Journal homepage: www.fia.usv.ro/fiajournal
Journal of Faculty of Food Engineering,

Ştefan cel Mare University of Suceava, Romania
Volume XII, Issue 1 – 2014, pag. 34 - 37

ION- SE LEC TIV E E LE C TRO DES BA SE D ON CAL CIUM

HYD ROXYL APA T IT E AS A TOOL FO R ANALYS IS OF V AR IOUS

EN VIRON M ENTA L OBJEC TS , FOOD A ND R AW MAT ER I AL S

Volodymyr DIICHUK1, Mariia VOROBETS1, *Igor KOBASA1

1Yu. Fedkovych National University of Chernivtsy, Ukraine, imk-11@hotmail.com

* Corresponding author
Received February 19th 2014, accepted accepted March 24th 2014

Abstract: A method of the potentiometric determination of Ca2+ and HPO42- ions in some food and
raw products using the original ion-selective electrodes with calcium hydroxylapatite has been devel-
oped. The plasticized ion-selective electrodes were constructed of calcium hydroxylapatite on PVC
substrate. Investigation of the electrode characteristics proved the linear parts on the Ca2+ and
HPO42- V-A dependencies. The slope coefficients also were calculated for the both ions. Preliminary
treatment of the electrodes in the Ca2+solutions results in significant changes in the both slope coeffi-
cients, which approach to the theoretical values.

Keywords: food; environmental objects; ion-selective electrodes; calcium hydroxylapatite

1. Introduction

Deep modernization of the food processing
branch is required in order to enhance its
competitive advances and to mitigate con-
sequences of the economical crisis, indus-
trial depression and decrease in the labour
productivity. A series of actions such as
economical changes along with technical
reequipment, development of resource-
and energy-saving technologies and pro-
cesses, ensuring thorough control of the
food and raw materials quality should be
taken to support this strategy.
Various physico-chemical and biological
methods are widely involved in such ana-
lytical technologies.
Potentiometry of the environmental ob-
jects, food and raw materials quality using
the ion-selective electrodes is an advanced
and powerful method of investigation. It is
express and accurate method that is capa-

ble to work with samples from one-two
tenth to 1 ml, usually no sample pretreat-
ment is needed, the method can be applied
directly to the turbid mixtures, viscous
substances without filtration, distillation or
extraction. This way, the method is highly
suitable to be employed in the automatic
and well-controllable mode [1, 2]. Howev-
er, new solid-ion selective electrodes with
high endurance and stable output signal
should be designed in order to perform an-
alytical determination of some compounds
including ions Ca2+ and HPO42-.
One of the advantageous methods of syn-
thesis for such electrodes presupposes
formation of the electroactive layer direct-
ly from calcium hydroxylapatite in course
of its synthesis.
Calcium hydroxylapatite (Ca-HOA) can be
found in nature and also synthesized. This
is the basic phosphate Са10(РО4)6(ОН)2, it

Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava
Volume XII, Issue 1 – 2014

Volodymyr DIICHUK, Mariia VOROBETS, Igor KOBASA, Ion-selective electrodes based on calcium hydroxylapatite
as a tool for analysis of various environmental objects, food and raw material, Volume XIII, Issue 1 - 2014, pag. 34 – 37

is non-toxic and almost dissoluble com-
pound [3].
This paper deals with investigation of pos-
sible utilization of the hydroxylapatite as
the electrode-active substrate for designing
of the ion-selective electrodes.

2. Experimental

A method of chemical depositions from the
solutions of calcium nitrate and ammonia
hydrophosphate at pH=9-10 according to
reaction 10Са(NO3)2 + 6(NН4)2НРO4 +
8NН4ОН = Са(РO4)6(ОН)2↓ + 20NН4NO3
+6Н2O has been used to synthesize the
stoichiometric calcium hydroxylapatite.
The crystals growth and deposition took
place gradually then the sediment was
washed by water, then ethanol and dried at
105 0C.
The electrode has been made using PVC as
substrate. This substance was dissolved in
cyclohexanone. Then the powder of pure
Ca-HOA was mixed with the 10 % solu-
tion of PVC to form a suspension with the
ratio Ca-HOA:PVC = 1:50. This suspen-
sion was applied on the graphite rod and
then dried at the room temperature during
a day and night. If required, the above op-
erations could be repeated several times.
A series of the sample solutions containing
of Ca(NO3)2 and (NH4)2HPO4 with concen-
trations of the both compounds ranged
from 10-5 to 10-1 mol/dm3 has been pre-
pared to investigate some electro-analytic
parameters of the system.
Two types of the electrodes were used to
find a pattern of dependence of the Ca-
HOA electrode’s potential on concentra-
tion of ions Ca2+ and HPO4

2-. First type of
electrode was untreated while the second
one was kept in the solution of Ca(NO3)2
and (NH4)2HPO4 with concentrations of the
compounds 10-3 mol/dm3 for a day and
night prior the investigation.

3. Results and Discussion

It is known [4] that the electrode-active
compounds should comply with some re-
quirements: have as low as possible value
of solubility product; exhibit ionic type of
conductance and be non-toxic and easily
accessible. Ca-HOA comply with all these
requirements: its solubility product is
1.0· 13410 [5], high ionic conductivity was
reported in [6] while easy and cheap syn-
theses were also shown. Toxicity investi-
gation of the substance proved its complete
safety [7]. It can be expected that the Ca-
HOA based electrode should be sensitive
to the both types of ions: calcium and
phosphates. That is why these ions were
selected to find experimentally a character
of the electrode potential dependence on
the ions’ concentration.
Dependence for the first type electrode
(without preliminary keeping in the solu-
tions) on Ca2+ concentration is shown in
Fig. 1. Three different parts with different
slopes can be identified in this line. The
first and the third parts exhibit low slope
meaning that the electrode potential de-
pends on Ca2+ concentration insignificantly
within the corresponding ranges of Ca2+
concentrations.

-5 -4 -3 -2 -1

0,12

0,14

0,20

0,22

0,24

lg[Ca
2+

]

E, V 1
2

Figure 1. Dependence of the equilibrium poten-
tial of the plasticized electrode on concentration

of Ca2+: 1 – Type 1 electrode (untreated); 2 –
Type 2 electrode (pretreated).

Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava
Volume XII, Issue 1 – 2014

The second part lies from concentrations
10-4 to 10-2 and exhibits the slope coeffi-
cient 0.0145 V/decade, which is close to
the theoretical value 0.01475 V/decade
calculated for the 4-charge ions. Since the
experimental dependence was determined
for the double-charged ions Ca2+, the slope
coefficient had to be close to the value
0.0295 V/decade. The significant discrep-
ancy fixed in our experiments can be
caused by the effect of some side ions on
the electrode potential.

-5 -4 -3 -2 -1
0,10

0,12

0,14

0,16

0,20

0,22

0,24

lg[HPO
4
2-]

E, V 1
2

Figure 2. Dependence of the equilibrium poten-
tial of the electrode on concentration of HPO4

2-:
1 – Type 1 electrode (untreated); 2 – Type 2 elec-
trode (pretreated in the 10-3 mol/dm3 solution of

Ca(NO3)2).

An ion-selective electrode can usually be
kept before the usage in the solution con-
taining the respective ions in order to en-
hance its characteristics [8]. The curve 2 in
Fig. 1 represents results of investigation of

such pretreated ion-selective electrode af-
ter the day and night long keeping in the
10-3 mol/dm3 solution of Ca(NO3)2. The
curve shows a dependence of the electrode
potential on Ca2+ concentration.
It can be seen that the slope coefficient of
the curve 2 is different and equal to 0.022
V/decade within 10-5–10-3 mol/dm3 (com-
pare with 0.0145 V/decade for the untreat-
ed electrode).
Therefore, it can be concluded that pre-
treatment of Ca-HOA electrode makes an
effect on its slope coefficient bringing it
closer to the theoretical value for the dou-
ble-charged ions (0.0295 V/decade).
Fig. 2 represents similar dependencies of
the electrode potential on the phosphate
ions concentration. This dependence has
two parts for the untreated electrode (Fig.
2, curve 1). The electrode potential chang-
es insignificantly within the first part (con-
centration of HPO42- 10-5–10-3 mol/dm3)
and slope coefficient is 0.0035 V/decade.
In the second part ranged from 10-3 to 10-1
mol/dm3 HPO42-, the slope coefficient is
0.015 V/decade. Similarly to the previous
case, these values are different from the
theoretical value for the double- and triple
charged ions (0.0295 V/decade and 0.0197
V/decade respectively).

Table 1
Linear ranges in dependencies of the Ca-HOA electrode potentials on concentrations of Ca2+ and

HPO4
2- and corresponding slope coefficients (S)

Potential de-
terminating

ion

Ca-HOA-electrode
Untreated After pretreatment (kept in 10-3

mol/dm3 Ca(NO3)2
Concentration

range, mol/dm3
S,

V/decade
Concentration

range, mol/dm3
S, V/decade

Ca2+ 10-4–10-2 0.0145 10-5–10-3 0.0220
HPO42- 10-3–10-1 0.0150 10-3–10-1 0.0225

The curve 2 represented in Fig. 2 repre-
sents a dependence of the electrode poten-
tial on concentration of HPO42- for the pre-
treated electrode. It can be seen that this

dependence is linear within the range of
HPO4

2- concentration 10-3–10-1 mol/dm3
and the slope coefficient is 0.0225

Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava
Volume XII, Issue 1 – 2014

V/decade (compare with 0.015 V/decade
for the untreated electrode).
Therefore, the electrode pretreatment in-
fluences its characteristics in the solutions
of Ca2+ and HPO42- and brings values of
the slope coefficients closer to the theoreti-
cal magnitudes.
Another samples of the plasticized Ca-
HOA electrode have been pretreated in the
10-3 mol/dm3 solution of (NH4)2HPO4 ac-
cording to the above mentioned methodics.
No influence of such pretreatment on the
electrode characteristic in solutions of Ca2+
and HPO42- was registered.
It can be summarized, that this method of
potentiometric determination of Ca2+ and
HPO42- ions concentration can be applied
to find contents of these ions in some natu-
ral objects, raw and food samples.
The plasticized electrodes based on the
PVC substrate and Ca-HOA has been de-
veloped and their electrode characteristics

for the Ca2+ and HPO42- solutions were
found, (Table 1).

4. Conclusion

Untreated electrodes show the linear de-
pendence of the electrode potential on con-
centration of Ca2+ ranged from 10-4 to 10-2
mol/dm3 with the slope coefficient 0.0145
V/decade. After pretreatment in the Ca2+
solution, the same electrode also showed
the linear dependence with the slope coef-
ficient 0.022 V/decade within the Ca2+
concentrations range 10-5–10-3 mol/dm3.
Similar parameters for the HPO42- solu-
tions were 0.015 V/decade within 10-3–10-1
mol/dm3 (untreated electrode) and 0.0225
V/decade within 10-3–10-1 mol/dm3 (treat-
ed electrode).
Therefore, Ca-HOA can be used in the ion-
selective electrodes for the food and raw
materials analysis methods.

5. References

[1]. DARST R., Ion-selective Electrodes. Mir,
Moscow. (1972).
[2]. Electroanalytical Methods. Theory and Prac-
tice. (Ed.: F. Scholz). Binom, Moscow. (2006).
[3]. ORLOVSKY V., SUKHANOVA G.,
EZHOVA Z.H., RODICHEVA G., Hydroxylapatite
Bioceramics. Bulletin of the USSR Chemical Socie-
ty, 36, 683-688 (1991).
[4]. VLASOV Y., BYCHKOV. E., A Composition
of the Chalcogenic Membrane for Determination of
Cadmium Ions. Journal of Analytical Chemistry,
40, 1067-1070 (1985).

[5]. ZHYBAREV A, E. ZHARYKOV, AHMET-
SHIN. E., A Method of Synthesis of Hydroxylap-
atite. Russian Federation Patent 2505479.
[6]. KIBATLTSEV V., KOMAROV V., Express
Synthesis of Hydroxylapatite Crystals. Journal of
Inorganic Chemistry, 25, 565-567 (1980).
[7]. FILIPPENKO V., ZYMAN Z., MEZENTSEV
V., Use of the Hydroxylapatite Ceramics in the
Musculoskeletal Systems Surgery. Orthopaedics,
Traumatology and Prostetics, 3, 141-142 (2000).
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