Indicator

173

Analytica Chimica Acta, 281(1993) 173-177 Elsevier Science Publishers B.V., Amsterdam

Indicator for the titrimetric determination of calcium and total calcium plus magnesium with ethylenediaminetetraacetate in water D.P.S. Rathore, PK. Bhargava, Manjeet Kumar and R.K Talra Chemical Laboratory, Atomic Minerals LXvision, Department of Atomic Energy, West Block-VII, RK &ram, New Delhi 110 066 (India) (Received 19th January 1993; revised manuscript received 9th March 1993)

Abstract The use of 2_[(4-phenylthioacetic acidJazoI-1,8-dihydroxynaphthalene-3,6-disulphonic acid as an indicator for the titrimetric determination of both calcium and total calcium plus magnesium is reported. The variables affecting the colour change at the end-point were optimized. The method is free from most metal interferences without the need for potassium cyanide and other masking agents. The method was applied to water samples and the results obtained compared favourably with those given by standard methods.

Keywordx Titrimetry; Azo dyes; Calcium; Indicators; Magnesium; Waters

Calcium and magnesium determinations in water are of importance in hydrogeochemical surveys [1,2] and for determining total hardness, potability and suitability for irrigation [3].

Complexometric determination of calcium and magnesium has been thoroughly investigated and is widely accepted [4,5]. Eriochrome Black T [6], murexide [71, calcon [8,9], the indicator of Patton and Reeder [lo], Arsenaxo I [ll] and other indi-. caters [12] change colour slowly at the end-point.

Moreover, traces of iron, copper, nickel and other metal ions dissolved in water “block� these indicators, thereby preventing a sharp-end point or seriously reducing its sharpness unless potassium cyanide and other masking agents are used.

Potassium cyanide is a potential safety hazard [ill and has to be handled with the utmost care.

Recently, p-aminophenyhnercaptoacetic acid was used as a diazotizable amine for the spectrophotometric determination of nitrite [13-161, the detection and determination of cerium [17], the indirect determination of chromium [18] based on azo dye formation, the identification of vanadium [19] as its schiff base using 2-furfuraldehyde and the rapid detection of vanadium [20] via its azo dye derived from chromotropic acid as coupling agent.

In this work, 2-[(4-phenylthioacetic acid&o]l,%dihydroxynaphthalene-3,6-disulphonic acid (PTAADNDA), an azo dye [15,20], was used for the titrimetric determination of both calcium and total calcium plus magnesium using disodium EDTA as titrant.

OH HOOC-H&--S

Correspcmdence to: D.P.S. Rathore, Chemical Laboratory, Atomic Minerals Division, Department of Atomic Energy, West Block-VII, RR. Puram, New Delhi 110 066 (India).

0003-2670/93/$06.00

Ca1993 - Elsevier Science Publishers B.V. All rights reserved

OH

N=N HO,S PTAADNDA

SO,H

174

D.P.S. Rathore et aL /Anal. Chim. Acta 281(1993) 173-l 77

EXPERIMENTAL

RESULTS AND DISCUSSION

Apparatus Spectral measurements were made with a Varian 634-S double-beam spectrophotometer with l.O-cm quartz cuvettes.

Preliminary experiments were conducted using known aliquots of standard calcium and magnesium solutions diluted to 50 ml with distilled water. Titrations were performed by adding 0.01 M EDTA solution from a lo-ml burette.

Reagents p-Aminophenylmercaptoacetic acid of 98.1% purity (Evans Chemetics, New York) was used as received. All other chemicals were of analyticalreagent grade and standard solutions were prepared by the usual methods [41.

The azo dye indicator PTAADNDA [15,20] was prepared by diazotization of p-aminophenylmercaptoacetic acid and coupling with chromotropic acid in alkaline media, as follows. A 5.5-g amount of p-aminophenyhnercaptoacetic acid was dissolved in 50 ml of hydrochloric acid 1 + 1 and 3.0 g of sodium nitrite were added with constant stirring at 0-5째C. Excess nitrite was destroyed by adding a pinch of sulphamic acid. The resulting salt was coupled by adding a solution of chromotropic acid disodium salt dihydrate (10.70 g in 10% sodium hydroxide), with stirring, while keeping the temperature at 0-5째C. When the addition was complete, stirring and cooling were continued for 30 min, then 40 ml of concentrated hydrochloric acid were added and the mixture was stored in a refrigerator overnight. The precipitate was filtered and washed with dilute hydrochloric acid and absolute ethanol, then dried for 6 h at 105째C (yield 4-5 g).

Axe dye (indicator) solution (O.l%, w/v> was prepared by dissolving 0.1 g of the azo dye in distilled water containing a pellet of sodium hydroxide. This solution is stable for at least 3 months.

Analytical procedures The procedures for the determination of calcium and total calcium plus magnesium were carried out as described [4], using PTAADNDA as indicator for both titrations. The difference between the two titrations gave the magnesium content in the sample.

Calcium The basis of the calcium determination is that magnesium is precipitated as magnesium hydroxide by addition of potassium hydroxide (pH 1213) and calcium forms a stronger complex with EDTA than does magnesium [6], hence calcium remaining in solution is then selectively titrated.

A 4-5-ml volume of 4 M potassium hydroxide solution per 50 ml final volume was the optimum to bring the desired pH to between 12 and 13 and to precipitate magnesium hydroxide virtually completely. After adding potassium hydroxide, the sample solution should be titrated within 3-5 min, otherwise there may be a loss of calcium as calcium carbonate at high alkalinity owing to atmospheric carbon dioxide, thereby resulting in a slightly lower titre for calcium. Any error in the calcium determination will result in an incorrect result for the magnesium content of the sample, although the total calcium and magnesium content remains the same. It was found that 5-7 drops of the 0.1% aqueous solution of the indicator per 50 ml final volume were optimum for the visual observation of the end-point. Near the end-point, a bluish tinge appears fiit, and then sharply changes to reddish violet at the end-point.

Hence the colour change is from orange to reddish violet.

If the calcium concentration is more than 10 mg per 50 ml final volume, then after adding potassium hydroxide some calcium is precipitated as the hydroxide [9]. Vigorous stirring is necessary to dissolve the Ca(OH), as the titration progresses. If the stirring is slow, false end-points are obtained. The return of the colour after each change as the stirring is continued might be due to a slow change of Ca(OH), to free Ca2+, responsible for the colour change. A smaller aliquot of the sample should then be taken and diluted with distilled water.

D.P.S. Rathore et al./AnaL Chim. Acta 281 (1993) 173-177

Total calcium and magnesium At pH = 10 both calcium and magnesium are titrated with EDTA solution. The colour change at the end-point is from reddish to bluish violet.

The end-point is sharper for the magnesium titration. An azo dye indicator concentration of 5-7 drops of 0.1% aqueous solution per 50 ml final volume was the optimum for visual observation of the end-point. A volume of 5 ml of buffer solution, pH = 10, was optimum for a 50-ml final volume.

Absorption spectra of the free azo dye, calcium-azo dye complex and magnesium-azo dye complex are illustrated in Fig. 1.

End-point The visual end-points in both titrations were compared with spectrophotometric titration curves for calcium or magnesium at pH = 10 and calcium at pH 12-13 by titration with 0.1 M EDTA using the azo dye as indicator. The titration curve shows that the sharp colour change occurs near to the equivalence point (Fig. 2).

Effects of foreign ions The effects of various foreign ions that generally accompany calcium and magnesium in water were studied using 5 ml of 1 mg ml- ’ each of

060

r

LOO

500

600

700

Wavelength / nm -

Fig. 1. Absorption spectra for (A) magnesium-azo dye complex; (B) calcium-azo dye complex and (Cl azo dye indicator.

Conditions: calcium (1 mg ml-‘), 5 ml, magnesium (1 mg ml-‘), 5 ml; buffer, pH = 10, 5 ml; PTAADNDA au, dye (O.l%), 1 ml; total volume, 50 ml.

175

,

1

OS0 -

OAO-

0

t

-r=?z

-

.

8 0.30 i g

0.20 -

s

0.10

1 1 0

1

O.&O

0.60

1.20

1.66

2.0

240

260

Volumr of EDTA/ml-

Fig. 2. Spectrophotometric titration curve of calcium by EDTA using PTAADNDA axe dye as indicator against distilled water at 495 mn. Conditions: calcium, 5 mg per 50 ml total volume; EDTA salution, 0.1 M, indicator (O.l%o),1 ml; potassium hydroxide (4 Ml, 4-5 ml; pH, 12-13.

TABLE 1 Effects of interfering ions (50 ml of sample) Interferent

Tolerated amount (pg) a

Copper

10 500 20 100 100 500 500 loo0 500 500 500 loo0 1000 2500 500 40 2000 5000 10000 10000 5ooo 1500 500 2000 5000 No interference No interference

Iron(II1) Cobalt(U) Nickel(I1) ManganeseJII) Barium(U) AIuminium(II1) Strontium(I1) ZindII) Cadmium(U) Lead(H) MercurylII) Chromate(I1) Silicate LJranyl(I1) Vanadium(V) Carbonate Hydrogencarbonate Sulphate Chloride Nitrite Nitrate Fluoride Phosphate Sulphite Sodium salts Potassium salts

a Amount of foreign ions causing a sharp end-point with a drop of titrant.

176

D.P.S. Rathore et aL /AnaL Chim. Acta 281 (1993) 173-l 77

TABLE 2 Standard addition for the determination of calcium contents in tap water (means of fiie analyses, SO-mlsamples) Sample No.

Calcium added per 50 ml (mg)

Calcium found (mg)

Recovery (%)

Magnesium added per 50 ml (mg)

Calcium (mgI

Recovery (%I

1 2 3

0 5 10

1.6 6.5 11.2

98 96

0 5 10

1.6 1.6 1.6

100 100 100

calcium and magnesium per 50 ml final volume at pH 12-13 and 10, respectively, by titration with 0.01 M EDTA solution. Unlike the common interferences due to co(B), NXII), C&I), Fe(III), M&I), Ba(II), Sr(II), Pb(I1) and Z&I) in complexometric methods [6-111, these ions are tolerated to a great extent with the present procedure without the need for potassium cyanide and other masking agents (as is evident from Table 1). This high tolerance can be attributed to the presence of the thio and carboxylic groups, which probably

facilitate chelate formation of these ions. This results in a decreased availability of these ions in solution to interfere in the determination of calcium and magnesium. The concentrations of interfering ions are almost same at pH = 10 and 12-13.

Application of the method In order to check the validity of the method for the determination of calcium and magnesium in water, two sets of experiments were done. In

TABLE 3 Standard addition for the determination of magnesium content in tap water (means of five analyses, 50-ml samples) Sample No.

Calcium added per 50 ml (mgl

Calcium found (mg)

Magnesium added per 50 ml (mg)

Magnesium found by difference (mg)

1 2 3 4 5 6

0 5 10 0 0 0

1.6 6.5 11.2 1.6 1.6 1.7

0 0 0 5 10 20

1.1 1.1 1.2 6.1 11.0 21.3

Recovery (%I

98 96 100 99 101

TABLE 4 Determination of calcium and magnesium in water samples (means of five analyses, 50-ml samples) Sample No.

1 2 3 4 5

Proposed method

Conventional method

Calcium (j.4gml-‘) (direct method)

Magnesium&g ml-‘) (by difference)

Calcium (fig ml-‘) (direct method Using Patton and Reeder’s indicator)

Magnesium @g ml-r) (by difference using Eriochrome Black-T indicator)

28 24 53 24 26

51 24 25 15 11

28 23 51 20 25

52 25 24 18 11

D.P.S. Rathme et al./Anal.

177

Chim. Acta 281 (1993) 173-177

TABLE 5 Comparison of the proposed method with other complexometric determinations

of calcium and magnesium Ref.

Indicator

Potential interferents

’

Eriochrome Black T Murexide Calcon Patton and Reeder’s indicator Arsenaxo I PTAADNDA axe dye

Ba2+ Sr2+ Pb2+ Zn2+ Cu2+ Co2+ Ni’+, Mn2+, Fe3+, A13+ Niz+‘coz; ,2; ~2;, BaZ~, SrZ+:PbZ+ Cu2’, Fe’+: Sr2+,‘Ba2+ Ba2+ Sr2+ Pb2+ Zn2+ Cu2+ Co2+, Ni’+, Mn’+, Fe3+ Mn2; ,2; Ni2; ~2; ,2;

t

,

,

None ’

6 7 899 10 11 This work

a Without the use. of potassium cyanide and other masking agents.

the first, the recoveries of calcium and magnesium were checked by adding varying concentrations of calcium and magnesium to tap water. In the other, various amounts of sample solutions were taken and diluted to constant volume with distilled water. For all samples accurate results were obtained. The results are summarized in Tables 2-4 and show that the proposed method is comparable to the conventional complexometric methods employing E&chrome Black T and the Patton and Reeder indicator.

The proposed method is a variant of the conventional complexometric method. Unlike other indicators, the same azo dye indicator is recommended for the determination of both calcium and magnesium. The sharp end-points and high tolerance to many foreign ions are the advantages of the proposed method.

Table 5 shows the comparative performances of the complexometric methods for the determination of calcium and magnesium. The simplicity and selectivity of the proposed procedure allows the routine on-site determination of calcium and magnesium over a wide range of concentrations.

The authors are grateful to Evans Chemetics, New York, for the gift of p-aminophenylmercaptoacetic acid. Thanks are due to Shri K.P. Cheria, Head of the Chemistry Group, Dr. S. Viswanathan, Director of the Atomic Minerals Division, and Shri Jagmer Singh, Regional Director, for providing the necessary facilities.

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