Volume 1 Paper 2
The Activity Coefficient of Sodium Chloride in a Simulated Pore Solution Environment
C.P. Atkins and J.D. Scantlebury
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JCSE Volume 1 Paper 2
Submitted 20 April 1995, published 28 June 1995
The Activity Coefficient of Sodium Chloride in a Simulated Pore Solution
C.P. Atkins and J.D. Scantlebury
Corrosion and Protection Centre, UMIST, P.O. Box 88, Manchester M60 1QD, UK.
Values for the activity coefficient of the chloride ion in pore water
solution when added as sodium chloride has been produced from electromotive
force data using a silver/silver chloride ion selective electrode.
Under normal circumstances steel reinforcement in concrete is in a passive
condition due to the high pH environment provided by the hydration of cement.
One of the causes of the break down of this passivity is attack by chloride
ions . Currently common methods of finding the
level of chlorides in a structure involve destructive techniques. The
potential of silver\silver chloride electrodes depends mainly on the chloride
ion activity in the surrounding electrolyte, and therefore it should be
possible to use these electrodes as a non-destructive method of obtaining the
chloride ion activity in cement and concrete structures. The term activity is
used to explain the deviation in electrolytic properties of real solutions
from ideal solutions and is related to the general concentration and
interaction of ions in solution, the more concentrated the solution the
further the activity will deviate from the concentration, the difference
usually being described in the form of an activity coefficient, i.e. activity
= coefficient x concentration. In very dilute solutions the activity
coefficient is assumed to be unity, but since concrete pore water is
approximately pH 13.6 this assumption may not hold and so the activity
coefficient must be determined experimentally from a knowledge of the
activity. Also before using a silver/silver chloride electrodes in
cementitious media it is necessary to ensure that they function in a
predictable manner. The purpose of this paper is to first prove this and then
to determine the activity coefficient in a simulated pore solution.
Silver/silver chloride electrodes were prepared in a manner similar to that
described in Ives and Janz. 25mm long sections
of 99.99% pure, 1mm diameter silver wire supplied by Goodfellow Metals of
Cambridge were taken and soldered onto screened cables. Heat shrink tubing was
cut to an appropriate length and applied to cover the connection between the
silver and the cable. These electrodes were then wiped with aceto ne to
degrease them and anodised at a current density of 0.4mA cm-2 for
approximately 30 to 40 minutes then moved to an aqueous solution of potassium
chloride for storage. A molar solution of sodium chloride was added to 100ml
of deionised water using a one ml graduated pipette with a least count of 0.01
ml. The exact amount of solution added was recorded and the solution was
stirred for approximately one minute. Four silver/silver chloride electrodes
were then rinsed with deionised water and immersed into this solution and
their potential against a saturated calomel electrode was recorded. The
saturated calomel used was in a Luggin probe arrangement, with the probe
containing agar agar with 0.1M ammonium nitrate as a salt bridge. Another
known quantity of 1M NaCl solution was added and the experiment was repeated
until a reasonable range of chloride concentrations were obtained. A simulated
pore solution, as used by Yonezawa et al  of a
similar composition to that found by Diamond,
containing 0.4M KOH and 0.2M NaOH was then used as the base electrolyte and
the experiment repeated. The complete procedure was then repeated to avoid any
cumulative or weighing errors that may have occurred.
§5 Figure 1 Potential versus concentration (larger
§6 Figure 2 Activity versus concentration (larger
§7 Table 1 Calculated Activity Coefficients for Sodium Chloride in Simulated
Pore Water Solution.
Concentration Activity Coefficients
Figure 1 shows the potentials of the silver/silver chloride electrodes
used, measured against a saturated calomel electrode (SCE). From this data the
activity of the chloride ion can be calculated by straightforward mathematical
manipulation of the Nernst equation:
§11 The potential of the saturated calomel electrode has been calculated at
values ranging from 0.2412 V to 0.2467 V.
The effect of this variation would be to change the gradient of the line
produced in a plot of activity against concentration. It was therefore decided
to take already published data on the activity coefficients ,
and calculate the potential of Eref by fitting the curve produced
from the deionised water data to the published activity coefficients. The
potential for Eref produced in this manner was then used in equation 2 to
calculate the activity of the chloride ion in simulated pore solution and
hence find the activity coefficients of the chloride ion in this solution. The
data produ ced in this manner is illustrated in figure 2, along with the data
the curve was fitted to. The resulting standard potential for the saturated
calomel electrode used was found to be 0.2412 V. The maximum deviation between
silver/silver chloride electrodes was 3mV at the lowest concentrations used,
at higher concentrations the maximum deviation between electrodes was less
than 1mV. This suggests a larger error in very dilute solutions, a reasonable
statement. This is also supported by the fact that the silver\silver chloride
electrode took longer to reach a stable potential in the most dilute solution,
yet were almost instantly stable in the higher concentrations.
§12 Table 1 shows the values of activity coefficients in the simulated pore
solution. These were found by dividing the calculated activity of the ions in
solution by the known concentration of ions in solution.
§13 The most common method of finding the concentration of chloride ions in use
is that of pore water expression, as described by Longuet
whereby a cylinder of concrete is subjected to triaxial loading and the
solution obtained in this manner is then analysed. Since the quantity of
solution obtained in this manner is small, usually not more than 5cc, it is
diluted between 25 and 100 times before analysis. The so lution is then
analysed for OH- and Cl- concentration in various ways.
Most commonly used is titration with nitric acid for the OH-
concentration and with silver nitrate for the chloride ions, as used by Page
in several papers .
Some authors 
however have used chloride sensitive electrodes in order to find the chloride
concentration, or pH electrodes  for the
hydroxide ion concentration. Both these methods give activity rather than
concentration, as mentioned by Tritthart who
found that a glass electrode gave a value of activity of 10-0.45,
whereas the measured concentration of hydroxide ions was 10-0.2
which would suggest an activity coefficient of approximately 0.56. Hausmann 
stated that the probability of steel corrosion in concrete was dependant
mainly on the ratio of chloride to hydroxide ions in solution. Since the above
authors all quote this figure an error with a magnitude of approximately 0.5
must occur in those that have determined the activity of one s pecies and
compared it with the concentration of the other. This does not occur when
comparing concentration with concentration because the activity coefficient is
in fact the mean activity coefficient of all ions in the solution.
The activity coefficient of the chloride ion when added as sodium chloride
to a simulated pore solution ranges from 0.75 at a concentration of 0.01M to
0.45 at 0.12M. Silver silver chloride electrodes provide the possibility of
measuring chloride concentrations in cement and concrete non destructively.
1. Page, C.L.; Lambert, P.; Vassie, P.R.W.; Materials and
Structures/Materiaux et Constructions, 24, p243 (1991).
2. Ives, D.J.G.; Janz, G.J.; Reference electrodes, Theory and practice,
Academic Press, London (1961).
3. Yonezawa, T.; Ashworth, V.; Procter, R.P.M.; Corrosion, 44 (7), p489
4. Diamond, S.; Cem. Concr. Res., 11 p383 (1981).
5. Chateau, H.; J. Chem. Phys., 55 p590 (1954).
6. Fales, H.A.; Mudge, W.A.; J. Am. Chem. Soc., 42 p2345 (1920).
7. Scratchard, G.; J. Am. Chem. Soc., 47 p641 (1925).
8. Lewis, D.A.; Randall, M.; Thermodynamics, McGraw Hill book company
p348, (As quoted in Scratchard7) (1923).
9. Longuet, P.; Burglen, L.; Zelwer, A.; Revue des Materiaux de
Construction et de Travaux Publics, 676 p35 (1973).
10. Page, C.L.; Vennesland, O.; Materiaux et Constructions, 16 (91)
11. Diamond, S.; Cement and Concrete Aggregates, CCAGDP, 8 (2) p97
12. Byfors., K.; Hansson, C.M.; Tritthart, J.; Cem. Concr. Res. 16
13. Rasheeduzafar; Hussain, S.E.; Al-Saadoun, S.S.; Cem. Concr. Res.
21 p777 (1991).
14. Tritthart, J.; Cem. Concr. Res. 19 p586 (1989).
15. Hauzmann, D.A.; Materials Protection, p19 (Nov. 1967).