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# Electrode Chemistry

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An ammonium ion selective electrode responds to both NH4+ and H+, but not to NH3. (pKa=9.244 for NH4+)

A) What pH conditions must be imposed to ensure the electode properly represents toe activity of NH4+

B) Why is the ionic strength an important factor when using this or any ISE? How can you compensate for this effect?

C) Using the Formula EISE = constant + RT/nF ln(ai +ki,jaj), what are the TWO pH conditions that present 10% error on a measurement of 1.00 x10-4M NH4+, if kNH4+ H+ = 0.035? You may assume identical activity coefficients

##### Solution Summary

This solution answers 3 electrode chemistry questions, addressing pH conditions, ionic strength, ISE and EISE

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An ammonium ion selective electrode responds to both NH4+ and H+, but not to NH3. (pKa=9.244 for NH4+)

A) What pH conditions must be imposed to ensure the electrode properly represents toe activity of NH4+ (SHOW ALL MATH).

pKa = -log (Ka)

Ka = -antilog (9.244) = 5.701 x 10-10.

The pH range should be 1-8.5

B) Why is the ionic strength an important factor when using this or any ISE? How can you compensate for this effect?
Ion selective electrodes measure the concentration of ions in equilibrium at the membrane surface. In dilute solutions this is directly related to the total number of ions in the solution but at higher concentrations, inter-ionic interactions between all ions in the solution (both positive and negative) tend to reduce the mobility and thus there are relatively fewer of the measured ions in the vicinity of the membrane than in the bulk solution. Thus the measured voltage is less than it would be if it reflected the total number in the solution and this causes an erroneously low estimate of the concentration in samples with a high concentration and/or a complex matrix.
Ionic Strength is a measure of the total effect of all the ions in a solution. It is the sum of the molar concentration multiplied by the square of the valency of all the ions. The effective concentration measured at the electrode head is known as the activity of the ion. In general chemical terms it is the number of ions taking place in any chemical reaction - measured in concentration units. The activity coefficient is the ratio of the activity ...

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