Journal of Analytical Chemistry., Vol.55, No. 11, 2000, pp. 1055-105Z Translatedfrom ZharnalAnaliticheskoi Khimii, Vol.55, No. 11, 2000, pp. 1176-1178.
Original Russian TextCopyright9 2000 by Alekseev, Gorelog Kornilov.
ARTICLES
Membrane Electrodes Selective
for Hydrogen Phosphate Ions 1
V. G. Alekseev, I. P. Gorelov, and M. V. Kornilov
Tver State University, Sadovyi per. 35, Tver, 170002 Russia
ReceivedDecember17, 1999;in finalform,March 22, 2000
Abstract--The possibility of using plasticized polyvinyl chloride film membranes with dispersed electroactive
substances (MgNH4PO4, BiPO4, and CrPO4) for developing electrodes selective for hydrogen phosphate ions
has been investigated. It is shown that a chromium(III) phosphate-based electrode is characterized by the linearity range 10-L10-6 M of the hydrogen phosphate function with a slope of 26.5 mV/pc and a satisfactory
selectivity in the presence of chloride, nitrate, sulfate, and hydrogen carbonate ions.
The determination of phosphoric acid salts is among
the first problems that arise in the analysis of natural
and waste waters. The spectrophotometric and gravimetric methods commonly used for determining phosphates are labor intensive and require laboratory conditions [1, 2]. In natural and waste waters, that is, in
media close to neutral, soluble phosphates commonly
occur as the hydrogen phosphate ion. Therefore, the
development of a simple and highly sensitive electrode
selective for hydrogen phosphate and suitable for direct
potentiometric analysis under both laboratory and field
conditions is needed.
only the electroactive substance (EAS) but also the
matrix material can affect the electrode properties [9].
Therefore, we decided to return to experiments with
solid heterogeneous membranes using an alternative
matrix material.
The goal of this work was to study the possibility of
developing an electrode selective for hydrogen phosphate ions based on a slightly soluble phosphate dispersed in a polyvinyl chloride matrix plasticized by
dioctyl phthalate. As the electroactive substance, we
used MgNH4PO4 (electrode I), BiPO4 (electrode II),
and CrPO4 (electrode III).
Only a few studies on phosphate-selective electrodes have been reported so far in the scientific literature. The following electrodes were suggested: an electrode with a liquid membrane based on organotin com-
EXPERIMENTAL
pounds (response to the HPO 2- and H2PO 4 ions) [3],
electrodes with solid membranes based on an Ag2SP2S3 alloy (response to the H2PO4 ion) [4] and a mixture of silver and bismuth(Ill) oxide reduced by hydrazine in a Teflon matrix (response to the PO 3- ion) [5],
a metallic cobalt electrode (response to the H2PO 4 ion)
[6], and an enzyme electrode with response to the PO4aion [7]. These electrodes exhibit satisfactory analytical
characteristics; however, they have not gained wide use
so far for various reasons. Experiments were also performed with electrodes based on slightly soluble phosphates dispersed in a paraffin or synthetic rubber
matrix, which are simpler to manufacture. A review of
these publications is given in the monograph [8]. The
author noted that all the electrodes studied were of low
selectivity. At the same time, there is evidence that not
Ipresented at the V All-RussianConferencewith the Participation
of CIS Countries on Electrochemical Methods of Analysis
(EMA-99), Moscow,December6-8, 1999.
Preparation of electroactive substances. Magnesium ammonium phosphate. A required amount of
1 M KEHPO4 was added to a solution containing 1 M
MgCI 2, 1 M NH4C1, and 1 M NH 3. A white curdy
MgNHaPO4 precipitate formed.
Bismuth(Ill) phosphate. A weighed portion of bismuth was dissolved in cone. HNO 3, and 1 M KEHPO4
was added. A white BiPO4 precipitate formed.
Chromium(llI) phosphate. A 1 M Cr(NO3) 3 solution was added to 1 M KzHPO4. A violet CrPO4 precipitate formed.
The precipitates obtained were washed by decantation, separated by centrifugation, and dried at a temperature of 80~ After these operations, CrPO4 passed
from the violet form to a green one.
Preparation of membranes. A weighed portion of
polyvinyl chloride (PVC) was dissolved on heating in
cyclohexanone, and a suspension of an electroactive
substance in dioctyl phthalate was added. The solution
was stirred until complete homogenization, poured into
a Petri dish, and dried in air until complete evaporation
of cyclohexanone. An elastic plasticized PVC film
about 0.5 mm thick with the electroactive substance
1061-9348/00/5511-1055525.00 9 2000 MAIK"Nauka/Interperiodica"
ALEKSEEV et al.
1056
Table 1. Electrochemical and analytical ~roperties of electrodes
Electrode
EAS
EAS pK,
[10]
Electrode function
slope, mV/pc
Electrode function
linearity range, M
Detection
limit, M
Response
time, s
I
MgNH4PO4
12.6
10.7
1001-1005
4x100 5
90
II
BiPO4
22.9
85.0
1001_10-5
6 x 10--6
60
III
CrPO 4
22.6
26.5
100L10-6
5x 10~
60
dispersed in it was obtained. The membrane composition was as follows: PVC, 52.5%; dioctyl phthalate,
45.0%; electroactive substance, 2.5%.
Preparation of electrodes. A disc 1 cm in diameter
was cut from the finished membrane. This disc was
pasted to an end of a PVC tube 1 cm in diameter and
10 cm in length with a solution of PVC in cyclohexanone. At the other end, the tube was closed with a
tightly fitted polyethylene stopper with a silver wire
covered with silver chloride that was passed through
the stopper. The electrode was flooded with a solution
containing 0.1 M KCI and 0.1 M K2HPO4 . Before performing measurements, the electrode was soaked in
0.1 M K2HPO4 for no less than 3 h.
All the potentiometric measurements were performed at 20~ in reference to a saturated calomel electrode using an 1-135 potentiometer with an ion-selective electrode.
To study the electrode function, a stock 0.5 M
K2HPO4 solution was prepared from an accurately
weighed portion; solutions with concentrations from
10-1 to 10-7 M were prepared by successively diluting
the stock solution. The selectivity of electrodes was
estimated by the separate solution method [9].
RESULTS AND DISCUSSION
The potential of each electrode was constant over
the pH range 6-10, which almost corresponds to the
region of existence of the HPO 2- ion. The electrode
function (Table 1) is linear over the range 10-1-10-5 M
for electrodes I and II and over the range 10-1-10 -6 M
for electrode III.
It is evident that all the electrodes are characterized
by low detection limits and short response times. Thus,
the linearity range for electrode III is from one to three
orders of magnitude wider and the detection limit is an
order of magnitude lower than for the electrodes
described in the literature [3-8]. This provides a reliable determination of HPO42- at a level of the maximum permissible concentration, which is equal to 3.5 x
10 -5 M [11]. The slope of the electrode function was
close to the theoretical one only for electrode III. The
selectivity coefficients of the electrodes relative to the
inorganic ions most commonly occurring in natural and
waste waters (CI-, NO~, HCO 3, and SO42-) are presented in Table 2.
Electrode I is characterized by very low selectivity
and is hardly suitable for the analysis of real multicomponent samples. The selectivity coefficients of electrodes II and III are acceptable, though they are worse
by approximately an order of magnitude than those of
the electrodes described in the literature. It is likely that
the selectivity of the electrodes proposed can be
improved by varying the ratio of membrane components.
Thus, electrode III (based on CrPO 4) showed the
best electrochemical and analytical properties; therefore, this electrode may be recommended for practical
use.
REFERENCES
Table 2. Potentiometric selectivity coefficients relative to
foreign anions (An)
t eo, A.
Electrode
CI-
so42-
NO~
HCO~
I
1.29
1.98
4.67
3.31
H
0.50
0.40
0.49
0.32
III
0.52
0.41
0.70
0.39
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Analysis: Determination of Nonmetals), Moscow:
Khimiya, 1974.
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JOURNAL OF ANALYTICALCHEMISTRY
Vol. 55
No. 11
2000
MEMBRANE ELECTRODES SELECTIVE FOR HYDROGEN PHOSPHATE IONS
5. Moskvin, L.N. and Ushenko, V.G., USSR Inventor's
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JOURNAL OF ANALYTICALCHEMISTRY
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1057
9. Koryta, J. and Stulic, K., lontove-Selektivni Elektrody
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