34 Amzlyst, January, 1973, Vol. 98, @@. 34-39 The Polarographic Determination of Tripolyphosphate Ions and of Tripolyphosphate and Nitrilotriacetic Acid in Admixture BY F. AL-SULIMANY AND ALAN TOWNSHEND (Department of Chemistry, University of Birmingham, P.O. Box 363, Birmingham, B16 2TT) Tripolyphosphate ions in amounts from 2 to 20pmol are determined by measurements of the decrease in the height of the cadmium polarographic wave by using a trimetaphosphate buffer at pH 7.0. Up to 6 mol per cent. of orthophosphate or pyrophosphate and 50 per cent. of trimetaphosphate or tetrametaphosphate ions do not interfere. Tripolyphosphate and nitrilo- triacetic acid in admixture can be determined by measuring the reduced height of the cadmium wave and the cadmium - nitrilotriacetic acid complex wave.THE present paper describes attempts to develop a rapid polarographic method for the determination of tripolyphosphate ions in the presence of other polyphosphates. It also describes attempts to determine tripolyphosphate and nitrilotriacetic acid in admixture. One of the most important chemical properties of polyphosphate ions is their ability to form complexes with metal ions, but the polarographic behaviour of these complexes has received surprisingly little attention. Existing polarographic methods for the determination of polyphosphates do not involve complex formation. Cahn and Kolthoff precipitated as little as 9mg of pyrophosphate ions with cadmium, and determined the cadmium content of the precipitate polarographically ; at pH 3.6, orthophosphate did not interfere.Tripoly- phosphate (350 mg) has been precipitated with tris(ethy1enediamine)cobalt dichloride solution and the excess of I precipitant determined polarographically.2 In the present study, the effect of polyphosphate ions on the d.c. polarographic behaviour of cadmium ions has been investigated. DETERMINATION OF TRIPOLYPHOSPHATE BY DECREASE OF CADMIUM WAVE HEIGHT- Initial experiments showed that at pH 7, tripolyphosphate at a concentration of M decreased the height of the polarographic wave of the cadmium ion although no precipitate was formed. The half-wave potential of the wave was unchanged, and no wave appeared at more negative potentials. It was assumed that a stable, soluble complex was formed and that the reduction of the cadmium from the complex occurred at potentials beyond the base electrolyte discharge potential. The metaphosphates examined (trimetaphosphate and tetrametaphosphate) had no effect on the cadmium wave, but pyrophosphate ions at a concentration of 1 0 - 3 ~ gave a precipitate with an equal concentration of cadmium ions.The height of the cadmium wave decreased with increasing tripolyphosphate concen- tration, but in the ammonium acetate buffer solution used initially the decrease was rather small (Fig. 1). This phenomenon was ascribed to competitive complexing of cadmium by ammonia and acetate ions3 In a non-complexing solution (2 x M sodium perchlorate at pH 7.0), a much greater decrease was obtained (Fig. 1). The metaphosphates again had no effect a t the 2 x M concentration level but this concentration of pyrophosphate also precipitated cadmium.Pyrophosphate ion concentrations of greater than 10 mol per cent. of that of tripolyphosphate also gave increased reduction of the height of the wave given by cadmium in the presence of tripolyphosphate (see Table I), although in this instance no precipitate was formed unless a large amount of pyrophosphate was present. Moreover, although the metaphosphates alone did not affect the height of the cadmium wave, they reduced the amount by which tripolyphosphate ions decreased the cadmium wave height (Table I). The metaphosphate effects are relatively small and level out at higher metaphos- phate concentrations. @ SAC and the authors.AL-SULIMANY AND TOWNSHEND 35 F Tripolyphosphate ion concentration/M x Fig.1. Effect of tripolyphosphate ions on the wave height of 1.0 x M cadmium in: 1, 2 per cent. ammonium acetate solution; 2, 2-0 x 10-4 M sodium perchlorate solution; and 3, 4.0 x lo-' M sodium trimetaphosphate solution The above findings suggested the use of a metaphosphate buffer solution to eliminate the interference by metaphosphates by ensuring the presence of a metaphosphate concen- tration that is sufficient to give a constant enhancement to the cadmium wave height. As trimetaphosphate had the greater effect, it was selected for further investigation as a buffer The use of a solution of trimetaphosphate at a concentration of 4 x 10-2 M and at pH 7.0. enabled a concentration of 4 x 10" M of tripolyphosphate to be determined without inter- ference from a less than loA3 M concentration of trimetaphosphate, but the solution became turbid before the polarogram had been recorded completely.A reduction in the concentration of trimetaphosphate (to 4 x M) eliminated this problem. In the latter solution, tripoly- phosphate ions had an increasingly large effect on the cadmium wave height as the pH increased to 7.0. At a pH of more than 7.5 cadmium again gave a precipitate with trimetaphosphate, so that pH 7-0 was selected as the optimum for the determination of tripolyphosphate. TABLE I EFFECT OF POLYPHOSPHATES ON CADMIUM WAVE HEIGHT IN THE PRESENCE oI; TRIPOLYPHOSPHATE AND 2 x 10-4 M SODIUM PERCHLORATE SOLUTION AT p~ 7.0 Tripolyphosphate concentration/M x 10-4 r 1 2.5 5.0 10.0 A - I \ - Pyro- Pyro- Tetrameta- Trimeta- phosphate phosphate phosphate phosphate concen- concen- concen- concen- trationl Wave tration/ Wave trationi Wave tration/ Wave M x 10-6 height/cm M x height/cm M x heightlcm M x heightlcm 0 6.5 0 5.7 0 5-7 0 3.9 2 6.6 6 5.6 1 6.2 1 4.3 4 6.1 8 6.2 10 6.2 2 4.6 6 6.7 10 4.7 100 6.2 5 4.6 The maximum concentrations of polyphosphate in the final solution that could be tolerated in the determination of various amounts of tripolyphosphate under the recom- mended conditions are summarised in Table 11.Orthophosphate and pyrophosphate ions cause serious interference if present in amounts greater than 6 mol per cent. of the tripoly- phosphate concentration.36 AL-SULIMANY AND TOWNSHEND : POLAROGRAPHIC DETERMINATION [Analyst, Vol. 98 M could be tolerated in the determination of a concentration of 4 x low4 M of tripolyphosphate.Nitrilotriacetic acid, which has been proposed as an alternative detergent to tripolyphosphate, decreased the cadmium wave both in the presence and absence of tripolyphosphate ions. This effect is described in more detail below. A nitrilotriacetic acid concentration of about 5 x 1 0 - 5 ~ is the maximum that can be tolerated in the determination of a concentration of 4 x 10-4 M of tripolyphosphate without correcting for the nitrilotriacetic acid concentration as described below. Chloride and sulphate ions at concentrations of M and less than 5 x Higher sulphate concentrations decreased the cadmium wave height. It was also established that calcium and magnesium in concentrations of less than M, respectively, had no effect.TABLE I1 APPROXIMATE MAXIMUM TOLERABLE CONCENTRATIONS OF POLYPHOSPHATES I N THE DETERMINATION OF TRIPOLYPHOSPHATE Maximum toleration/M x A Tripolyphosphate r 7 concentration/ Pyro- Trimeta- Tetrameta- Ortho- M x 10-4 phosphate phosphate phosphate phosphate 2.0 0.7 5 7 - 4.0 2 60 50 3 8.0 3 60 60 - A typical calibration graph is given in Fig. 1, line 3. The standard deviation for six determinations of tripolyphosphate concentration between 2.85 x and 8.1 x low4 M was 0-2 x M. Although a number of interferences could not be eliminated, the method proved to be satisfactory for the rapid analysis of solid tripolyphosphate samples. The method was tested on polyphosphate samples previously analysed by an ion-exchange - molybdenum-blue procedure and the results are summarised in Table 111.All were within one standard deviation of the results obtained by the reference method. TABLE I11 ANALYSIS OF POLYPHOSPHATE SAMPLES Sample content, per cent. of pyrophosphate* tripolyphosphate* 0.7 64.1 0.7 53-2 0.8 62.2 1.2 62.2 1.4 62.6 1-0 62.4 0.0 41.9 A f \ Tripolyphosphate found, per cent. 64.8 63.6 63.6 63.6 63.6 64.2 41-9 1.3 per cent. of tripolyphosphate is approximately equivalent to 0.1 cm of wave height. * Calculated as phosphorus pentoxide. DETERMINATION OF TRIPOLYPHOSPHATE AND NITRILOTRIACETIC ACID IN ADMIXTURE- Nitrilotriacetic acid forms a complex with cadmium that shifts the half-wave potential from -0.58 V for free cadmium ions to -1.10 V. In the presence of an excess of cadmium, two waves are observed, E+ = -0.58 and -1.10 V (see Fig.2). The increase in height of the wave at -1.10 V with increasing nitrilotriacetic acid concentration has been used by a number of workers4-' as the basis for the determination of small amounts of nitrilotriacetic acid. When tripolyphosphate ions are also present the wave at -0.58 V is further depressed. The total wave height is also reduced somewhat, but unexpectedly the height of the cadmium - nitrilotriacetic acid wave is increased (Fig. 2). The heights of both waves obtained for various nitrilotriacetic acid and tripolyphosphate concentrations are given in Table IV. The results in Table IV show that any given combination of nitrilotriacetic acid and tripolyphosphate concentrations below certain limits gives rise to a unique pair of wave heights.A graph of the first wave height against the second wave height can be plottedJanuary, 19731 OF TRIPOLYPHOSPHATE IONS AND NITRILOTRIACETIC ACID (C1 (dl (4 ( f 1 Fig. 2. Effect of tripolyphosphate ions on the wave of 1.0 x M cadmium in the presence of 5.0 x M ; (d), 6.0 x lo-"; (e), 8.0 x 1 0 - 4 ~ ; and (A, 1.0 x M. All waves start a t -0.4V. Sensitivity = 7 pA full-scale deflection, when using a ,drop-life timer M nitrilotriacetic acid: (a) 0.0 M ; ( b ) , 2.0 x lo-' M ; (c), 4.0 x 37 and the points of equal nitrilotriacetic acid concentration and of equal tripolyphospliate concentration joined by tie lines, as illustrated in Fig. 3. Such a graph can be used as a calibration graph for the determination of nitrilotriacetic acid and tripolyphosphate in admixture in any combination of concentrations up to 5 x M of each component.TABLE IV EFFECT OF NITRILOTRIACETIC ACID AND TRIPOLYPHOSPHATE ON CADMIUM WAVE HEIGHTS Wave heigh t/cm Tripol yphosphate Nitrilotriacetic acid r-----h-----7 concentration/M x concentration/M x Wave 1 Wave 2 0.0 2.0 4.0 6.0 8.0 10.0 0.0 2.5 6.0 0.0 2.6 6.0 0.0 2.6 5.0 0.0 2.5 5.0 0.0 2.5 5.0 0.0 2.5 5.0 9.0 0.0 6.9 0.4 4.6 2.4 6.8 0.0 4.3 1.1 2.2 3.3 4.5 2.9 1.1 3.1 1.4 0.45 1.8 0*56 0.4 0.0 1.7 4.0 0.0 2.3 4.5 0.0 2-86 4.1 0.8 0.0 0.6 2.5 0.6 3.9 EXPERIMENTAL APPARATUS- A standard calomel reference electrode was used. REAGENTS- Wilson (Mfg) Ltd. ; other chemicals were of analytical-reagent grade. Polarograms were obtained on a Polariter PO4 polarograph fitted with a drop-life timer.Polyphosphate solutions were prepared from sodium salts provided by Albright and38 AL-SULIMANY AND TOWNSHEND : POLAROGRAPHIC DETERMINATION [Andyst, Vol. 98 PROCEDURE FOR THE DETERMINATION OF TRIPOLYPHOSPHATE I N A SOLID SODIUM POLY- PHOSPHATE SAMPLE- Dissolve 0.2 g of the sodium polyphosphate sample (containing less than 5 mol per cent. of orthophosphate or pyrophosphate and less than 50 per cent. of trimetaphosphate or tetra- metaphosphate) in 50 ml of distilled water. Transfer by pipette 0.8 ml of this solution into the polarographic cell, followed by exactly 2.0 ml of cadmium nitrate solution (1.0 x M), 10.0 ml of trimetaphosphate solution (8.0 x lo4 M, pH 7-0), and 5.2 ml of water. Adjust the solution to pH 7.0 (by using a pH meter) with 1.0 x M perchloric acid, noting the volume of acid added, and make the volume up to exactly 20 ml with water.After passing nitrogen through the solution for 2 minutes, record the cadmium polarographic wave, starting at -0.40 V v e i s m S.C.E. and measure the wave height. Determine the tripolyphosphate con- centration from a calibration graph obtained by applying the above procedure to standard tripolyphosphate samples. PROCEDURE FOR THE DETERMINATION OF TRIPOLYPHOSPHATE IN AQUEOUS SOLUTIONS- 4 x lo-' mol of tripolyphosphate. Use the above procedure to analyse 6.0 ml or less of solution containing not less than Height of second wave/cm 0.0 1.0 2.0 3.0 4.0 5.0 6.0 Fig. 3. Relation between height of first and second cadmium waves ( E , = -0.66 and - 1.08 V) for various amounts of tripolyphosphate and nitrilo- triacetic acid.Solid lines : concentrations of nitrilotri- acetic acid are M x and broken lines: concen- trations of tripolyphosphate are M xJanuary, 19731 OF TRIPOLYPHOSPHATE IONS AND NITRILOTRIACETIC ACID 39 PROCEDURE FOR THE DETERMINATION OF NITRILOTRIACETIC ACID AND TRIPOLYPHOSPHATE IN Dissolve 0-5g of the solid mixture (containing less than 25 per cent. of nitrilotriacetic acid) in 50 ml of water. Transfer into the polarographic cell, by pipette, 0.8 ml of this solution and follow the procedure given above for tripolyphosphate. Measure the height of both polarographic waves and obtain the nitrilotriacetic acid and tripolyphosphate concentrations from a calibration chart similar to Fig.3, prepared from the results obtained with suitable standard mixtures subjected to the procedure. As for tripolyphosphate alone, aqueous solutions can be analysed by taking a sample of 6.0 ml or less, containing at least mol each of nitrilotriacetic acid and tripolyphosphate, the exact limits depending on the relative concentrations (Le., smaller amounts of nitrilo- triacetic acid can be determined when the tripolyphosphate concentration is relatively high). The authors thank Professor R. Belcher for his interest and encouragement. They also thank Mr. I. L. Jones and Mr. S. Greenfield, Albright and Wilson (Mfg) Ltd., Oldbury, for their advice and for the provision of polyphosphate samples. F. Al-Sulimany thanks the University of Birmingham for the provision of research facilities and the University of Riyadh, Saudi Arabia, for financial support. REFERENCES ADMIXTURE- 1. 2. 3. 4. 5. 6. 7. Cahn, G., and Kolthoff, I. M., Ind. Engng Chem., Analyt. Edn, 1942, 14, 886. Golubchik, E. M., Zav. Lab., 1969, 35, 926; Analyt. Abstr., 1970, 19, 2193. Andress, K. R., and Nachtrab, R., 2. anorg. allg. Chem., 1961, 311, 22. Asplund, J., and Wanninen, E., Analyt. Lett.. 1971, 4, 267. Daniel, R. L., and LeBlanc, R. B., Analyt. Chem., 1959, 31, 1221. LeBlanc, R. B., Ibid., 1959, 31, 1840. Farrow, R. N. P., and Hill, A. G., Analyst, 1965, 90, 241. Received January loth, 1972 Accepted September 21st, 1972