination take therefrom an appropriate aliquot (to which 3 ml of tartaric acid solution should be added) estimated after analyzing the other major elements.

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analyte solution; 10 ml of it suffices to precipitate 5 30 mg of Sn alone.

2.4.10. Note 10 If Mo or V or both are present, the mass of the precipitate in the sintered glass crucible on drying is the sum of these two or three chelates. Hence, a correction for this co-precipitation should be made and Eq. (1) is modified as follows: Sn%=

(m2 − m1 − 4.382ma − 7.390mb )× 0.1687 G × 100

(2)

where ma is the mass of Mo co-precipitated, mg; 4.382 is its conversion factor to MoO2L2 (L stands for the TDTC anion here), i.e. 420.453/ 95.94; mb is the mass of V co-precipitated, mg; 7.390 is its conversion factor to VO(OH)L2, i.e. 376.463/50.942. Once m1 and m2 are known, ma and mb can be determined as follows. Dissolve the precipitate in aqua regia, dilute to a definite volume and take aliquots for determining Mo by photometry with thiocyanate and V by oxidimetry or 2,2%iminodibenzoic acid method [8], respectively.

2.4.5. Note 5 Omit its addition whenever colored species is present.

3. Results and discussion

2.4.6. Note 6 Omit or reduce its addition if the analyte solution contains Al. This addition may raise the pH of the analyte solution.

Sn(IV) is the stable species in the aqueous medium, ligands of O,O-or S,S-donor type can react with it. Unfortunately, these reactions are poorly selective. Ligands of the latter type would be preferred, however, because reaction selectivity can be achieved by making use of the fact that they, like sulfide ion, do not precipitate Sn(IV) in an alkaline medium; but do most of the other precipitable species. Among them dithiocarbamates would be the candidates. It should be noted that those of the mono-substituted group tend to decompose readily in acidic and alkaline media [9], whereas the disubstituted group do so in the acidic medium only. Hence, the latter group would seem recommendable. The Sn(IV)-DDTC chelate was reported to be a mixture as the result

2.4.7. Note 7 If V is present, add 10 ml of ethanol to the filtrate. 2.4.8. Note 8 Its amount depends on that of the species to be masked. 2.4.9. Note 9 Its amount depends on: (a) mg of Sn(IV) to be determined; (b) mg of Mo or V or both in the

3.1. Choice of the precipitant for Sn(IV)

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of its decomposition or simultaneous redox reaction between its components [10,11]. On the other hand, TDTC has the following special features: (1) very soluble in water, much more so than the corresponding ammonium salt [12]; (2) slight solubility of its chelates [13]; (3) stable to a considerable extent toward gently heating in the weakly acidic medium; (4) much less oxidizable than other dithiocarbamates, hence, precipitation in the absence of any accompanying redox reaction. Accordingly it was chosen as the precipitant.

3.2. Optimum conditions for quantitati6e precipitation of Sn(IV) Our preliminary experiments showed that TDTC precipitates at pH5 6. Generally the decomposition rate of dithiocarbamates increases with increasing acidity of the medium, a pH range over 5.0 – 5.5 was chosen for the precipitation. This is easily controlled by the use of an acetate buffer of pH 5.2 with the aid of a screened indicator. Under such conditions the protonation effects of TDTC and the masking agent used would be minimized too. It is an added advantage. An auxiliary complexing agent should be first chosen to meet the following requirements: It can prevent the hydrolysis of Sn(IV) and keep all the concomitant species in solution, yet exerts little effect on the main reaction. It was found that tartaric acid serves the purpose and in the presence of 0.5 – 1.0 g of it the precipitation of Sn(IV) runs normally. As it complexes cationic species in moles, its amount of addition vary widely for different kinds of alloys, as specified in note 4 of Section 2.4. As revealed by our preliminary experiments, the mass of the precipitate¬6 × the mass of Sn added. Therefore, it can be inferred that the precipitate would be Sn(C5H8NS2)4, because its relative molecular mass, calculated on the basis of values of relative atomic mass of elements recently recommended by IUPAC [13] would be 703.74, which divided by 118.71, the relative atomic mass of Sn, gives the quotient 5.928.

This inference seems theoretically sound, because DDTC reacts with Sn(IV) in the same mole ratio [11]. Hence, the conversion factor from SnL4 (L stands for the TDTC anion here) to Sn should be 1/5.928 = 0.1687. With a view to applying this theoretical factor the reaction conditions were optimized as follows. Different temperatures for drying the precipitate to constant mass were tested and their results are shown in Table 1. Evidently 95°C would be the optimum one, whereas low recovery of Sn was found if dried at higher temperatures, presumably owing to thermal decomposition of the precipitate. Drying at B 95°C takes much more time. The optimum volume of the analyte solution for precipitating was found to be 80–100 ml. The precipitate formed at room temperature is difficult to filter. On the other hand, high precipitation temperature favors its filtrability, but accelerates the decomposition of the precipitant. As a compromise, 70°C was specified and found to be optimum. The precipitate should be aged for 15 min at this temperature and filtered only after cooling to room temperature. Otherwise the results may be poorly reproducible. Different amounts of the precipitant were added for precipitating different amounts of Sn(IV) and the results are shown in Table 2. Obviously addition of a large excess of TDTC does no harm. Its optimal amounts were specified to be 1–3 g. One g of it suffices to precipitate quantitatively 5 30 mg of Sn when present alone.

Table 1 Effect of drying temperature on determination of 15 mg of Sn Drying temperature (°C)

Mass of precipitate (mg)

Sn found (mg)

105

87.7 88.3

14.8 14.9

100

88.1 88.5

14.9 14.9

95

89.2 88.9

15.0 15.0

Z. Nan / Talanta 46 (1998) 1237–1243

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Table 2 Effect of amount of TDTC on the determination of Sn Sn added (mg)

TDTC added (g)

Mass of precipitate found (mg)

Mass of Sn found (mg)

15.0

0.5

88.9 89.1

15.0 15.0

30.0

0.8

178.4 177.5

30.1 30.0

75.0

1.7 3.0

443.9 444.3 444.9

74.9 75.0 75.0

3.3. Effects of di6erse ions and preliminary separation As the preparation reaction with TDTC is not selective, most of other cationic species, if present, should be removed in advance. DDTC was reported to precipitative Sn(IV) over the whole pH range of 2–14 [14]. It is surely invalid. As an analog of TDTC, it has the same donor system, hence should behave similarly. Our experiments confirmed this since at pH] 7 no precipitate was found. Accordingly a preliminary group separation can be made by precipitation with DDTC at pH 9 in the presence of tartrate to remove at a single stroke: Ag(I), Au(III), Bi(III), Cd(II), Co(II), Fe(III), Hg(II), Mn(II), Ni(II), Pb(II), Sb(III), Te(IV), Tl(I) and Zn [14]. Although the removal of Se(IV) and Te(VI) is not quantitative, yet only a part of their residual portions would react with TDTC under the specified conditions [14]. Fortunately, they are usually present in very small amounts, hence, their interference might be ignored. The ionic species of platinum metals, if present, would be removed also. Once the filtrate is acidified to pH 2 – 6, the excess of DDTC left would precipitate Sn(IV) immediately. Therefore, it is advisable to overstep this pH region by pouring the alkaline filtrate portionwise to strong acids. The possibly remaining species after this group separation would be: (a) cationic species of alkali and alkaline earth metals; (b) soluble tartrate complexes of B(III), Be(II), Al(III) and Ti(IV). All of them are tolerable even in macro amounts. Interferents are Cr(III), Ga(III), In(III) [14]. They can be masked, however, by EDTA, the excess of

which then reacts with Al(III) to constitute a ligand buffer, as their formation constants lg Kf were reported to be 23.4, 20.3, 25.0 and 16.3, respectively. Zr(IV), Hf(IV), Th(IV), U(VI) and most of the cationic species of rare earth metals would behave similarly, since they also form stronger chelates [15] with EDTA than Al(III). It was found that the precipitation of Sn(IV) runs normally in the presence of this ligand buffer, and that in the presence of Ce(III) or La(III), which itself is a non-interferent, a slight turbidity appears sometimes, and addition of EDTA helps to clear the solution. Nb as an interferent may be present in some Ti alloys. If HCl alone is used for sample decomposition, it would remain undissolved and thus removed from the analyte solution [7]. So do W, Si and the platinum metals [7]. If necessary, the trace amount of Sn which might be occluded in the insoluble matter, can be recovered and determined separately as reported elsewhere [16]. Up to 1% of As may be present in some alloys of interest. As it is not separated by the preliminary group precipitation and reacts with TDTC, it would cause a positive error. Even 0.1 mg of it would increase the mass of the Sn-precipitate by 0.7 mg. Hence, it seems advisable to keep it in the trivalent state and distil off as AsCl3. Ge(IV) is an interferent [14] of rare occurrence. If present, it can be removed together with As. Generally speaking, oxidants can oxidize TDTC, and hence interfere. The only oxidant introduced to the analyte solution is nitric acid. Accordingly its addition should be kept as small as possible. Its remnant can be completely removed by adding Ti(III):

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4H + + NO3− +3Ti3 + “No  +3Ti4 + +2H2O (3) Ti(III) was chosen for this purpose, since it serves to reduce also the higher valence states of As and Sb quantitatively to their trivalent ones, the presence of its excess is indicated by its own color and both Ti(III) and Ti(IV) are non-interferents. If sodium sulfite were used instead, the separation of Sb(III) would be incomplete owing to reoxidation [17]. Moreover, sulfite would react with hydrogen sulfide, one of the acid decomposition products of DDTC [18] to form colloidal elemental sulfur, and hence is the only interferent possibly used among the reductants. The reaction between Mo(VI) and DDTC was reported to take place over the pH range of 2–9 [14]. This statement is questionable, as our experiments showed that it, like Sn(IV), reacts with either DDTC or TDTC at pH56 only. Hence, Mo(VI) would accompany Sn(IV) in the filtrate after the preliminary separation at pH 9. This interference from Mo can not be eliminated by masking with EDTA or hydroxylamine (which was used to mask the precipitation of MoS3 in acid solution) [19] alone, nor as Mo(VI)– NH2OH–EDTA [20]. V, like Mo, would accompany Sn(IV) in the filtrate too. As shown in its predominance-zone diagram, it is at pH 5.0 – 5.5 in the state of vanadyl vanadate, i.e. partly as V(IV) and partly as V(V) [21]. Hence, effects of both V(V) and V(IV) should be studied. It was found that in the presence of EDTA, TDTC does not precipitate either of them at the stated pH to form the same yellow product. The only difference lies in that this precipitation takes place at once in the case of V(V), but slows down somewhat in the case of V(IV). It should be noted that at pH 9 V(V) is the predominate species in the filtrate; whereas at pH 50 V(IV) is [21]. Therefore, subsequent acidification of the alkaline filtrate makes the unstable V(V) an oxidant. Under such conditions V(V) would oxidize DDTC or its decomposition products and thus render the analyte solution colloidal. In order to avoid this additional interference ethanol is purposely added prior to acidifi-

cation and preferentially oxidized. It was chosen since it is very pure, harmless and can be readily boiled off. Attempts were made to eliminate interferences from Mo and V, but failed. Thus, we can not help taking another approach: to precipitate them altogether, determine their co-precipitated amounts and make corrections for them (Eq. (2)). Fortunately, their presence or absence can be readily detected by the naked eye: in their absence the precipitate is pure white; while in their presence it is colored (yellow if V is present and scarlet if Mo present). Their determinations are also an easy matter. Indeed, a clear-cut separation of Mo and V from Sn seems very much difficult, if not impossible, to incorporate conveniently in the proposed method. For anions, some alloys may contain 5 1% of P which is tolerable. Effects of some ionic species on the determination of Sn are shown in Table 3.

3.4. Applications It should be noted that only Sn(IV) forms a precipitate with TDTC in the highest mole ratio of 1:4. Hence, its conversion factor is very small Table 3 Effects of cations on the determination of 30.0 mg of Sn Cation added (mg)

Mass of precipitate found (mg)

Mass of Sn found (mg)

As(III) 2 Bi(III) 100 Cu(II) 200 Fe(III) 30 Ni(II) 50 Pb(II) 200 Sb(III ) 50 Zn(II) 80 Al(III) 200 Ti(IV) 200 Cr(III) 10 In(III) 23 Mo(VI) 5 V(V) 5

177.7 177.5 176.8 177.9 177.2 177.0 178.0 177.4 178.5 178.3 178.1 178.2 200.6 216.9

30.0b 29.9c 29.8c 30.0c 29.9c 29.9c 30.0c 29.9c 30.1 30.1 30.0d 30.1d 30.1a 30.2a

a

Corrected data by Eq. (2). After volatilization. c After DDTC separation. d Masked by EDTA. b

Z. Nan / Talanta 46 (1998) 1237–1243 Table 4 Analytical data of some simulated samples Sample composition (%)

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Table 5 Determination of Sn in some certified reference materials (CRM) Sn found (%)

Cu 5, Sb 12, Sn 73, Sb 10 72.8 As 0.5, Cd 2, Cu 3, Pb 73, Sb 15, Sn 6.5 6.55 Cu 83, Pb 4, Zn 6, Sn 7 6.95 Bi 45, In 10, Pb 25, Sn 20 20.10

(0.1687), even smaller than that of the well known nickel diacetyldioximate (0.2031). The proposed method is very suitable for the determination of 10–70 mg of Sn. Its standard deviation (n= 10) at the level of 20 mg of Sn was found to be 0.08 mg. Its special features may be summarized as follows: 1. Simplicity: ]5% of Sn can be determined by taking 0.2 g of sample or even an aliquot there from. This makes the separation simpler and easier. 2. Selectivity: a single clear-cut precipitation in combination with masking suffices to eliminate most of the interferences and render the determination specific for Sn in most cases. 3. Versatility: provision is made for eliminating interferences in special cases, such as correction for Mo and V. This makes it also applicable to the analysis of Ti alloys. 4. Flexibility: it is flexible in the technique of sample decomposition and simplifications are possible in the absence of certain interferents. Owing to these features the proposed method will find many industrial applications. The results for analysis of some simulated and industrial samples are shown in Tables 4 and 5 respectively.

4. Conclusion A new gravimetric method is proposed for the macro-determination of Sn. It is simple, selective, versatile and flexible, hence would be very promising.

Acknowledgements Grateful thanks are due to all members of the

CRM sample of alloys

Sn found (%)

Its certified value (%)

Al-Snb Cu-base

15.60 18.55 9.83 7.39 8.81 87.7 5.20

15.66 18.47c 9.80a 7.34 8.88 87.6 5.13

Mg-basec Pb-basec Sn-basec Ti – 5Al – 5Sn – 5Zrb

Obtained from: a Bureau of Analysed Samples, UK; b Jinan Metallurgical Institute, China; c SRIM, a well known supplier of CRMs in China since 1952.

Directorate of SRIM for permission to publish this paper.

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