Talcum, Vol 39, No 7, pp 765-768, 1992 0039-9140/92 $5 00 + 0 00 Pnnted m Great Bntam All nghts reserved Copynght 0 1992 Pergamon Press Ltd

OSCILLOPOLAROGRAPHIC ADSORPTIVE WAVE DETERMINATION OF MICROAMOUNTS

OF ORTHOPHOSPHATE

LIYI CHBN

Department of Chemistry, Jtshou Umversrty, Hunan, People’s Repubhc of China

(Recewed 8 February 1989. Rewed 4 November 1991 Accepted 27 November 1991)

Summary-In the system HCl-Sb(III)-ammomum molybdate-acetonebutanone, IQ-, Sb(II1) and MofVI) form a ternary heteropoly acrd, which can be adsorbed on a dropping mercury electrode and reduced to a heteropoly blue The electrochemtcal reaction produces a senstttve polarographtc wave with peak potential of -0 42 V us SCE. The hmrt of determmatton of phosphate IS 6 x IO-‘M The coeffictents of variance are 10 and 3% for 0 1 and 1pM phosphate, respecttvely

Spectrophotometric determination of mlcro- amounts of phosphate has been recommended as a standard method,’ but because of its low sensitivity, a preconcentration step IS necessary for the determination of phosphorus at pg/ml level. Indirect determmations by polarography and semidifferential electroanalysis have been reported.2 Here we advance an oscillo- polarographic method based on adsorption of a ternary heteropoly acid on a droppmg mercury electrode.

EXPERIMENTAL

Apparatus

A model JP- 1 A oscillopolarographic analyser (made m China) equipped with a dropping mercury electrode was used (drop time 7 set).

Reagents

Acetone-butanone mixture, I:1 v/v. Ketone mixture.

Standard phosphorus solution, 10 pglml. Pre- pared from potassium dihydrogen phosphate.

Medium mixture. Dissolve 0.67 g of potass- mm antimony tartrate and 8.8 g of ammomum heptamolybate tetrahydrate in 677 ml of 6M hydrochloric acid and dilute the solution to the mark in a 1-litre standard flask with water.

General procedure

Transfer about 15 ml of distilled water into a 25ml standard flask, then add 1 .O ml of medium mixture, 4.0 ml of ketone mixture, a known volume of standard phosphorus solution, and dilute to volume with distilled water. Mix well

and transfer to the polarographic cell. Record the polarogram from -0.3 to -0.8 V (vs. S.C.E.) at a scan-rate of 250 mV/sec, with an oscillopolarographic analyser. Typical waves are shown m Fig. 1. Prepare a calibration graph.

Determination of phosphorus in silicate, iron ore and manganite

Weigh out exactly 0.1-0.2 g of sample into a nickel crucible and add 4 g of powdered sodium hydroxide. Cover the crucible and heat it for 15 min in a muffle furnace at 650-700”. Cool the crucible and leach the melt with small portions of hot distilled water, transferring the leachate into a loo-ml standard flask. Clean the crucible with small portions of 6M hydrochloric acid, adding the washings to the flask. If the green colour of manganate appears, add a little ethanol to remove it. Dilute to volume, let any insoluble material settle, then pipette 5 ml of the clear solution into a 50-ml beaker. Add one drop of phenolphthalein solution as indicator and then add 6M hydrochloric acid dropwise until the colour IS discharged. Transfer into a 25-ml standard flask, add 1.0 ml of medium mixture, 1 ml of 50-mg/ml L-ascorbic acid sol- ution to mask iron, 4 ml of ketone mixture, and dilute to the mark. Transfer the solution into the polarographic cell, and record the polarogram as above.

Determination of phosphorus in carbonates

Weigh out accurately about 0.1 g of sample into a loo-ml beaker. Moisten with water, add 10 ml of 6M hydrochloric acid, cover the beaker, boil gently for l-2 min. Then remove

765

LIYI &EN 766

I

-0.42V

/

V Fig 1 Adsorptive wave for the heteropoly acid (1) conven-

tlonal curve, (2) denvatlve curve

the cover and heat gently until the solution has evaporated to about 1 ml. Let cool, then dilute to volume in a 50-ml standard flask with water, mix and let settle. Analyse 5 ml of the clear solution in the same way as the sample solution prepared for silicates etc.

Determination of phosphorus in tea leaves

Weigh accurately about 1 g of finely ground dry tea leaves into a porcelain crucible. Slowly carbonize the sample by heating with a Bunsen burner, let cool, add 0.5 ml of sulphuric acid (1 + 2), heat gently until white fumes appear, then heat for 2 hr in a muffle furnace at 550”. Cool the crucible and dissolve the contents with 5 ml of 2M hydrochloric acid. Transfer this solution to a 250-ml standard flask and dilute to the mark Pipette 1.0 ml into a 25-ml standard flask, add 1.0 ml of medium mixture, 4.0 ml of ketone mixture, dilute to the mark and record the polarogram.

Fig. 2

HCI, M Dependence of peak current on andity

P, 0 40 rg/ml.

0 2466 10 12

Sb (Ill), 10-5M Rg. 3 RelatIonstip of antimony concentration to peak

current. P, 2 pg/ml.

Determination of phosphorus in natural waters

To 20 ml of sample (that has been filtered through 0.45 pm Millipore membrane filter) add 1.0 ml of medium mixture and 4.0 ml of ketone mixture, and record the polarogram.

RESULTS AND DISCUSSION

Selection of conditions

The acidity, the concentration of antimony and ammonium molybdate, and the presence of organic solvents such as ethanol, acetone and butanone were all found to affect the peak height of the polarogram, as shown in Figs. 2-5.

The recommended conditions are 0.16M hydrochloric acid, 8 x lo-‘M antimony(III), 2 x 10d3M molybdenum(VI), and 8% v/v of both acetone and butanone (i.e., 4.0 ml of 1: 1 v/v mixed solvent per 25 ml of solution polarographed).

400 -

0 1 2 3 4 5

MO (VI), 10-3M Fig 4. The effect of concentration of MO on peak current

P, 2 &ml.

Oscdlopolarograptic determmation of orthophosphate 767

I I I I I 0 12 3 4 5

Ketone mixture (ml) Fig. 5. The effect of amount of ketone mixture (1.1 v/v) on

peak currenv P, 0 4 &ml, total volume 25 ml

60-

.= ._/g9 60- a 1

20-

I I I I 0 10 20 30 40

7-K) Rg 6 The effect of temperature on the peak current

P, 0 40 fig/ml

The effect of temperature was also examined. The peak current reaches a maximum of 25” (Fig. 6), and the change in the temperature coefficient indicates that the wave is adsorptive in nature.

Analytical characteristrcs

The concentration of phosphorus is linearly related to peak current up to 0.2 pg/ml, and non-linearly at higher concentrations but a calibration graph is usable over the range up to 1 mg/ml phosphorus.

Recovery experiments with known amounts of phosphorus added to aliquots of a solution obtained from a silicate rock gave the results shown in Table 1.

Results from the analysis of various samples are shown in Table 2.

Table 1 Recovery of phosphorus (4.0 gg ongmally present)

P added, ccg 0.5 1 0 2.0 5.0 100 Recovery, % 94 100 100 103 99

Table 2. Analytical results

P,* %

Sample type A B P,t PM

Silicate 0.043 0.041 Carbonate 0.024 0.023 Sod 0031 0 032 PylYteS 0.185 0.172 Manganese spar 0 030 0 035 Tea leaves 0.30 0.30 Tea leaves 0.40 0.38 Tea leaves 0.28 0.29 bver water 0 13*001 Rwer water 028*001 bver water 0 68 f 0.02 Sea-water 0 13 f 0.003 Sea-water 0 24 f 0.01 Standard sea-water 024&001

*A, polarographlc method; B, reference method I tMean f standard dewatlon.

E#ect of other ions

Zn’+, CdZ+, Co*+, Ni*+, Mn*+, Cr(III), W(VI), V(V), A13+, SiO$-, NO;, Br-, So’,-, in hundredfold mass-ratio to phosphorus, and Sn(II,IV), Fe(II,III) and Cu*+ at twentyfold ratio did not interfere with the determination of phosphorus. Since the Pb(I1) wave is diffusion- controlled, its sensitivity is lower and a tenfold ratio of lead has no influence.

As(V) behaves similarly to P(V), and can react with Sb(II1) and Mo(V1) to form an analogous ternary heterology acid, giving a sharp wave at - 0.37 V, so the method can also be applied to the determination of As(V).

DISCUSSION

The mmimum in the electrocapillary curve at the peak potential (Fig. 7) and the change in

I I I I I I

’ -0.1 -0.2 -0.3 44 -0.5 -0.6

V Fig. 7. ElectrocapilIary curves: (1) HCl 0 16M, Sb(III) 8 x 10-5M, Mo(VI), 2 x IO-‘&f; (2) P(V) 1 x 10-5M+ (1)

768 LIYI CHEN

i I I

1 -040 -0.80

peak current from 72 to 12 ,uA, from which it is concluded that the unreduced ternary hetero- poly acid is adsorbed. Cyclic voltammetry showed a reduction wave at the peak potential but no anodic wave in the reverse scan (Fig. 8), showing that the electrode reaction is irreversible.

Earlier work3 has shown that the molar ratio of the phosphorus, antimony and molybdenum IS 2: 3 : 24 in both the oxidized and reduced forms, so the structure of the complex is un- changed in the electrode reaction.

The organic solvent presumably has a stabil- izing effect on the heteropoly acid, thereby increasing the sensitivity of the method.

REFERENCES

Amencan Pubhc Health Assoaatlon, Amencan Water Works Assoclatlon and Water Pollution Control Feder- ation, Standard Methods for the Examznatron of Water and Wustewater, M J Taras (ed.), 13th Ed., 223D APHA, Washmgton, 1971 M Goto, Y Mwa, H Yoshlda and D. Ishil, Mzkrochtm. Acta, 1983 I, 121 Chen Llyl, J Jrshou CJmu , 1986, No 1, 56

v Rg 8 Cychc voltammetnc curve

1 sign of temperature coefficient (Fig. 6), indicate that the polarographic wave is an adsorptive one.

Continued electrolysis at the peak potential 2 ’ gave a blue colour after 20 min and a blue

deposit after 160 min, with a steady decrease in 3