1. Silver/silver chloride electrode
This electrode is more rarely used but its robustness may make it worth
using. Its simplicity also makes it useful for miniaturized assemblies. It is
most often used in combination with a glass electrode or with ion
selective electrodes.
This electrode makes it possible to work at temperatures ranging from
−30 to +135°C. At 25°C, when filled with saturated potassium chloride,
15.2.4 Electrometric Checking and Calibration
The pH meter has to be calibrated before measurements are made. The
apparatus chosen must be able to correct slope and temperature. In most
cases, a precision of ± 0.1 pH unit is sufficient as other causes of errors
are greater (e.g. heterogeneity of the sample). For measurements on
suspensions, it is preferable to choose a device with a separate reference
electrode as the risk of errors caused by clogging of the porous junction is
reduced and it is easier to access when problems occur.
The apparatus should be switched on some time before beginning
the measurements (there is often a waiting position which maintains the
circuits in equilibrium), and then the electrodes are immersed in the
appropriate buffer. It is advisable to begin with buffer T4AC (cf.
Appendix 3, at the end of this chapter), which is particularly stable with
respect to temperature. Adjust the pH meter to the appropriate value, then
move on to the second buffer, which should be very close to the values to
be measured. The apparatus should then display the value for the new
buffer (corrected for temperature, see Appendix 3). Compensate for any
slight variation while correcting slope. If correction is not possible, the
coupling of the electrodes may not be satisfactory, particularly due to the
glass electrode which does not display a strictly linear response between
the two pH values selected. If this is the case, choose a narrower range of
pH: for example, instead of pH 4 and 9, choose 7 and 9 or 6 and 8. The
closer the value of the standard buffer to the value to be measured, the
better the measurement. Agitation is not advised during measurement. In
the case of problems of linearity, carefully clean the porous junction of
the electrode by brushing it with a hard brush. It can also be cleaned by
suction using a filter pump.
Inorganic Analysis564
its potential is +200 mV compared to a standard hydrogen electrode (see
appendix 1). Its potential is −45 mV compared to the calomel electrode
°at 25 C. It is advisable to monitor the stability of this electrode, as
measurement currents can cause the transformation of silver by micro-
electrolysis; it can be improved by a thin coating of Teflon film.

2. Measurements of soil pH can be classified in four main types:
1. On aqueous suspensions of soils
2. On saturated pastes
3. On extracts of saturation
4. “In situ” measurements
15.2.5 Measurement on Aqueous Soil Suspensions
Procedure
Fig. 15.2. Recommended
position of the electrodes
– Weigh 10 g of soil dried at room temperature and sieved to 2 mm
(a test specimen of at least 5 mL being the ISO standard), add 25 mL
(or five times the volume of the test specimen, ISO standard) of boiled
distilled water (CO2 removed); shake for 1 h on an oscillating table
(energetic agitation for 5 min, ISO standard).
– let decant for 30 min (at least 2 h but not more than 24 h, ISO standard),
immerse the electrodes taking care that the porous part of the reference
electrode is submerged in the clear part of the suspension (Fig. 15.2).
Read the pH value after stabilization of the measurement. Note the
temperature, and check correction for temperature on the pH meter.
Remarks
As the value of the pH drops with a rise in temperature, it is usual
to bring back all the values to 25°C. For precise measurements, a
thermostatic bath should be used; as soil suspensions often have a buffering
effect that it is impossible to correct by calculation.
pH Measurement 565
(1998) recommends a 1:2.5 soil:water ratio. The International standard NF
ISO 10390 (1994) recommends a ratio of 1:5.
Specifications vary with the soil:solution ratio due to the diversity of soil
materials. In France, the experimental AFNOR standard NF X-31-103

3. Measurements should be made without agitation. For certain soils, the
indications provided by the pH meter may not be stable, giving a
permanent drift. In such cases, it is advisable to read the pH after a
specific time interval, for example, 3 min, and to use the same time
interval for the other measurements, not forgetting to mention the fact
when noting the results.
Measurements on soil suspensions and more importantly on saturated
pastes induce a phenomenon called “paste effect” or “suspension effect”
which can modify the results by ± 1 pH unit (Fig. 15.3). This effect is
especially significant when the electrodes – and particularly the reference
electrode – are in contact with the sediment. This effect could be due to
the difference in mobility of the K+
and Cl−
ions of the diffusion solution
of the reference electrode when colloids (charged particles, strong cation
exchange capacity) are present. Grewling and Peech (1960) reported that
this phenomenon could be minimized by taking measurements with a
0.01 molar solution of CaCl2 (pHCa).
Although the soil:solution ratio influences the results of measurement
of pH in water, it has very little effect in saline solutions such as CaCl2
0.01 mol L−1
(Conyers and Davey 1988). Nilsson et al. (1995)
recommended water: soil ratios higher than 10 (v:w or 2 in v:v) to obtain
reliable measurements of pHwater on organic soils.
The time of contact between water and soil also influences the
pH. According to Conyers and Davey (1988) agitation times of more than a
few hours are not advisable because they generate variations and make it
impossible to obtain a stable value. Qiu and Zhu (1986) reported
stabilization of their measurements after 30 min (and up to 1 h) of agitation.
Fig. 15.3. Possible errors due to the effect of suspension for a soil in distilled
water (Bates 1973)
Inorganic Analysis566

4. 15.2.6 Determination of the pH-K and pH-Ca
When measured in water, the pH value does not take total acidity into
account, particularly the protons and the aluminium forms fixed on the
exchange complex which represent potential acidity. In addition to the
first measurement in water, it is consequently necessary to carry out a
measurement in 1 mol (KCl) L−1
aqueous solution with the same soil:
solution ratio (1:2.5 for the procedure described in “Procedure” under
Sect. 15.2.5) and using the same technique. This gives pHK.
pHK measurements usually give lower pH values2
than pHH2O
measurements. The difference (ΔpH) can be as much as one pH unit. A
value of ΔpH > 0 indicates that the cation exchange capacity is higher than
the anion exchange capacity. There is a significant correlation between a
positive ΔpH and exchange acidity. However, although this is true in the
case of one family of soils, it is not true of a general comparison.
Measurements in 0.01 mol (CaCl2) L−1
aqueous solution are also used
because Ca ions cause flocculation of the solution and minimize the paste
and dilution effect. pH expression is thus less random and comparison is
easier between different soils and particularly saline soils. pHCa
measurements induce lower ΔpH than pHK measurement The study of
Conyers and Davey (1988) showed that pHwater, pHK, and pHCa are closely
correlated on soils with a broad range of pH. For non-saline soils with a
negative net charge, they found the relation:
pHCa = 1.05 pHeau − 0.9
The procedure described in “Procedure” under Sect. 15.2.5 is valid in
all cases, whether in water or saline solution.
15.2.7 Measurement on “Saturated Pastes”
pH measurement on saturated paste aims to reproduce the conditions of
the natural environment as closely as possible. This technique is very
delicate to implement. It is necessary to start by preparing a “saturated
paste”:
2
In some Andosols rich in allophane, pHKCl can be higher than pHH2O (amphoteric
medium).
pH Measurement 567
This measurement was the subject of an experimental an AFNOR
standard NF-X31-104 (1988) and became part of the international NF
ISO 10390 standard (1994).