The Influence of Electrolyte Solutions
on Soil pH Measurements
TRAIAN GAVRILOAIEI*
“Al. I. Cuza” University of Iasi, Faculty of Geography and Geology, Department of Geology, 20A Carol I Blv., 700505 Iasi, Romania
In order to study the influence of electrolyte solution on the value of soil pH, it was investigated the relationship
between pH in water (pHH2O) and pH in 0.01 M CaCl2 (pHCaCl2) or 1 M KCl (pHKCl) electrolyte solution, for 94
samples from Iasi City, Romania. The pH values in salt solutions are lower than those in water and the
differences between the pH measurements are dependent on the nature of salt solution: the average value
of pHCaCl2 is lower with 0.604±0.146 pH units than pHH2O, while the average value of pHKCl is lower with
0.895± 0.104 pH units than pHH2O. The paper also describes the relationships between pHH2O and pH
measured in other electrolyte solutions by linear regression. A strong correlation coefficient was obtained:
r=0.984 for CaCl2 solution and r=0.992 for KCl solution, respectively. Both salt solutions are useful in such
determinations.
Keywords: soil pH methods, pH water, pH calcium chloride, pH potassium chloride
The soil pH represents a measure of the acidity which
plays an important role in determining the solubility of
important elements and processes from soils. Some
problems are clearly understood in pH soil measurements
[1, 2]:
- the pH measurement is influenced by soil/electrolyte
ratio, that means if this ratio increase, the pH values also
increase;
- the measured values for any soil sample depend on
the electrolyte solution; small increases in the electrolyte
concentration cause a decrease in pH value;
- pH values are less dependent on stirring/non-stirring
conditions during the measurements.
In the literature, the soil pH is appreciated either in water:
soil mixture (pHH2O) or in other electrolytes with different
ionic strength, like CaCl2 (pHCaCl2) or KCl (pHKCl). The
influence of salt solution on the soil pH measurements is
also called ”suspension effect” on pH [2]. In U.S. the
measurements of soil pH are realized in 1:1 mixture of
soil: water, but in Europe the researchers use a 1:5 soil:
water mixture. To compare these different laboratory
methods is problematic, because they use different
electrolytes, different ratio of soil/electrolyte or different
extraction time. The pH measurements which are realized
in water are comparable with the measurements in other
electrolytes. When the soil is diluted with water, most of
the protons tend to remain fixed to the soil particles and
are not released into the soil solution. The measurement
of pH in 0.01 M CaCl2 solution was proposed by Schofield
and Taylor, 1955 [1], because its concentration is almost
equal to the salt concentration in soil solution. The
measurements of pH in a calcium or potassium chloride
solution are recommended, the use of a dilute salt solution
will give more consistent results than using distilled water.
Low concentrations of calcium chloride (10-5-10-4 M) had a
small effect on the soil pH measurements, while
concentrations higher then 10-3 M had a direct influence on
the soil pH measurements [1]. The addition of salt solution
release cations, which replace some of the protons from
the soil particles. These processes force the hydrogen ions
to pass into the solution and make their concentration in
the bulk solution closer to the value found in the field.
Consequently, the pH measured in chloride solution is
* email: [email protected]
396
always lower than pH measured in water, due to the higher
concentration of protons.
Considering that calcium and potassium are the cations
of highest concentrations in soil solution, it was studied
how the soil pH was influenced by calcium and potassium
chloride. This paper presents two models relating the soil
pH measurements, in water and in 0.01 M CaCl2 / 1M KCl
electrolyte solutions, for a number of urban / rural soil
samples from Iasi City, Romania.
In the Southern part of the Iasi City - Romania the shapes
of the land and geomorphologic processes are the
determinant factors of pedogenesis [3]. Cambisoils from
Sourthern part of the city are characterized by a weak
alkaline reaction, while in the higher areas, soils are supplied
with humus and have a slightly acidic reaction. Industrial
activities from the city contribute to the creation of new
parental materials, through the inclusion of different
materials (bricks, concrete, glass etc.) in the soil profile.
The soils samples from inside the city are characterized by
large amounts of organic carbon, oxides, carbonates and
clays which may influence the chemical element content.
The soils from the Northern part of the city were formed in
an excessive continental climate, through intense
bioaccumulation processes and are predominantly formed
by chernozems [3]. The samples from this part of the city
belong to the residential areas dominated by garden
houses, while the samples from the Southern part belong
to the industrial areas, with industrial objectives and blocks
of flats. The latest soil samples contain specific and (non)biodegradable wastes, hard rocks fragments or domestic
wastes which contribute to the formation of entiantrosoils
[3].
Experimental part
Materials and method
The study was focused on a number of 94 soil samples
from Iasi City, Romania which were collected within a 500
m grid (max. 25 cm depth). Three series of samples were
taken into account: samples from 1 to 30 were collected
from the Southern part of the city, samples from 31 to 62
were collected from inside of Iasi City and the samples
from 63 to 94 belong to the Northern part of the city (fig. 1).
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Fig. 1. Location of the studied area
The samples were dried in air (at 400C) and sieved to < 1mm fractions and then the pH values were determined
using a Corning M555 pH meter device, which had
previously been calibrated.
For pH determination 10 g soil were mixed with 50 mL
of deionized water (pHH2O) or 50 mL of 0.01 M CaCl2 / 1 M
KCl solution (pHCaCl2, pHKCl), respectively. The mixtures
were shaken for 10 min. on a magnetic stirrer and were
left for 15 min., to reach the equilibrium. The experiments
for pH determination were carried out under laboratory
conditions, on duplicate; the temperature was kept
constant (25 ± 10C) and all chemicals were of reagentgrade quality obtained from commercial sources.
Results and discussions
For the samples considered in the study, the range of
pHH2O variation is not so large: from a minimum pH value
of 5.97 to a maximum pH value of 8.97 (in the Northern
part of the city) the dominant soil reaction being neutral to
weak alkaline. For those three series of samples, the soil
reaction is characterized by an alkaline reaction for
samples 31-62 (average pH=8.39) and by a weak alkaline
reaction for samples from 1 to 30 (average pH=7.96) and
for samples from 63 to 94 (average pH=7.79), respectively.
In the Northern and Sourthern parts of the territory, the soil
reaction varied from slightly acidic to slightly alkaline, but
the dominat reaction of the soils is a moderate alkaline
one. To the acid domain, with pH values up to 6.9
correspond almost 16% of the investigated samples, only
of rural soils (no samples of urban soil) and the difference
belongs from neutral to weakly alkaline domain. The acidic
reaction of soil for these 94 samples is given by the the
presence of carbonates, soluble salts and (for some
samples) by the presence of exchangeable Na within the
exchange complex [3].
There are a lot of factors which influence the variability
of soil pH measurements: soil-solution ratio, the nature
and concentration of electrolyte in the solution, the
procedure used for determinations, the soil properties, the
strirred/non-stirred conditions etc. [2, 4, 5]. For the data
presented in the paper we can discuss about a period of
time between the sampling and the measurement
moments of soil pH. The sampling was realized in Spring
2007 [3] and the measurements were realizedrange in
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Spring 2009. The differences are in the range of
experimental errors and should not be ignored for precious
measurements of soil pH. Due to a long period of storage
(5 to 9 years) of dried soil samples, some authors have
found that soil pH measured in 0.01 M CalCl2 increased
with 0.23 pH units, while for pH measured in water appears
an increase with 0.55 pH units [5].
Comparison of pHH2O vs pHCaCl2
The importance of electrolyte solution in routine pH
measurements is emphasized in many papers and
Schofield and Taylor, 1955 [1] found that 0.01M CaCl2
solution is the most satisfactory solution for use in nonsaline soils. The obtained results show that the values of
soil pH in water (pHH2O) were significantly higher than the
values obtained in CaCl2 solution (pHCaCl2). As can be seen
(fig. 2) there is a good R-squared value between these two
series of measurements (R2 = 0.969). The pH in CaCl2
solution is lower with 0.604 ± 0.146 pH units (average
value) than in water and the min-max differences were
0.317 – 1.008 pH units for all analyzed samples.
The highest differences (average values) between pHH2O
and pHCaCl2 were found for samples 31 to 62, which pass
through the Iasi City (0.648±0.137 pH units) and the lowest
differences were found for samples 63 to 94 from the
Northern part of the city (0.541± 0.084 pH units). It is also
of interest to mention that 12.76% of the samples have
small differences (0.3-0.4 pH units) and only 5.31% of the
samples have great differences (0.9 - 1.0 or more pH units)
between these two kinds of measurements. Almost 16%
of the samples had a pHH2O ≤ 7.0 and 22.34% of the samples
had a pHCaCl2 ≤ 7.0.
The pH measurement in water is a routine method and
could be used in-situ while the method in CaCl2 solution is
more precise and gives us the possibility to avoid high
variation of the results during some period [6]. The equation
of linear regression is given in table 1 and could be used to
recalculate the pH values. Linear relationship with slope
close to unity and intercept different from zero had been
obtained and this involves a difference between these two
variables over the pH range of interest. A slope less than
unity indicate that the difference between pHH2O and pHCaCl2
decreases with the decreasing of pH values.
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397
Figure 2. Comparison between soil pH in water and in 0.01 M CaCl2
solution (average difference of 0.604 pH units, for 94 samples)
Figure 3. Comparison between soil pH in water and in 1 M
KCl solution (average difference of 0.895 pH units, for
94 samples)
Table 1
LINEAR AND POLYNOMIAL EQUATIONS FOR pH IN WATER AND IN 0.01M CaCl2 SOLUTION
Table 2
LINEAR AND POLYNOMIAL EQUATIONS FOR pH IN WATER AND IN 1M KCl SOLUTION
Although the relationship between these two variables
is described by linear regression, a second order polynomial
relationship was fitted and gave a good representation of
the data, which means that the soils considered here
present variable charge characteristics. This slight
improvement in the coefficient of determination indicates
that the relationship could be curvilinear and a different
mechanism governs the differences between pHH2O and
pHCaCl2. The second order polynomial equation is given in
table 1.
The shape of the curve is very close to the linear one for
these two variables and describes the data better than the
linear fit only for the high pH values (pHH2O ≥ 8), where
there are more sample points. The estimated correlation
coefficient between these two series of measurements is
very high and significant for all soil types considered in the
experiment (table 1).
Other equations were fitted (data not shown); third order
polynomial (R 2=0.970) and logarithmic (R 2=0.965)
relationships gave not a significant improvement, while
the exponential (R2=0.972) relationships gave a slight
improvement. All the plots were very close to linear
equation for the samples considered in the study, but for
the samples with pHH2O ≥ 8 it appears that a complex
model would describe the relationship between pHH2O and
pHCaCl2.
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Similar studies used linear regression to relate pHH2O to
pHCaCl2. For most researchers, the data have permitted the
examination of the relationship between these two kinds
of measurements across a wide range of soils types,
through linear regression. Some authors [4] have studied
the values of pHH2O for a large number of samples and they
have compared the data for pH above 8.0 with those below
5.0. They found a curvilinear relationship for 91 acidic soils
samples between pH in water and in CaCl2 solution, in 1:5
soil:solution ratio. The authors have attributed this
relationship to the acidic pH soil, very close to the point of
zero salt effect, with small differences between these two
measurements. For non-acidic soils, they found a higher
difference between pH in water and in calcium chloride
solution. Other authors [7] have worked with the same
electrolytes (CaCl2 and KCl) and in different dilution ratios
and they found a good correlation over a wide range of soil
pH. They also found the regresion equation between pHH2O
and pH CaCl2 which was valid only for non-saline, net
negatively soils samples. The polynomial relationship
between the measurements is limited in practice by the
soil type and by the range of the pH values. Beside a linear
relationship, it was found also a polynomial one for 1342
surface soil samples with pHH2O between 4.6-8.85 and was
considered that special mechanisms have governed the
differences between these two kinds of measurements
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REV. CHIM. (Bucharest) ♦ 63 ♦ No.4 ♦ 2012
[8]. The deviation from linearity of the sigmoidal curve was
due to the buffering effect of Al at low pH and to the
presence of carbonate at high pH. It has been clearly shown
in the literature [9] that polynomial function is not suitable
for all domain of soil pH and this function can be useful for
a restricted range of pH.
Comparison of pHH2O vs pHKCl
The use of high ionic-strength media, like KCl solution in
measurements of soil pH has been limited primarily to acid
soils. Some authors [10] have worked with 1 M KCl solution
for calcareous soils and pHKCl has found to be inversely
related to potassium concentration of KCl extract and also
to the cation exchange capacity of the samples. They
concluded that KCl solution may provide important
information concerning the chemical properties of the soils.
The presence of soluble salts in a soil sample may
influence the pH value and some analysts prefer to
measure it in a mixture with other chlorides to mask the
differences between these two variables. The values of pH
for analyzed samples measured in 1 M KCl solution are
lower than the values measured in water, due to the higher
concentration of H+ in solution. As can be seen (fig. 3),
there is a good R-squared value between these two series
of measurements (R2 = 0.984). The pH in KCl solution is
lower with 0.895± 0.104 pH units (average value) than the
values measured in water, very close to the value for the
differences in CaCl2 solution. The min-max differences
between pH in water and in KCl like electrolyte were 0.680
– 1.171 pH units, higher than in the case of CaCl2 solution
like electrolyte.
The highest differences (average values) between pHH2O
and pHKCl were found for samples 63 to 94 from the
Northern part of the city (0.905±0.106 pH units) and the
lowest differences were found for the samples 1 to 30
from the Southern part of the city (0.890±0.113 pH units).
For all the data, 13.88% of the samples have the difference
between pHH2O and pHKCl higher than pH=1 and 12.77% of
the sample have the differences between pHH2O and pHKCl
smaller than pH=0.8. As it was mentioned above almost
15.95% of the samples had a pHH2O ≤ 7.0 and only 23.40%
of the samples had a pHKCl ≤ 7.0. For all soil samples, the
estimated correlation coefficient between the
measurements is very high and higher than the value
obtained for CaCl2 solution (table 2), that’s why it can be
said that KCl is also a good electrolyte for soil pH
measurements.
Although the relationships between pHH2O and pH
measured in the other electrolytes could be described by
linear regression, the fitting of other equations improved
the coefficients of determination, which indicate that the
relationships could be curvilinear.
The equation of linear regression is given in table 2 and
shows a very good slope of the straight line (close to unity)
and this involves a constant difference between these two
variables. A slope more than unity indicates that the
difference between pHH2O and pHKCl decreases with the
increasing of pH values. The second order polynomial
equation is given in table 2 and shows a minimal increase
in the coefficient of determination, because of the narrower
pHH2O range (5.97-8.97). The shape of the curve is also
very close to the linear one and is an improvement over
the linear regression line for high pH values (pHH2O ≥ 8),
where there are more sample points, as it describes the
data better than a linear fit.
Although the linear regressions gives a good correlation
between the variables, other mathematical functions were
REV. CHIM. (Bucharest) ♦ 63 ♦ No. 4 ♦ 2012
fitted (third order polynomial and logarithmic relationships),
but they gave not a significant correlation comparable with
the linear equation (data not shown).
Similar studies used linear regression to relate these two
parameters, but the data about the use of KCl solution like
electrolyte solution in measurements of soil pH are less
consistent. It has been shown in the literature that the
deviation from linearity is due to the buffering effect of Al
at low pH and to the presence of carbonate at high pH,
while others have shown that ionic strength, exchangeable
cations (Na, K, Ca, Mg and Al) and Al and Mn extracted
could influence the measurements of soil pH [8]. Some
bigger differences for pHH2O and pHKCl were obtained for
acidic to slightly acidic samples [11]. Other authors have
used some electrolytes (CaCl2, KCl, BaCl2) and different
dilution ratios in determinations of soil pH and they shown
that the measurements were less influenced by the
suspension, with and without stirring [2].
Addition of CaCl2 solution to the mixture has a smaller
effect on solution ionic strength than KCl (or other salt)
solution has and ionic strength has an inverse relationship
with soil pH [2]. The use of electrolyte solutions has the
advantage of decreasing the variability in content of soluble
salts between soils and the pH values obtained are less
dependent on the soil solution ratio. In addition, CaCl2
solution is able to stabilize the dissolution of soil minerals,
because of the common ion effect, more than in the case
of the KCl solution. The values of pHH2O and of pHKCl (1M KCl
solution) were studied and some authors have found a
curvilinear relationship between these two variables [4].
Conclusions
Several factors influence the soil pH, namely the ionic
strength of the 1:5 soil:water suspension, exchangeable
cations (Na, K, Ca, Mg and Al) and Al-Mn extracted with
0.01 M CaCl2 [8]. Addition of electrolytes (CaCl2 or KCl
solutions) to measurements of soil pH decreases the pH
of soils comparable with the pH in distilled water and the
variation is due to the release of protons when salts are
added to the soil suspension.
In order to study the influence of electrolyte solution on
the values of soil pH, the measurements were realized in
water (pHH2O) and in 0.01 M CaCl2 or in 1M KCl solution
(pHCaCl2, pHKCl) respectively, for 94 samples from Iasi City,
Romania, in similar laboratory conditions. A strong
correlation of the pH values in water and in these two
electrolytes was observed: for CaCl2 solution the pH is lower
with 0.604 ± 0.146 pH units, while for KCl solution the pH
is lower with 0.895± 0.104 pH units than in water. Among
all the samples, almost 16% of the samples had a pHH2O≤
7.0 and for the electrolytes 22.34% had a pHCaCl2 ≤ 7.0 and
23.40% had a pHKCl ≤ 7.0, respectively. A strong correlation
coefficient was obtained, between the pH values in water
and in these two electrolytes (r= 0.984 for CaCl2 solution
and r=0.992 for KCl solution, respectively). In order to
convert between these two methods it was used linear
and second polynomial relationships. The equation of linear
regression show a very good slope of the straight line for
both electrolytes and the second order polynomial
relationship provides a quantitative comparison between
these two methods. It appears that a more complex model
(linear model with a slope close to unity for all the samples,
but curving at high pH) could describe the relationship
between these two variables, pHH2O and pHCaCl2/pHKCl. In
conclusion, both electrolytes (CaCl2 and KCl) are suitable
for this kind of measurements.
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399
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Manuscript received: 20.06.2011
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