Spectrophotometric determination of tin in copper-based alloys using pyrocatechol violet

Talanta ELSEVIER

Talanta 42 (1995) 1973-1978

Spectrophotometric determination of tin in copper-based alloys using Pyrocatechol Violet A.C. Spinola Costa*, Leonardo S.G. Teixeira, Srrgio L.C. Ferreira Universidade Federal da Bahia, lnstituto de Quimica, Salvador, Bahia 40170-290, Brazil

Received 30 May 1995; accepted 21 June 1995

Abstract

In the present paper, a new procedure using Pyrocatechol Violet (PCV) for the determination of tin in copper-based alloys is proposed. The use of HEDTA as masking agent allowed tin to be determined in the presence of large amounts of copper, without any separation procedure. The method is more selective than previous methods. Cetyltrimethylammonium bromide (CTAB) and Tween-20 are used to increase the stability of the system. The method can be applied directly to an acidic solution of Sn(IV) in the range 2.0-60.0 lag with a final volume of 50 ml. The pH is adjusted to 2.0 + 0.2 with glycine buffer and, after 30 min, the absorbance is measured at 660 nm. AI(III), Cd(II), Co(II), Mg(II), Ca(II), Mn(II), Ni(II) and Pb(II) do not interfere at the 500 mg level; 20 000 ~tg of Cu(II) and 400 lag of NaC1 can be present. The interference at 100 lag of Fe(III) can be masked with ascorbic acid. Bi(III), Sb(V), Ti(IV), Mo(VI), EDTA, tartrate, citrate and iodide interfere. The proposed method was used for tin determination in several copper-based alloys and a comparison of the analytical results with certified values indicates that the procedure provides accurate and precise results. Keywords: Copper-based alloys; Pyrocatechol Violet; Spectrophotometric analysis; Tin determination

1. Introduction

Tin is an element frequently present in copper-based alloys. However, the speetrophotometric determination of this element in these matrices is problematic because m a n y of the reagents used also react with the copper and other elements present. Numerous procedures have been published for this determination, and normally these methods are not simple and usually require extensive and laborious steps for the separation of tin from the matrices, using procedures that involve operations such as extraction [1,2], ionic exchange [3,4], precipi-

* Corresponding author.

tation of tin as metastannic acid [5], coprecipitation of tin using copper sulfide [6], manganese dioxide [7], or beryllium hydroxide [8] and until distillation [9], the tin being separated as stannic bromide. Table 1 describes the application of some of the reagents during tin determination in copper matrices. Pyrocatechol Violet (PCV) [10] is one of the main reagents used in the spectrophotometric determination of tin. Some procedures [11] were proposed that used gelatin to stabilize the color; however, the addition of gelatin causes the absorption band of the Pyrocatechol Violet-tin(IV) complex to become broadened and to extend to longer wavelengths. Therefore, the absorbance measurements must be made at 619 rim, and a loss of sensitivity is observed.

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1974

A.C. Spinola Costa et al. / Talanta 42 (1995) 1973-1978

Table 1 Reagents used for tin determination in copper and its matrices Reagent

Method of separation of tin

Applications

Reference

8-Hydroxyquinoline Propylfluorone PCV Phenylfluorone Rhodamine 6Zh Gallein Hematein Gallein Silicomolybdotungstate Quercetina PCVb

Extraction Extraction Ion exchange Ion exchange Tin precipitation as metastannic acid Tin coprecipitation with copper sulfide Tin coprecipitation with manganese dioxide Tin coprecipitation with berylium hydroxide Distillation -

Copper and zinc-based alloys Bronzes Bronze Copper-based alloys Brass Copper metal and brasses Copper-based alloys Copper Brass Brass and bronze Brasses and bronzes

[1] [2] [3] [4] [5] [6] [7] [8] [9] [8] This work

a With copper masking with use of thiourea. b With masking using HEDTA. 0,9 0.8

•

•

II

•

•

II

•

•

•

0.7 0.6

8

.~. 0,5

~ 0.4 0.3 0.2 0.1 0 1,4

I

I

I

I

I

I

I

I

1.6

1.8

2

2.2

2.4

2.6

2.8

3

pH

Fig. 1. Effect of the pH on the Sn(IV)-PCV system. [Sn(IV)], 50.00 ~g per 50 mi; PCV amount, I ml; PCV solution, 0.1%. Dagnall et al. [12], studied the effect o f several surfactants in relation to an increase in stability o f the Pyrocatechol V i o l e t - t i n ( I V ) system and f o u n d that cetyltrimethylamm o n i u m bromide ( C T A B ) is the most satisfactory. However, the use o f this surfactant is not possible in the extraction step, because tetraa l k y l a m m o n i u m iodide was precipitated. Pyrocatechol Violet was p r o p o s e d for tin determination in bronze by K a r n a u k h o v a [3], the procedure is based on tin separation on an anionic resin in the chloride form. In the present paper, we propose a procedure that uses P C V for tin determination in brasses and bronzes, using H E D T A as masking agent for several ions, allowing tin determination

without separation. The p r o p o s e d m e t h o d is simple, highly selective and reproducible. A mixture o f two surfactants, cetyltrimethylamm o n i u m bromide and Tween-20, increases the system stability.

2. Experimental 2.1. R e a g e n t s

All reagents were analytical grade unless otherwise stated. The tin solution (100.0 lag ml -~) was prepared by dissolving 0.1 g o f granulated tin metal (Baker) in 20 ml o f concentrated sulfuric acid with heating, and dilution to l l with

A . C Spinola Costa et al. / Talanta 42 (1995) 1973-1978

60 ml of concentrated sulfuric acid and demineralized water. The PCV solution (0.1%) was prepared by dissolving 0.1 g in demineralized water to 100 ml. Tween-20 (1%) was prepared by dissolving 1.0 g in demineralized water to 100 ml. Glycine buffer solution (pH 2.0) was prepared with glycine solution (1.0 M) and the pH was adjusted to 2.0 with hydrochloric acid and/or sodium hydroxide solution. H E D T A solution (0.10%) was prepared by dissolving 0.10g of N-(carboxymethyl)-N'-(2-hydroxyethyl)-N,N'-ethylenediglycine in demineralized water to 100 ml.

2.2. Apparatus The absorption spectra were recorded and measured with a Varian DMS-80 spectrophotometer and an Intralab recorder using 1.0 cm cells. A 300 Analyser pH meter was used to measure the pH values.

1975

Table 2 Effect of the amount of glycine buffer solution (M) Concentration

Absorbance

0.04 0. ! 4 0.20 0.30 0.40 0.50

0.787 0.786 0.795 0.796 0.796 0.797

3. Results and discussion

3. I. Characteristics of the reagent and the complex

2.3. General procedure

The reaction of PCV with the tin(IV) cation in the presence of CTAB and Tween-20 results in a green complex with an absorption maximum at 660nm. The PCV reagent has an absorption maximum at 441 rim. The complex without surfactants is red, with a maximum absorption at 555 nm.

Spectrophotometric determination of tin

3.2. Effect of the pH

Transfer to a 50 ml volumetric flask 1.0 ml of the PCV solution, 2.0 ml of the Tween-20 solution and 2.0 ml of the CTAB solution. Mix and add a portion of tin(IV) solution containing 10-60 lag of tin, 10.0 ml of glycine buffer and 5.0 ml o f H E D T A solution. Mix, and after 30 min, dilute to the mark with water and measure the absorbance at 660 nm in a 1 cm cell, using water as a blank.

The effect of pH on the tin(IV)-PCV system was studied and the results demonstrated that the absorbance signal is at a maximum and constant in the pH range 1.60-2.7 as can be seen in Fig. 1. In the described procedure the use of glycine buffer at pH 2 is recommended because at this pH, the selectivity is higher. A glycine buffer

Table 3 Effect of the order of addition of reagents Order of addition

Absorbance

PCV + Tween-20 + CTAB + Sn(1V) + buffer + H E D T A PCV + CTAB + Tween-20 + Sn(IV) + buffer + H E D T A PCV + Tween-20 + CTAB + Sn(IV) + H E D T A + buffer PCV + Tween-20 + CTAB + buffer + Sn(IV) + H E D T A PCV + Tween-20 + CTAB + H E D T A + Sn(1V) + buffer Tween-20 + CTAB + PCV + Sn(IV) + buffer + H E D T A Tween-20 + CTAB + Sn(IV) + PCV + buffer + H E D T A Buffer + H E D T A + PCV + Tween-20 + CTAB + Sn(IV) Sn(IV) + Tween-20 + CTAB + PCV + buffer + H E D T A Sn(IV) + PCV + Tween-20 + CTAB + H E D T A + buffer Sn(IV) + buffer + PCV + Tween-20 + CTAB + H E D T A Sn(IV) + H E D T A + buffer + Tween-20 + CTAB + PCV Sn(IV) + PCV + HEDTA + Tween-20 + CTAB + buffer Sn(IV) + PCV + H E D T A + buffer + Tween-20 + CTAB Sn(IV) + H E D T A + PCV + Tween-20 + CTAB + buffer

0.802 0.796 0.790 0.720 0.021 0.803 0.789 0.082 0.640 0.606 0.376 0.019 0.015 0.011 0.011

1976

A.C Spinola Costa et al. / Talanta 42 (1995) 1973-1978

Table 4 Effect of the amount of HEDTA on the tin(IV)-PCV system ( I0 -4 M) Concentration

Absorbance

3.6 7.2 10.8 14.4

0.805 0.801 0.799 0.795

Table 5 Analytical characteristics of the procedures

i m e n t a l section 2.3, using water as the blank. Beer's law was obeyed in the c o n c e n t r a t i o n range 0 - 1 . 2 0 lag m l - ~ o f tin a n d the curve does n o t pass t h r o u g h the origin. The m o l a r a b s o r p tivity was 1.03 x 1051mol -~ cm -~ at 660rim. The analytical sensitivity [13], the c a l i b r a t i o n sensitivity [13], the limit o f detection a n d the limit o f q u a n t i t a t i o n [14] as well as other a n a lytical characteristics are s u m m a r i z e d in T a b l e 5.

Characteristic

Value

3.6. Effect of interfering ions

Molar absorptivity (I mol-t cm-t) Calibration sensitivity (ml lag-~) Analytical sensitivity (mi Ixg- t) Limit of detection, CL; 3a (ng ml-t) Limit of quantitation, CQ; 10a (ng ml-t) Linear dynamic range (lagml-i) Coefficient of variation (%)

1.03 x 105 0.867 86.73 7 25 0.02-1.20 1.25

Solutions c o n t a i n i n g 50.00 lag o f tin(IV) a n d various p r o p o r t i o n s o f several cations a n d anions were prepared, a n d the general procedure was followed. T a b l e 6 shows interference levels d u r i n g tin d e t e r m i n a t i o n in the absence a n d presence o f H E D T A . T h e m a i n interferences are v a n a d i um(V), m o l y b d e n u m ( V I ) , t i t a n i u m ( I V ) a n d iron(Ill). I r o n ( I l l ) interference can be eliminated b y r e d u c t i o n to iron(II) with ascorbic acid; 50 m g

c o n c e n t r a t i o n in the range 0 . 0 4 - 0 . 5 0 M does n o t affect the a b s o r b a n c e signal o f the t i n ( I V ) - P C V system (Table 2).

3.3. Effect of the order of addition of the reagents on complex formation T h e order o f reagent a d d i t i o n was studied. This is a critical stage in the reaction. The results d e m o n s t r a t e d that the order strongly affects the complex f o r m a t i o n , as c a n be seen in T a b l e 3. T h e reagent a n d surfactants m u s t be added before the tin(IV) or sample solution, a n d only after this can the p H be adjusted.

3.4. Effect of the HEDTA t i n ( I V ) - P C V complex

amount on the

P C V is n o t a selective reagent for tin determ i n a t i o n . However, the use o f H E D T A as m a s k i n g agent solves this problem. T h e effect o f the H E D T A a m o u n t o n t i n ( I V ) - P C V complex f o r m a t i o n was studied, a n d the results show that it does n o t affect the complex formation when present at the least in the range 3.6 x 1 0 - 4 - 1 4 . 4 x 10 - 4 M (Table 4). T h e H E D T A m u s t be a d d e d after the f o r m a t i o n o f the complex.

3.5. Analytical characteristics of the method A c a l i b r a t i o n curve was prepared according to the general p r o c e d u r e described in the exper-

Table 6 Tin determination with PCV in the presence and absence of HEDTA Cation Amount

Interference level

C~g) Ag(1) 500 Ca(lI) 500 Mg(ll) 500 Ba(ll) 500 Sr(ll) 500 Th(1V) 500 AI(Ill) 500 Mn(ll) 500 Bi(lll) 500 Cd(II) 500 Pb(ll) 500 Sb(V) 500 Zn(ll) 500 Ni(II) 500 Ti(IV) 500 Hg(ll) 500 Co(li) 5 0 0 Fe(Ill) 500 Mo(VI) 500 V(V) 500 Cu(ll) 20000 Cu(ll) 5000 Cu(ll) 500

Without HEDTA

With HEDTA

+ 7.03 - 1.35% -0.90% -0.54% - 1.35% - 1.50% +5.75% +21.43% - 13.15% + 1.99% +0.57°/,, + 7.03% +2.00% +23.43% +209.32% + 5.41% +4.86% - 60.54% + 319.2% --48.51Y¼,

-0.38% -0.62% -0.87 - 1.12% -4.17% -0.13% +0.63% - 1.77% - 1.62% - 1.37% +9.11% + 2.02% -0.25% - 73.28% + 63.72% -- 32.49% + 2.92% + 2.54% + 0.12%

-

+ 3.93%

Conditions: [Sn(IV)], 50.00lag per 50ml; [HEDTA], 7 x 10-4 M, pH 2.00.

A.C. Spinola Costa et al. / Talanta 42 (1995) 1973-1978

The results shown in Table 7 demonstrate that there is no significant difference between the certified value and the value obtained with PCV at the 95% confidence level for four different alloys. Application of the paired t test [15] in the results obtained by the PCV procedure and the ICP method (Table 8) showed that there is no significant difference between the two methods at the 95% confidence level.

Table 7 Tin determination in copper-based alloys (standards) Standard

Brass Brass Brass Brass

NIST 62d NIST 164 C E P E D 486 NIST 52c

Certified value

Value found ~

(%)

(%)

0.38 0.63 2.81 7.85

0.38 0.64 2.82 7.82

+ + + _

0.01 0.01 0.03 0.02

1977

(5) (5) (5) (5)

a At the 95% confidence level. Table 8 Tin determination in copper-based alloys (samples)

4. Conclusions Sample

Brass Brass Brass Brass

40 37 10 1

1CP method

PCV method a

(%)

(%)

0.14 0.90 4.36 5.50

0.13 0.91 4.38 5.50

+0.01 (5) + 0.02 (5) + 0.02 (5) + 0.02 (5)

a At the 95% confidence level.

of ascorbic acid are sufficient for masking 1000 lag of iron(III) during the determination of 50.0 lag of tin(IV). The effect of the ionic strength on the system is negligible up to a sodium chloride concentration of 0.1 M.

3. 7. Application Tin determination in copper-based alloys The proposed procedure was applied to tin determinations in several standards and samples of copper-based alloys. The sample solutions were prepared using nitric and hydrochloric acids. The results are described in Tables 7 and 8, and the matrix compositions are shown in Table 9.

The spectrophotometric determination of tin using Pyrocatechol Violet can be improved with the use of HEDTA as masking agent, and Tween-20 and cetyltrimethylammonium bromide as sensitizing and stabilizing agents. This new proposed method can be used for the spectrophotometric determination of tin in copper-based alloys without carrying out a separation procedure.

Acknowledgments The authors acknowledge the financial support of the CNPq, FINEP and CAPES.

References [1] A.E. Eberle and M.W. Lerner, Anal. Chem., 34 (1962) 627. [2] N.L. Olenovich and G.I. Savenko, Zavod. Lab., 41 (1975) 658; Anal. Abstr., 30 (1976) 1B33. [3] N.N. Karnaukhova, Zavod. Lab., 36 (1970) 1047; Anal. Abstr., 21 (1971) 63.

Table 9 Compositions of the standards and samples analyzed Standard/sample

Brass Brass Brass Brass Brass Brass Brass Brass

NIST ~ 62d NIST ~ 164 C E P E D b 486 NIST ~ 52c 40 37 10 1

Sn

Cu

Ni

Pb

Zn

Fe

(%)

(%)

(%)

(%)

(%)

(%)

0.38 0.63 2.81 7.85 0.14 0.90 4.36 5.50

59.07 63.76 85.86 89.25 58.10 70.78 85.13 86.75

0.28 0.046 0.29 0.76 0.001 0.58 0.33 0.50

0.25 0.22 5.32 0.011 2.45 0.94 4.72 1.97

37.14 21.89 2.87 2.12 39.10 26.65 4.71 4.80

0.86 2.52 0.21 0.004 0.007 0.076 0.211

" National Institute of Standards and Technology, USA. b Centro de Pesquisa e Desenvolvimento da Bahia, Brasil.

1978

A.C. Spmola Costa et al. / Talanta 42 (1995) 1973-1978

[4] VT Kurbatova and V.V. Stepin, Tr. Vses. Nauchn.issled. Inst. Stand. Obraztsov. Spektr. Etalonov., I (1964) 14; Anal. Abstr., 13 (1966) 6802. [5] N.L. Shestidesyatna, N.M. Milyaeva and LT Kotelyankaya, Zavod. Lab., 41 (1975) 653. [6] S. Ambujavalli and N. Premavathi, Anal. Chem., 48 (1976) 2152. [7] R. Tanaka, Jpn. Anal., 10 (1961) 336; Anal. Abstr., l0 (1963) 2161. [8] F.D. Snell, Photometric and Fluorimetric Methods of Analysis, Wiley-lnterscience, New York, 1978. [9] E.F. Tkach, Uch. Zap. Kishinev. Gos. Univ., 68 (1963) 61; Anal. Abstr., 12 (1965) 5662.

[10] Z. Marczenko, Spectrophotometric Determination of Elements, Wiley, New York, 1973. [11] A. Ashton, A.G. Fogg and D.T. Burns, Z. Anal. Chem., 264 (1973) 133. [12] RM. Dagnall, T.S. West and P. Young, Analyst, 92 (1967) 27. [13] D.A. Skoog and J.J. Leafy, Principles of Instrumental Analysis, Saunders College Publishing, Florida, 1992. [14] F.A. Medinilla and F.G. Sanchez, Talanta, 33 (1986) 329. [15] G. Chirstian, Anal. Chem., Wiley, New York, 1980, p. 75.