I use calcium ion selective electrode. I calibrate it with 5 standard solutions and draw the mV vs. Calcium concentrations related to the standard solution calibration graph.
Then I mix 100mM calcium chloride and 100mM sodium sulphate to precipitate calcium sulphate, from beginning I put calcium ion selective electrode in mixing solution to monitor the calcium ion concentration changes. and Log the data by computer. But the data are in mV, How can I change the mV to calcium ion concentration? is there any linear relation between mV and calcium ion concentration to use the calibration graph?
I would appreciate if you could send me any necessary equations.
Electrode Operation
The calcium electrode consists of an electrode body containing a liquid internal filling solution in contact with a gelled organophilic membrane containing a calcium ion exchanger. When the membrane is in contact with a solution containing free calcium ions, an electrode potential develops across the membrane. This electrode potential is measured against a constant reference potential, using a standard pH/mV meter or an ion meter. The level of calcium ions, corresponding to the measured potential, is described by the Nernst equation:
E = E' - S log X
where: E = measured electrode potential
E’ = reference potential (a constant)
S = electrode slope (-26 mV/decade)
X = level of calcium ions in solution
The activity, X, represents the effective concentration of the ions in solution. The total calcium ion concentration, Ct, is the sum of free calcium ion, Cf, and complexed or bound calcium ion, Cb. The electrode is able to respond to only the free ions, whose concentration is :
Cf = Ct - Cb
Since calcium ions form very few stable complexes, the free ion concentration may be equated to the total ion concentration.
The activity is related to the free ion concentration, Cf, by the activity coefficient, γ, by:
X = γ Cf
Activity coefficients vary, depending on total ions strength, I, defined as:
I = 1/2 ∑CxZx2
where: Cx = concentration of ion X
Zx = charge of ion X
∑ = sum of all of the types of ions in the solution
In the case of high and constant ionic strength relative to the sensed ion concentration, the activity coefficient, γ, is constant and the activity, X, is directly proportional to the concentration.
Electrode Operation
The calcium electrode consists of an electrode body containing a liquid internal filling solution in contact with a gelled organophilic membrane containing a calcium ion exchanger. When the membrane is in contact with a solution containing free calcium ions, an electrode potential develops across the membrane. This electrode potential is measured against a constant reference potential, using a standard pH/mV meter or an ion meter. The level of calcium ions, corresponding to the measured potential, is described by the Nernst equation:
E = E' - S log X
where: E = measured electrode potential
E’ = reference potential (a constant)
S = electrode slope (-26 mV/decade)
X = level of calcium ions in solution
The activity, X, represents the effective concentration of the ions in solution. The total calcium ion concentration, Ct, is the sum of free calcium ion, Cf, and complexed or bound calcium ion, Cb. The electrode is able to respond to only the free ions, whose concentration is :
Cf = Ct - Cb
Since calcium ions form very few stable complexes, the free ion concentration may be equated to the total ion concentration.
The activity is related to the free ion concentration, Cf, by the activity coefficient, γ, by:
X = γ Cf
Activity coefficients vary, depending on total ions strength, I, defined as:
I = 1/2 ∑CxZx2
where: Cx = concentration of ion X
Zx = charge of ion X
∑ = sum of all of the types of ions in the solution
In the case of high and constant ionic strength relative to the sensed ion concentration, the activity coefficient, γ, is constant and the activity, X, is directly proportional to the concentration.
Jeremiah basically told all. Summarazing, the easier approximation is that E (mV) = +29 *(-log ([Ca2+](M)) + cte. i.e. you should get 29 mV per dec, it is a logaritmic relationship.
In order to linearize the logarithmic relationship between the free calcium ion concentration and the ISE-reference electrode cell voltage the Gran’s plot paper was developed many years ago (see also:http://de.scribd.com/doc/38516302/LOCATION-OF-END-POINT-IN-POTENTIOMETRIC-ARGENTOMETRIC-TITRATION-USING-GRAN-PLOT-AND-TITRATION-ERRORS). This method overcomes certain disturbing effects in precipitation titrations.
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