Masking of manganese in relation to stepwise complexometric determination of calcium, magnesium and manganese

693

SHORT COMMUNICATIONS 7do~itu. Vol.3. pp 6Y3 695

MASKING OF MANGANESE IN RELATION TO STEPWISE COMPLEXOMETRIC DETERMINATION OF CALCIUM, MAGNESIUM AND MANGANESE B. C. SINHA and Analytical

(Received

Chemistry

Division,

19 December

S. DASGUPTA

Central Glass and Ceramic Calcutta-32, India

1977. Reoised

Summary-A critical study has been made of alkaline medium for complexometric titration there is incomplete oxidation to manganese(III) or air is bubbled through the solution at 35 reduced with ascorbic acid for titration with magnesium, manganese and calcium in silicate end-points.

17 April

Research

1978. Accepted

Institute,

2 June

1978)

the masking of manganese with potassium cyanide in of calcium and magnesium. It has been found that unless the solution is aged for a sufficient period k 5” for 10 min. The manganese(lI1) complex can be EDTA. Procedures are given for stepwise titration of materials. Mixed indicators are used, to improve the

Calcium. magnesium and manganese are often found present together in ores, slags and certain ceramic products such as enamels and amber glasses. Determination of calcium and magnesium in such materials is difficult owing to interference by manganese. Therefore, prior separation of manganese with hydrogen sulphide, solvent extraction or mercury cathode electrolysis is usually an essential step for gravimetric or titrimetric determination of calcium and magnesium. Manganese also interferes with the EDTA titration of calcium and magnesium by precipitating as manganese hydroxide, which absorbs metallochromic indicators. Thus masking or separation of manganese is again essential. The literature on the masking of manganese is confusing. The use of TEA (triethanolamine) was first proposed by PFibil ‘,’ for masking up to 3 mg of manganese; with larger amounts the manganese(IIItTEA complex formed by aerial oxidation interferes by imparting a green colour to the solution and then slowly forming manganese hydroxide. The addition of reducing agents such as ascorbic acid, tartrate or hydroxylamine hydrochloride keeps the manganese in the bivalent state, which is co-titrated3 with calcium and magnesium. Povondra and Pfibi14 used TEA and potassium cyanide in strongly alkaline medium in the absence of iron for complexing up to 80mg of manganese, as Mn(CN)z-. The sum of calcium and magnesium was then found by adding excess of EDTA and backtitrating. Yotsuyanagi er aL5 also masked manganese with TEA and cyanide, but titrated calcium and magnesium directly. Manganese was then determined by a back-titration method after reduction to Mn(I1) with ascorbic acid. One of the main limitations of both methods4,5 is that aluminium and iron interfere in the back-titration methods and must be separated first. We have found6 that positive errors occur in the Yotsuyanagi method because of incomplete oxidation of Mn(II), unless the concentration of manganese is low (I.0 mg/250 ml). Asaoka’ reported that in 0.2M sodium hydroxide/0,2M TEA solution as medium, manganese(II1) (10m3M) is slowly reduced to manganese(I1) (within 10 hr) at high cyanide concentration (0.4M) but not at low (0.02M). We have now observed that at cyanide concentrations up to O.lM, no significant reduction of Mn(II1) to Mn(I1) takes place in alkaline TEA medium during 6 hr. and so there is no effect on EDTA titration of calcium at pH 12.5. However, when the pH

is lowered to 10 for titration of Ca + Mg, positive errors are obtained because of reduction of Mn(II1) to Mn(I1) unless it is kept oxidized either by aging or passage of air. Potassium cyanide in conjunction with hydrogen peroxide was used by Fogs for masking manganese in EDTA titration of magnesium. The use of hydrogen peroxide in alkaline medium was reported’ to be objectionable because it oxidized EDTA in the presence of iron. Further it was found6 that the peroxo-cyano-complex of manganese could not be demasked for subsequent titration of manganese with EDTA in presence of ascorbic acid. West’ rightly described the masking of manganese as unsatisfactory. PFibil and Vesely” preferred separation of manganese with hydrogen peroxide in alkaline medium and titration of calcium and magnesium in the filtrate with EDTA or DCTA. Thus the literature reveals no completely satisfactory procedures for masking manganese at pH 10 for titration of Ca + Mg with EDTA. The present communication reports the results of studies on quantitative masking of manganese for direct EDTA titration of calcium and magnesium and for demasking manganese from its complex for direct titration with EDTA without involving separation of aluminium and iron. EXPERIMENTAL Reagents

All reagents grade.

used were of analytical

or general

reagent

Mixed indicator I.’ ’ Prepared by mixing 0.08 g of Naphthol Green, 0.05 g of o-cresolphthalein complexone and 20 g of ammonium chloride. Calcein indicator. Calcein (0.2 g) mixed with potassium chloride (20 g). Thymolphthalein, 0.1% in 3094 TEA solution. Mixed indicator II.” Eriochrome Black T (0.03 g), Titan Yellow (0.04 g) and Naphthol Green (0.01 g) dissolved in 25 ml of TEA. Preparation

of silicate

sample solutions

A dried (105-l 10”) and weighed (0.5 g) sample is moistened with water in a platinum basin and treated with 10 ml of hydrofluoric acid and 1 ml of sulphuric acid

694

SHORT

VJMMUNICATIONS

(18N). The mixture is heated on a sand-bath till fumes of sulphur trioxide are evolved. This treatment is repeated with a further IO ml of hydrofluoric acid (after cooling) and the solution is evaporated to dryness. The residue is fused with 2-3 g of potassium hydrogen sulphate, cooled and dissolved in 5”,, hydrochloric acid (iron, titanium or zirconium, if more than 5 mg present, are removed by precipitating as hydroxides with ZOO<,hexamine solution and then filtering). The solution is diluted to 250 ml in a standard flask.

A 25ml or other suitable portion of the solution is diluted to about 100 ml with water. 20 ml of 30”1,; TEA solution are added and the solution is thoroughly mixed. Then a mixture of 30 ml of buffer (pH IO) and 30 ml of IO’:, potassium cyanide solution is slowly added with swirling. The solution is diluted to 200 ml, air is bubbled through it at 3@40” for IO min and then calcium and magnesium are titrated with EDTA, 0.2 g of mixed indicator I being used. The end-point is indicated by a sharp change of colour from pink to green. 3O

After the titration of Ca and Mg. 2-3 g of ascorbic acid are added and dissolved. Manganese(l1) is then titrated with EDTA, a few drops of mixed indicator II being used. The titration is done slowly up to the first change of colour from pink to blue. The solution is then warmed to about 40’ and if a pink colour reappears is titrated to a permanent blue.

A 25-ml or other suitable portion of solution is diluted to 150 ml. Thymolphthalein solution (2 ml) is added followed by 10 ml of TEA added slowly with swirling. Then I M potassium hydroxide is added dropwise till the solution turns blue or green. Then 5 ml of 10:; potassium cyanide solution and I5 ml of 5N potassium hydroxide are added, followed by titration with EDTA in presence of 0.2 g of calcein indicator; the end-point is shown by a colour change to red with disappearance of the green fluorescence RESLLTS

AVD

DISCLrSSIOI\I

It is well known that in alkaline medium the manganese(I1) cyanide complex is titrated with EDTA along with alkaline earth elements whereas the manganese(II1) cyanide complex. being very stable, is not. The conflict of opinion about masking of manganese at pH IO with potassium cyanide alone or in conjunction with TEA is caused by the incomplete oxidation to the manganese(II1) cyanide complex. as discussed in the introduction. The effect of

30

0

I 5

I IO

I 15 Time,

I 20 min

Fig. I.

I 25

I 30

I 35

+

I

I

I

I

I

5

IO

15

20

25

30

Time,

1

min

Fig. 2. time and temperature on the oxidation was therefore critically studied. The results (Fig. I) indicate that solutions aged for at least 15 min at 3(f40’ (curves C, D and E) give correct results for magnesium in presence of managanese (13.7 ng), indicating that the latter is quantitatively oxidized to form the manganese(II1) cyanide complex. When the aging temperature is raised to 50” or above, erratic results (curve F) are obtained owing to hydrolysis to form a manganese hydroxide precipitate. When the temperature of aging is lowered much below 25” (curve A) the rate of oxidation is too slow to be analytically useful. Therefore. 30-40” is the optimum temperature range. The period of aging needed increases. however, with the concentration of manganese, e.g., 15 min for up to 28 mg/200 ml (Fig. 2, curve A) and 30 min for up to 56 mg/200 ml (curve B). When the concentration is much above 56 mg/200, the Mn(I1) cyanide complex slowly hydrolyses before it is all oxidized. To increase the rate of oxidation. and check the hydrolysis. air was bubbled through the solution at 35 + 5” and it was observed that up to I10 mg of manganese could be quantitatively oxidized within IO min (curve C), compared to 55 mg in 30 min (curve B) without air bubbling. The oxidation behaviour at pH 10 was found to be the same whether borate, ammonia or sodium hydroxide/TEA medium was used and irrespective of whether the pH was raised or lowered to pH IO. The literature does not reveal whether TEA is essential for the oxidation. The experiments were therefore repeated without the addition of TEA and similar results were obtained, indicating that TEA is not essential. TEA is. however, found to be effective and useful for removal of the interference due to aluminium in EDTA titrations of alkaline earth elements and for keeping the manganese in solution at high pH (2 12). During the oxidation of large amounts of manganese in ammonia-ammonium chloride medium. an interesting effect was the gradual formation of a pink complex which faded away before the oxidation was complete. The same phenomenon was not observed, however, when sodium hydroxide or borate buffer (pH IO) was used. This suggests formation of an intermediate mixed ligand amminecyanide complex. The oxidized complex can be reduced with ascorbic acid for EDTA titration. These observations on the masking of manganese with cyanide have been utilized for working out the procedure described here for stepwise titration of calcium plus mag-

SHORT COMMUNICATIONS

Table

I. Determination

of calcium. magnesium and manganese tions containing 5 mg of iron

Mg

Mg found.

@?

taken, my

40.27 20.04 IO.00 2.0 I 10.00 2.05 40.09 9.93 2.11

10.49 20.98 10.49 5.25 5.25 1.05 10.49 1.05 5.25

IO.39 20.85 IO.59 5.28 5.33 0.93 10.35

Ca taken.

Ca found.

nrg 40.08 20.04 10.02 2.00 IO.02 2.00 40.08 10.02 2.00

mg

nesium and then manganese with EDTA. Eriochrome Black T is oxidized by the manganese(III) cyanide complex. so cannot be used for the titration of Ca + Mg. ~Cresolphthalein complexone is not suitable for EDTA titration of Mn(II). ft has been observed, in agreement with earlier workers,” that the use of calcein alone does not give a satisfactory end-point for calcium in the presence of a substantial amount of magnesium hydroxide percipitate. This is overcome by the addition of thymolphthalein,

Table

2. Determination of CaO, MgO and MnO and enamel samples

Sample Vitrate CaO

Vitrate CaO MgG MnO Enamel CaO MgO MnO

Found 0, /D

*Certified values 0: 10

1.95 1.60 4.80

2.10 I .53 4.69

7.90 5.09 10.31

7.83 5.21 10.29

7.22 0.30 3.60

7.15 0.32 3.51

15.55 1.16 14.37

15.64 1.18 14.25

8.99 3.78 4.00

9.01 3.75 4.15

Mn taken. r%?

Mn found,

26.92 26.92 26.92 26.92 13.46 5.38 5.38 2.69 2.69

26.75 26.90 26.78 26.89 13.50 5.45 5.38 2.65 2.60

solu-

rng

which improves the end-point and yields satisfactory results. The procedures given do not require prior separation of aluminium, even when this is present in large amounts. However, more than a few mg of titanium or zirconium will precipitate as the hydroxide, which adsorbs the indicator, and iron (rS mg) seriously affects the end-point. Therefore iron, titanium or zirconium, if present in more than 5-mg amounts. require at least semiquantitative separation. The results of determinations of Ca. Mg and Mn in synthetic solutions and in a few glass and enamel samples compare favourably with the certified values (Tables 1 and 2). Ack,lowludgenlent-The authors are thankful to Mr. K. D. Sharma. Director of the Institute. for giving permission to publish the paper.

REFERESCES

glass(H)

MgO MnO Coloured cao MgO MnO

in glass

1.O? 5.37

in synthetic

glass(I)

MgO MnO Vitrate CaO

695

I.

glass

glass(III)

* MnO by conventional bismuthate method, CaO MgO by EDTA after separation of Mn by H,S.

and

2. Idem. ibid., 1954, 19, 465. 3. T. S. West, Comple.~ometr~ with EDTA atld Related Reagetrrs, p. 106, BDH Poole. 4. P. Povondra and R. Pfibil. Colfectiotr Czech. Cizem. Commtm., t 96 I. 26, 3 I 1. 5. T. Yotsuyanagi. T. Yamaguchi. K. Goto and M. Nagayama. Burtseki Kuguku. 1967. 16, 1056. 6. B. C. Sinha and S. Dasgupta, unpublished work. 7. H. Asaoka, Burrseki Kagaku, 1963. 13, 1144. 8. H. M. Fog, Acta Cltetn. Sound.. 1968, 22. 791. 9. B. C. Sinha and S. K. Roy. 1. I~Lv. Chem. I&a. 1974. 46, 19. IO. R. Pfibil and V. Veseij. Clretnisr-Aizrrl~~Ft, 1966, 55, 68, I t. R. M. Sales. British Glass ftzdusrry Research Association, Techtzicul Note No. 6, 1957. 12. B. C. Sinha and S. K. Roy. Analysf. 1973. 98, 289. 13. Glass Techno/.. 1963. 4. 109.