INFLUENCE OF PRESSURE IN THE 0.1–100 MPa INTERVAL
1077
mal cell with a liquid junction consisting of a measurꢀ trode of original design [1] (such electrodes can work
ing glass electrode and a silver chloride reference elecꢀ for long periods of time independent of pressure):
Ag, AgCl|0.1m HCl|pH membrane|solution under study||3m KCl |AgCl, Ag
Measuring electrode
Reference electrode
°
It should be noted that the glass electrodes were
unloaded with respect to pressure, i.e., the pressure
inside and outside the Н+ꢀsensitive membrane was the
same. Two measuring glass electrodes and two referꢀ
ence electrodes were used simultaneously.
of determining pH (0.01 units). The values logK1
=
–
°
°
– ΔfGP(H3AsO3(sol))/2.303RT are
–(ΔfGP(H2AsO3 )
°
given in Table 2. The error logK1 at elevated pressure
is determined mainly by the error in the accepted
The electrode system was calibrated at 298.15 K
and pressures of 0.1, 20, 50, 70, and 100 MPa on two
buffer solutions: an acetic–acetate solution
°
logK1 value at a pressure of 1 bar.
(0.25m CH3COOH + 0.25m CH3COONa) with
Table 1. pH values of buffer solutions 0.025mHAc + 0.025m
NaAc (A)a 0.01mNa2B4O7 (B) used to calibrate the potenꢀ
tiometric cell (298.15 K)
рН0.1 МPа 4.69, and a borate solution with рН0.1 МPа
9.18 before each experiment. The pH values at
the elevated pressures for the acetate and borate
buffers were taken from [1] and [2], respectively,
and are given in Table 1. Calibration showed that
the standard electrode potential of the cell and the
slope of the straight line in the E–рН coordinates
was –58.69 mV/units рН and was independent of the
pressure over the studied range.
P, MPa
А [1]
В [2]
0.1
50
4.69
4.59
4.56
4.51
9.18
8.91
8.81
8.65
70
100
Table 2. Results from pH measurements of solutions
0.010 NaOH – xm As (OH)3 at 298.15 K and pressures of
RESULTS AND DISCUSSION
m
Table 2 shows the results of рН measurements in
H3AsO3 solutions at pressures of 0.1 to 100 MPa. To
increase the accuracy of our calculations when deterꢀ
mining the pressure dependence of balance constant
Eq. (1), we used not the pH values themselves but their
°
0.1 to 100 MPa, along with the values of logK1
pHexp
= 0.031
pH
–logK1
°
Р, MPa
ΔE
, mV
–
Δ
x
0.1
50
8.950
9.32
9.19
9.15
9.08
changes (ΔрН) upon a pressure increase, as compared
7.2 0.2 0.123 0.003 8.827
10.0 0.2 0.170 0.003 8.780
14.0 0.3 0.238 0.005 8.712
to the pH values at 0.1 MPa (рН0.1). We should note
that in such an approach, the importance of the diffuꢀ
sion potential is eliminated due to a slight change in
the solution composition and the mobility of ions with
an increase in pressure. The рН0.1 values were calcuꢀ
70
100
x
= 0.021
0.1
9.243
9.32
9.19
9.15
9.07
°
lated on the basis of the value logK1 = –9.32 [3].
50
70
7.4 0.3 0.126 0.005 9.117
10.1 0.3 0.172 0.005 9.071
14.5 0.4 0.247 0.007 8.996
The рН0.1 values we measured differ from the calcuꢀ
lated values by less than 0.01, except for one solution
100
(0.03 units pH). The error in determining
ΔpH =
x
= 0.016
pHP – pH0.1 is within the limits of 0.003–0.007. Using
the OptimA computer program [4], we then miniꢀ
mized the difference between the experimental and
0.1
9.526
9.32
9.19
9.14
9.07
50
70
7.5 0.2 0.128 0.003 9.398
10.3 0.3 0.175 0.005 9.351
14.6 0.3 0.249 0.005 9.277
calculated ΔfGP values of the H2AsO–3 ion at three
°
100
pressure values (50, 70, and 100 MPa). The properties
of all other particles in the As–Na–O–H system
under study, except for H2AsO–3 , were taken from the
Note: Δ
E
=
E
–
E
, change in EMF, observed upon a 0.1 MPa to P
P
0.1
exp
increase in pressure; change in ΔpH as calculated from experꢀ
calc
Slop98 base of thermodynamic data [5]. The error
imental values of Δ
E
; the calculated values for pH = pH
+
0.1
–
exp
°
in determining ΔfGP (H2AsO3 ) at a confidence level
°
ΔpH are given; logK1 values at P = 0.1 MPa, as calculated
of 95% is 0.065 kJ/mol at the accepted error
according to [3].
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 84
No. 6
2010
Shikina
