Theoretical Prediction of the Water Activity of Standard
Saturated Salt Solutions at Various Temperatures
DORA KITIC, DENISE C. PEREIRA JARDIM, GUILLERMO J. FAVETTO, SILVIA L. RESNIK, and JORGE CHIRIFE
ABSTRACT
A theoretical approach was used to predict the water activity (a,) at
15°C and 35°C of selected saturated salt solutions used as standards
in the range of microbial growth (20.57-0.97).
For this purpose a
rigorous thermodynamic model to predict osmotic coefficient was used.
The results agreed very well with experimental measurements at 15°C
and 35°C for most salts studied, which included NaBr, NaCl,
(NH&S04,
KCl, BaC12, and KzS04. For KNOs the agreement at the
higher temperature was somewhat less satisfactory and the reasons for
this behavior are discussed.
INTRODUCTION
IT IS WELL KNOWN that adjustment of the water activity
(a,) of foods is an important method of controlling spoilage
by microorganisms (Troller and Christian, 1978). This has been
recognized by regulatory agencies in several countries who
have incorporated the a, principle in the definition of standards
for various preserved foods.
The U.S. Food & Drug Administration has adopted the water
activity specification in the definition of low-acid foods (FDA,
1973) as has the FAO/WHO Codex Alimentarius Commission
(FAO/WHO, 1979) and the European Community for the trade
of meats products (EEC, 1976). The reliability of most a,
measuring devices such as electric or fiber-dimensional hygrometers, strongly depends on proper calibration using reference
sources of a,. Saturated salt solutions are used by a large
number of people as a convenient way to provide solutions of
known a,. They are reproducible reference standards because
no measurement of concentration is needed and if the salts are
properly chosen no interfering vapors are present (Stoloff, 1978;
Gal, 1981).
Considerable data and several reviews are available on the
a, of standard salt slurries (Stokes and Robinson, 1949; Wexler and Hasegawa, 1954; Young, 1967; Rockland, 1960; International Critical Tables, 1926; Labuza et al., 1976; Greenspan,
1977). Unfortunately, most reports in the literature do not agree
on the exact a, of each saturated salt solution. Considerable
doubt has arisen about the accuracy of water activity of saturated salt solutions in view of the recent review by Greenspan
(1977). His work reflects very well the uncertainties of literature values from various workers. Recently, Resnik et al.
(1984) reported the results of a world survey of a, values at
25°C of selected saturated salt solutions used as standards in
the range of a, 0.57-0.97, which is of most interest to microbial growth. Their results confirmed the discrepanciesbetween
laboratories regarding the correct a, to assign to each salt. The
Authors Kitic, Favetto, and Chirife are with PRO/PA (CONICETFCEyN), Depto. de Industrias, Facultad de Ciencias Exactas y
Naturales, Univ. de Buenos Aires, 1428 Buenos Aires, Republica
Argentina. Author Pereira Jardin is with lnstituto de Tecnologia
de Alimentos (ITAL), Campinas, S.P., Brasii. Author Favetto is a
member of CONICET, Argentina. Author Resnik is a member of
Comisi& de lnvestigaciones
Cientificas de la Provincia de Bue-
nos Aires, Argentina.
survey was limited to 25°C but it was expected that the discrepancies would be even larger if other temperatures were
considered. It is obvious that for different regulatory agencies
to adopt an a, specification for foods, and for different researchers to reproduce each others a, measurements, there
must be an “universal” agreement on the values to be assigned
to reference standards of saturated salt solutions. This agreement should include not only 25°C but also other temperatures
of interest of microbial growth.
The purpose of this study was to use a theoretical approach
based on the thermodynamic properties of strong electrolyte
aqueous solutions to calculate the a, of selected saturated salt
solutions in the temperature range 15-35°C. The study covered
the range of a, of most interest to microbial growth (=0.570.97). The predicted results were compared with experimental
data obtained with an electric hygrometer previously calibrated
with unsaturated salt solutions for which agreement on their
a, at 15°C and 35°C existed.
MATERIALS
& METHODS
Materials
Saturated salt solutions were prepared by mixing a large excess of
each salt and distilled water and stored at 15°C (?0.2”C) or 35°C
(kO.2”C) in a forced convection low-temperature incubator for various days before using. Seven reagent grade salts were used: sodium
bromide, sodium chloride, ammonium sulfate, potassium chloride,
barium chloride, potassium nitrate and potassium sulfate. These salts
have been used extensively as reference standards of water activity in
the range of interest to the present study. Unsaturated solutions of
sodium chloride and lithium chloride were prepared using anhydrous
reagent grade salts for calibration of the hygrometer at 15°C and 35°C.
Measurement
of a,
The a, of the six saturated salt solutions at 15°C and 35°C was
measured using a Novasina Humidat-TH2 Thermoconstanter hygrometer (Novasina AC, Ch-8050 Zurich, Switzerland). This instrument
has a built-in accurate temperature control device which maintains the
sensor and the sample at constant temperature in the range 0-50°C.
The hygrometer was operated following the procedure described in
detail by Kitic et al. (1986). Each a, measurement represents the
average of three determinations. As shown previously (Kitic et al.,
1986), three replicates are enough to obtain a precision of f 0.005
a, when using the Novasina Thermoconstanter for measuring the a,
of saturated salt solutions in the range of interest of this study. To
measure the a, of saturated salt solutions at 15°C and 35°C the instrument was first calibrated at these temperatures using other standards for which agreement exists on their a, at 15°C and 35’C. Chirife
and Resnik (1984) and Kitic et al. (1986) proposed the use of unsaturated solutions of NaCl and LiCl as isopiestic standards for calibration of a, measuring devices at various temperatures and showed that
there was excellent agreement (theoretical and experimental values)
on the exact value of the a, of these solutions; thus these values were
used for the calibration of the Novasina Thermoconstanter at 15°C
and 35°C. Table 1 shows the data used for the calibration; at least six
points were used at each temperature. NaCL solutions are useful as
standards only for values of a, of about 0.75 and above (saturation
value); for this reason LiCl solutions were used as standards for lower
a, values. Equilibration times for measuring a, of each saturated salt
were previously determined and ranged between 30-90 min.
Volume 51, No. 4, 1986JOURNAL
OF FOOD SCIENCE-1037
a, OF SALT SOLNS AT VARIOUS TEMPS . . .
Table l--Unsaturated
salt solutions used as isopiestic standards for calibration of the Novasina Thermoconstanter
hygrometer at 15°C and 35°C
Salt
LiCP
Concentratio+
% (w/w)
a,
a,
(15°C)
(35°C)
0.505
0.536
0.568
0.597
0.598
0.612
0.732
0.744
0.745
0.748
0.756
0.770
25.97
25.00
23.98
23.00
22.98
22.51
18.01
17.49
17.48
17.36
16.98
16.39
0.521
0.550
0.581
0.609
0.609
0.623
0.739
0.751
0.751
0.754
0.762
0.775
NaCIc
Concentratior@
% (w/w)
(15°C)
0.795
0.808
0.821
0.833
0.850
0.857
0.864
0.872
0.889
0.906
0.920
0.929
0.948
0.962
0.986
0 Kitic et al. 0966)
bg of anhydrous salt per 1OOgof solution
CChirife and Resnik (1964)
RESULTS & DISCUSSION
Theoretical prediction of the a, of aqueous
strong electrolyte solutions
The thermodynamic properties of aqueous strong electrolyte
solutions have been extensively studied both experimentally
and theoretically. Excellent examples may be found in the
standard monographs by Hamed and Owen (1958) and Robinson and Stokes (1965) or in the more recent works of Scatchard
(1968), Scatchard et al. (1970), Pitzer (1973) and Bromley
(1973). One of the most useful and accurate theoretical models
to predict the a, of salt solutions is that of Pitzer (1973), (Pitzer
and Mayorga, 1973). Pitzer (1973) developed a system of
equations (ion-interaction model) for the thermodynamic properties of strong electrolytes which yield agreement within experimental error up to concentrations of several mol/kg (in
numerous cases very close to saturation). It is interesting to
note that the fit was particularly good for l-l and 2-l electrolytes, which are those of concern in the present study.
The osmotic coefficient, 4, is defined as
-55,51 In a,
Vi ml
(1)
where vi is the number of particles into which each solute of
molality mi dissociates. The osmotic coefficient is given by
the equations developed by Pitzer (1973) as:
BMX
\ - I
+ m*2 03/2
V
cMx
(2)
where, w and ux are the number of M and X ions in the
formula and ZM and Zx give their respective charges in elec1038-JOURNAL
OF FOOD
p
0.0973
0.0785
0.0409
0.04835
0.2628
-0.0816
0.04995
in Eq. (2) for some selected
pll,
0.2791
0.2664
0.8585
0.2122
1.49825
0.0494
0.77925
CMX
0.00116
0.00127
-0.00116
- 0.00084
-0.01938
0.00660
__-
salt sommaxb
4
8
5.5
4.8
1.8
3.8
0.7
a Pitzer and Mayorga (1973)
b Maximum molality for which agreement with experimental data was attained to
0.01 in $ (or for which data were available).
tronic units; also v = u, + x; the other quantities have the
form:
(35°C)
0.795
0.808
0.821
0.833
0.852
0.859
0.867
0.874
0.891
0.908
0.921
0.930
0.949
0.963
0.986
4=
NaBr
NaCl
NW,)304
KCI
BatI&
KNOB
K.S.0,
of the parameters
aw
a,
23.50
22.50
21.50
20.50
19.02
18.38
17.70
17.01
15.25
13.42
11.98
10.71
8.25
6.18
2.44
Table 2-Values
lutions at 25”Cs
SCIENCE-Volume
51, No. 4, 1986
BMX =. p& + p& exp (- ~1I)“2
(0) (1)
pMx, pMx and CMx are parameters which are fixed for each
electrolyte, A, is the Debye-Hiickel coefficient for the osmotic
function, I is the ionic strength % Z miZi*; b is equal to 1.2
for all solutes and (Y = 2.0 was found to be satisfactory for
all solutes considered by Pitzer and Mayorga (1973).
The ion-interaction model of Pitzer (1973) has been very
successful in describing osmotic results, both at 25°C (Pitzer
and Mayorga, 1973) and at elevated temperatures (Silvester
and Pitzer, 1977; Holmes and Mesmer, 1983) for different
electrolyte solutions.
Table 2 shows .the values of the parameters which describe
the a, at 25°C of the salt solutions considered in this work,
namely NaBr, NaCl, (NH4)*S04, KCl, BaC12, KN03 and
K2S04, as reported by Pitzer and Mayorga (1973). These values were mostly obtained from careful isopiestic deterrninations of vapor pressures in such a way that they would allow
accuracy to the third (or even fourth) decimal figure in predicted a,. In order to predict the a, of the corresponding
saturated salt solution at 25”C, one only needs to have a precise
knowledge of the molality at saturation (m,), and then the a,
at saturation may be calculated through the relationship
(aJsat = exp( - 4 0.018 m,v)
(3)
Tran and Lenzi (1974) demonstrated that Pitzer’s equation may
be safely extrapolated even to predict a, of various supersaturated salt solutions. Chirife et al. (1983) successfully applied
Pitzer’s model to predict the a, of saturated NaCl, (NH&S04,
KCl, BaC12, KN03, and K2S04 at 25°C.
a, of saturated salt solutions at temperatures
other than 25°C
Chirife et al. (1983) have shown that the a, of certain saturated salt solutions at 25°C may be predicted accurately. In
order to predict the a, at different temperatures two aspects
have to be taken into account: (1) the effect of temperature on
the salt solubility, usually, an increase of temperature will
increase the amount of soluble salt thereby decreasing a, if
other temperature effects are absent; (2) the thermodynamic
effect of temperature on a, at constant concentration of the
solution; this im lies that the temperature dependence of parameters p(O), p( P) and C MX in the predictive equation (Eq. 2)
must be known. Thus, the effect of a temperature change on
the a, of a saturated salt solution will be the result of these
two factors.
Silvester and Pitzer (1977) extended the ion-interaction model
(Eq. 2) for the calculation of activity and osmotic coefficients
of NaCl in a wide range and applied the equations to an extensive array of the thermodynamic data of aqueous NaCl from
0” to 300°C. The general forms to describe the temperature
Table 3-Prediction
of a,,, of NaCI solutions
Coefficients for Eq. (4), (5) and (6)
q1
q2
q3
q4
%
0,
=
=
=
=
=
=
at different
0.0765
- 777.03
- 4.4706
0.008946
-3.3158
x 10-e
0.2664
temperatures*
6.1608 x IO-5
1.0715 x 10-e
0.00127
33.317
0.09421
-4.655 x 10-s
=
=
=
=
=
=
q9
%o
911
cl12
q13
0.”
-
* Silvester and Pitzer (1977)
Table 4-Prediction
of a, of KCI solutions at different
Coefficients for Eq. (7)
p(l)
pw
Pl
P2
P3
P4
P5
on
0.04808
-758.48
-4.7062
0.010072
-3.7599 x 10-s
___
temperaturese
-
CMX
-7.88 x 1O-4
91.270
0.58643
-0.0012980
4.9567 x lO-7
_--
0.0476
303.9
1.066
--_
___
0.0470
B Holmes and Mesmar (1983)
dependenceof B(O),B(l) and C&x for NaCl solutions are given
as:
p(O) = qt + 92 (: - f)
= q6 + q9(T
-
Salt
NaBr
NaCl
W ’W,SOa
KCI
BaCI,
KNO,
KzSQ,
=
$1
+
%2($
-
T,) + qlo(T’
-
‘L2,
(4)
Table S-Solubility
oeratures
NaBr
q13 ln f
+
qdT
-
Tr)
(6)
NaCI
r
standard
mbmax
9
6
___
4.5
1.8
2.4
0.1
46.58
26.35
42.53
24.64
25.68
20.50
9.21
it
f
k
k
f
f
+
0.10
0.03
0.17
0.20
0.10
0.15
0.04
salt solutions
at various
tem-
Solubilitya
salt par IOOg of solution)
25°C
35°C
(g of anhydrous
15°C
Temp
I%)
15
25
35
15
25
35
where Tn is 298.15”C and the values of the parameters
qr . . . . q14 are given in Table 3. These values, along with
Bq. (3), (4), (5), (2) and (1) allow a rigorous and accurate
prediction of the a, of NaCl solutions for the whole concentration range (up to saturation) and any desired temperature.
The values of the Debye-Htickel parameters(Eq. 2) at different
temperaturesare given by Ananthaswamy and Atkinson (1984)
and those of b and 01are taken as temperature independent.
Recently, Holmes and Mesmer (1983) applied the ion-interaction model of Pitzer (1973) to aqueous solutions of the
alkali metal chlorides to 250°C. Their resulting set of equations
provide a thermodynamic description within the accuracy of
the reported experimental data. The parameters B(O),B(l) and
CMx are described by arbitrary functions of temperature, f(T)
of the form
+ p3 ln f
in Eq. (7) for some
acMx
x 105
7
-9.30
-10.54
--_
- 5.095
-2.3
39.7
-__
48.61
26.50
43.42
26.44
27.04
27.54
10.73
f
f
e
k
f
+
+
0.10
0.10
0.10
0.10
0.15
0.24
0.06
Table 7-Comparison
of predicted and measured
saturated salt solutions at various temoeratures
r
f(T) = pl + PZ ($ - f)
apfl)
x 104
7
10.7s
7.005
___
10.71
4.3,
64.5
8.g3
of selected
Salt
NaBr
NaCl
W-L&SO.,
KCI
BaCI,
KNO,
KzSO4
(5)
f)
+
ap@)
x 104
7
7.6g2
7.159
___
5.794
0.854
2.06
1.92
derivatives
a Silvester and Pitzer (1978).
b Maximum molalities to which the enthalpy equations were fined.
Salt
CMX
of the temperature
50.48
26.61
44.22
27.93
27.90
34.84
12.17
f
f
+
k
f
-c
+
0.48
0.05
0.20
0.10
0.15
0.24
0.05
s Literature mean value (calculated from several different sources) and estimated
standard deviation.
+ q3 ln(+)
; q4(T - '6 + qs(T* - T,*,
p(l)
Table 5-Parametersa
selected salt solutions
+ ~4 (T - ‘G)
(7)
+ ps (T2 - Gr2) + p6 ln;T - 260)
where p,,. . . ., p6 are parameters.
with the exception of the term envolving p6 this equation
is identical with the form used by Silvester and Pitzer (1977).
The values of the parameters pt, . . . ., p6 for aqueous solutions of potassium chloride are listed in Table 4; they allow
a very accurate calculation of the a, of KC1 solutions for the
whole concentration range (up to saturation) at any temperature .
Silvester and Pitzer (1978) gave the change with temperature
of the activity and osmotic coefficients. They reported results
for 84 electrolytes of 1- 1, 2- 1, 3- 1 and 2-2 valence types. The
parameters allow the convenient calculation of properties at
other temperatures not too different from 25°C. The values of
B(O),B(l) and CMX at temperaturesnot too different from 25°C
Predicteda
Ib1
0.562
values of a, of selected
Predicted*
lc)
___
f 0.002
---
0.752 k 0.001
0.751 +- 0001
0.750
.._. f 0.001
__--.
Measured
Id1
0.601
f 0.002
___
0.525 T 0.008
0.603
___
0.537
0.753
0.753
___
0.751
k 0.001
___
0.749 I 0.001
PJH&S04
15
25
35
0.803
--_
___
___
__-
0.804
___
0 797
KCI
15
25
35
0.858
0.842
0.827
+ 0.002
+ 0.001
+ 0.001
0.859
0.842
0.827
f 0.002
-+ 0.001
f 0.001
0.860
--0.828
BaCls
15
25
35
0.903
___
f 0.001
--_
0.910
f 0.001
___
0.894 k 0.002
0.908
___
0.893
KNOs
15
25
35
0.926
--_
IT 0.001
___
0.948
f 0.001
-__
0.889 f 0.002
0.943
___
0.898
KzSQI
15
25
35
0.975
___
-c 0.000
___
0.979
0.977
___
0.971
k 0.001
___
-+ 0.000
-__
0.972 f 0.000
OThe fluctuations in a, values arise from the standard deviation of the solubility
data.
b Predicted using Pitzer’s model (Eq. 2) with parameters for 25°C or with the rigorous
temperature dependence for 15’C and 35°C [Eq. (4). (5). 0%. and (7)l.
c Predicted using Pitzefs model with approximation for the temperature dependence
IEq. WI.
d Measured with the Novasina Thermoconstanter (average of triplicate determinations).
(i.e., 15°C or 35°C as is the case in the present paper) can be
approximated by
fcr, = 4x-c) + -&CAT
(8)
where f(r) is the value of the function at temperature T. The
values of the corresponding derivatives for the salts of interest
to the present study are given in Table 5. The most important
conclusion is that for almost all salts considered here the temperature derivatives are relatively small. Thus, for NaBr, NaCl,
KCl, BaC12, and K2S04 where B(O)and B(l) at 25°C had a
magnitude of about 0.05 or a few tenths (Table 2), the derivVolume 51, No. 4, 1986--JOURNAL
OF FOOD SCIENCE-1039
a, OF SALT SOLNS AT VARIOUS TEMPS . . .
Table 8-Prediction
of the a,,, of selected saturated salt solutions at 15°C
and 35°C neglecting
the effect of temperature
on the thermodynamic
parameters
of the ion-interaction
model8
b
a,
a,
Salt
(35°C)
NaBr
NaCl
0.601
0.752
0.808
0.856
0.910
0.944
0.979
N-L&S04
KCI
BaCI,
KNOB
K&L,
‘- 0.002
k 0.001
f 0.002
AZ 0.002
f 0.001
k 0.001
e 0.000
0.522
0.750
0.798
0.830
0.895
0.907
0.972
f
f
e
e
k
f
55
0.008
0.001
0.002
0.001
0.002
0.002
0.000
a Pitzer (1973)
b Fluctuations in the predicted aw values arise from the standard deviation of the
solubility data.
Table *The
37-c
a, of selected
Temp
(“a
NaCl
15
17
19
21
23
25
27
29
31
33
35
37
0.753
0.753
0.752
0.752
0.751
0.751
0.750
0.750
0.750
0.749
0.749
0.748
saturated
NL,LSOz,
0.808
0.806
0.805
0.804
0.803
0.803
0.802
0.801
0.800
0.799
0.798
0.797
salt solutions
between
15°C and
KCI
BaCI,
WOa
0.859
0.856
0.852
0.849
0.846
0.842
0.840
0.836
0.833
0.830
0.827
0.823
0.910
0.909
0.907
0.906
0.905
0.903
0.902
0.900
0.899
0.898
0.895
0.894
0.979
0.978
0.977
0.977
0.976
0.975
0.975
0.974
0.973
0.973
0.972
0.971
atives were the order of 1O-4 to 10-3. Hence, a temperature
change from 25°C of lO-20°C will cause little change in these
parameters and thus in a,. A different picture was observed
for KN03 where the temperature derivatives were relatively
more important.
An important prerequisite for the prediction of the a, of
saturated salts at different temperatures is precise knowledge
of the salt concentration at saturation. For this purpose a compilation of solubility data for NaBr, NaCl, (NH4)2S04, KCl,
BaC12, KN03, and K2S04 at various temperatures, was made.
Table 6 gives the results of the literature review on the solubility of the salts examined at 15”C, 25°C and 35°C (sources
of data are not reported here due to the very large number of
references considered).
The solubility of salts studied showed a significant increase
with increasing temperature, with KNO, showing the largest
increase. The exception was NaCl where solubility increased
only slightly with increasing temperature (1% change between
15°C and 35°C). Considering the small temperature dependence of the parameters for the ion-interaction model (Table 5),
it could be anticipated that the a, of saturated NaCl solutions
will be only slightly affected by temperature (in the range
studied).
It is now possible to combine the data of solubilities at
various temperatures with the concentration dependence of the
osmotic coefficient for each salt (calculated from the ion-interaction model) at different temperatures and to predict the
a, of saturated salt solutions at 15°C and 35°C. For NaCl and
KC1 the ion-interaction model of Pitzer with the rigorous temperature dependenceof parametersp(O), /3(l) and Cm [as given
by Eq. (4), (5), (6) and (7)] were used. For NaBr, BaCl,,
KN03 and K2S04 the temperature derivatives (Eq. 8) were
used. No information is available on the temperature dependence of ion-interaction parameters for (NH4)$S04. The results
of the prediction are shown in Table 7 which compares predieted values at 15°C and 35°C with those experimentally determined using the Novasina Thermoconst$nter previously
calibrated as described.
For all salts studied, the a, (either predicted or measured)
decreased with increasing temperature; this effect has been
104O-JOURNAL
Of
FOOD
SCIENCE-Volume
51, No. 4, 1986
experimentally found in the literature for several salts slurries;
however, as reported by Scott and Barnard (1983), the “physicochemical reasons for the decrease in a,,, were not fully
understood.” The a, of saturated NaCl remained almost constant between 15°C and 35°C (ha, = 0.002) and this was
expected on the basis of the solubility changes and very small
effect of the temperature on the characteristic parameters in
the model of Pitzer (1973). It was noteworthy that the predicted
a, values at 15°C and 35°C were in very good agreement with
the values measured with the Novasina Thermoconstanter previously calibrated with unsaturated NaCl and LiCl solutions;
differences between theoretical and experimental values were
only about 0.002 a,. The exception was KN03 for which a
larger difference was observed, these differences being 0.003
at 15°C and 0.009 at 35°C. This may be attributed to the
relatively large values of the temperatures derivatives (Table
5) which make more uncertain the accurate theoretical prediction of the effect of temperature. KN03 also exhibited a large
increase of solubility with increasing temperature; extrapolation of Pitzer’s (1973) model to the saturation concentration at
35°C may be somewhat less reliable.
Labuza et al. (1985) used the vapor pressure manometer to
measure the a, of saturated NaCl and KC1 solutions and reported values of 0.765, 0.748 and 0.727 for NaCl and 0.846,
0.841 and 0.786 for KC1 at 25”C, 30°C and 45”C, respectively.
The values for NaCl were not in good agreement with the
theoretical predictions or with present experimental determinations; the values for KC1 at 25°C and 30°C are in better
agreement but the value for 45°C seems to be somewhat low.
A simplified theoretical approach was also tested to predict
the effect of a temperature change (from 25°C to 15°C or 35°C)
on the a, of salt slurries studied in this work. For this purpose
it was assumedthat the effect of temperature may be accounted
for by only considering the effect on solubility and neglecting
the intrinsic variation of a, with temperature (i.e., the temperature derivatives of ion-interaction parameters are negligible). Thus, Eq. (2) with parameters p(O), p(l), CMx
corresponding to 25°C was used to predict the a, values at
saturation concentration at 15°C or 35°C. The Debye-Hiickel
parameters were those at the corresponding temperature 15°C
or 35°C (Ananthaswamy and Atkinson, 1984). The results are
shown in Table 8. Predicted values for NaBr, NaCl, KCl,
BaCl* and K2S04 agree quite well with the values obtained
with the more rigorous approach (Table 7). This method allowed the prediction of a, at 15°C and 35°C of (NH&S04;
there is a good agreement with measured values (Table 7). As
found before, KN03 at a higher temperature (35°C) exhibited
the greatest deviation. These results indicated that for the salts
studied (with the exception of KN03) the increase in solubility
was the dominant factor which regulated the change in a, when
temperature shifted from 25°C to 15°C or 35°C.
Having demonstrated the reliability of the theoretical predictions, the different equations and solubility data were used
to calculate the a, of saturated NaCl, (NH&S04, KCl, BaCl*
and K2S04 every 2°C between 15°C and 37°C (a slight extrapolation above 35°C was done because of the microbiological interest in the temperature 37°C). The results are shown
in Table 9; it is believed that these data are accurate enough
for almost all needs in food microbiology research or quality
control. NaBr and KN03 were not included here because the
predictions were somewhat less accurate than those for the
others. This theoretical approach may help to find ways to
reach a general agreement on the a, of standards used to callibrate a, measuring devices.
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. .From page 1016
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Volume
51, No. 4, 1986-JOURNAL
OF FOOD
SCIENCE-1041