Hello,

I've been attempting to work out the following question and have run into some problems. Here is the question:

You are unsure about the ideality of response of a liquid membrane electrode containing an ion carrier responsive to K

^{+}. You therefore make the following measurements: E

_{ISE}=0.1131V in 0.00800M KCl, E

_{ISE}=0.0824V in 0.00200M KCl. Just before you are to measure your unknown, a fellow student breaks the reference electrode and ruins the 0.00800M standard solution. Another reference electrode is found (but you are not sure whether it is the same kind as the one that was broken). The voltage of the ISE in your unknown is 0.0237V vs. the new reference and the voltage of a mixture of 10.00mL of your unknown and 5.00mL of the 0.00200M KCl solution standard is 0.0318V. What is the concentratin of potassium in your unknown?

I attempted to solve this problem by ignoring the measurements that were made on the original electrode and setting up 2 equations for the 2 measurements made on the new electrode and subtracting those equations to solve for [K

^{+}] I was able to get an answer but it wasn't the correct answer. (The correct answer is 8.60E-4M.) I must be overlooking something, but I can't think what it is, so if anyone has any ideas, I'd really appreciate any help