In a lab we did potentiometric titrations with a mixture of NaCl/KI and AgNO3. We plotted a titration curve and found the 2 endpoints which indicate where I is and where Cl is. We are looking to find the concentration of both of the halides present, and we have the equation Eind=E0(of Ag+) + 0.0592log(aAg+). I know that the acitivty in this case is linear to the concentration but whenever I try to put my values into this equation I get an answer of 0. Any help in how to figure this out would be greatly appreciated. The values I obtained were endpoint 1 is at 2.3mL with Eind being 347.9mv, and endpoint 2 is at 34.23ml with Eind being -12.8mv.
To find the concentration of halides (I- and Cl-) present in the mixture, you can use the Nernst equation for the standard electrode potential (E0) of Ag+ and the potentiometric titration data you obtained.
The Nernst equation relates the electrode potential (Eind) to the concentration of the analyte ion present. The equation you provided is:
Eind = E0(Ag+) + 0.0592log(aAg+)
To use this equation, you need to calculate the activity (a) of Ag+. The activity is related to the concentration of the ion through an activity coefficient (γ).
However, in the given equation, the activity is assumed to be linear to the concentration (aAg+ = [Ag+]), which simplifies the calculation. Therefore, you can directly use the concentration instead of the activity in this case.
Let's first calculate the concentration of Ag+ at the first endpoint:
Endpoint 1: Volume = 2.3 mL, Eind = 347.9 mV
Now, use the Nernst equation to calculate E0(Ag+):
Eind = E0(Ag+) + 0.0592log([Ag+])
Rearranging the equation:
E0(Ag+) = Eind - 0.0592log([Ag+])
Substituting the values:
E0(Ag+) = 347.9 mV - 0.0592log([Ag+])
Next, calculate the concentration of Ag+ at the second endpoint:
Endpoint 2: Volume = 34.23 mL, Eind = -12.8 mV
Using the same procedure:
E0(Ag+) = Eind - 0.0592log([Ag+])
E0(Ag+) = -12.8 mV - 0.0592log([Ag+])
Now, you have two equations with two unknowns, [Ag+] at endpoint 1 and endpoint 2. You can solve these equations simultaneously to find the concentration of Ag+ at both endpoints.
Once you have the concentration of Ag+ at each endpoint, you can calculate the concentration of halides (I- and Cl-) present in the mixture. Since you know that the mixture contains NaCl/KI, you can assume that the total concentration of halides is equal to the initial concentration of Ag+ added to the mixture.
For example, if the initial concentration of Ag+ added was c mol/L, then the concentration of I- and Cl- in the mixture would also be c mol/L.
Remember to convert mL to L (divide by 1000) when calculating concentrations.