stayelectrolyte solutionThe ion interaction makes the ion can not fully play its role.The concentration at which ions actually act is called effective concentration, or activity. Obviously, the activity value is usually smaller than its corresponding concentration value.[1]
Ionic activityMeanselectrolyte solutionParticipation inelectrochemical reactionIonicEffective concentration。There is a quantitative relationship between ion activity (α) and concentration (c), and the expression is: αi=γiciWhere: αiIs that of the ith ionactivity；γiIs of i ionsactivity coefficient；ciIs the concentration of i ions.γiUsually less than 1, the interaction between the infinitely diluted ions in the solution tends to zero, at this time, the activity coefficient tends to 1, and the activity is equal to the actual concentration of the solution.Different kindsion selective electrodeIt is convenient for the selective determination of ion activity.according toNernst equation, logarithm of ion activity andelectrode potential Linear correlation, so the electrode potential andactivityAt this time, the potential is measured to determine the ion activity.The measuring instrument isPotentiometerOr dedicatedIon activity meter。
The name activity (symbol is a) was originally used to indicate the effective concentration of a substance.For example, if a salt solution with a concentration of 0.2 gram molecule has an effective concentration of 0.18 gram molecule compared with the theoretical value at the time of infinite dilution in terms of reducing the freezing point, its activity a value is equal to 0.18.But before the theory of strong electrolyte solution was put forward, the nature of activity was not clear.The theory of electrolyte solution points out that strong electrolytes are completely dissociated in the solution. Since the same ions repel each other and the different ions attract each other, any ion is the aggregation center of the different ions attracted by it, which is called "central ion", while the counter ions around it are called "ion fog",Each central ion has a certain activity according to the size of the ion concentration around it, or has the ability to move as an ion. Therefore, someone used activity to represent the concentration that has corrected the interaction between ions, and derived the concept of activity coefficient (f) from the letter of activity (a) and concentration (c).However, no strict thermodynamic definition is given.Lewis pointed out that the ion activity is related to the degree of chemistry, so he proposed the following definition of ion activity:
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Ion activity coefficient
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Ion activity coefficient;Ion activity coefficient: an expression of the degree of ion imperfection in solution, with the symbol γ.If the concentration of an ion i is ci, then ci γ i=ai, which is called theactivity。Using activity instead of concentration for ionsEquilibrium reaction，Ideal solutionThe relationship of is still applicable.butelectrolyteThe positive and negative ions of are present at the same time, so the activity a+of the positive ion and the activity a - of the negative ion will appear in the balance equation at the same time.For convenience of thinking, average ions are introducedactivity coefficientThe concept of γ ± and average ion activity a ± (both refer toGeometric average）。For the 1-1 price of NaClelectrolyte solutionIf its concentration is c, then a ± 2=a+a -=(c+γ+) (c - γ -)=c2 γ ± 2, or a ±=c γ ±.Because of theelectrostatic interactions Very strong, even for extremely dilute solution, the activity coefficient cannot be ignored, but it can be determined according to P. Debye-Huckel Theoretical calculation.
Ion activity measurement
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Standard comparison method
It is similar to the method of measuring pH value of solution with glass electrode.First, prepare a standard solution whose concentration is as close as possible to the solution to be measured, add a certain amount of TISAB to the standard solution and the solution to be measured, and then measure the electromotive force ε of the standard solution under the same conditionss。And the electromotive force ε of the solution to be measuredx。The concentration of the ion to be measured can be calculated according to the following formula:[3]
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Standard curve method
Under the same conditions, measure the electromotive force of each concentration of a series of standard solutions, and make ε - lga or ε - lgc standard curves. The standard curves show a linear relationship.Then, measure the electromotive force of the liquid to be measured under the same conditions, and find out the concentration of the liquid to be measured from the standard curve.[3]
standard addition method
The method of adding standard solution to the solution to be tested for determination.This method can avoid the measurement error caused by the change of activity coefficient.
Assuming that the concentration of an ion in the solution to be measured is c (x), the volume of the solution during measurement is V, and the measured electromotive force is εoneAfter the standard solution of the ion is accurately added, its concentration increases to △ c, and the measured electromotive force is εtwo。If the increased volume is △ V and △ V<<V, then according to Nernst equation:
