Potentiometery Lab Purpose The purpose of this lab is to

Potentiometery Lab

Purpose

The purpose of this lab is to determine 2 out of the 3 endpoints found in a HCl and phosphoric acid compound. This is achieved through the use of a potentiometer to determine the amount of acidic compounds in the solution and then the amount of HCl and H2PO4 can be determined through excel calculations.

The second and third titration data points will be printed out and attached in the back with the carbon copy sheets.

Observations

The first titration went pretty fast because the purpose of the first titrations was purely qualitative and to see where the endpoints maybe. The second titration took much longer. That was because the rate of change in the pH began to increase near 10 mL, before that it would not change much and a lot of NaOH could be added. Once the pH started increasing by .1 or more, the goal became to record the volume of NaOH necessary to increase the pH by just .1. The main problem was that in the second titration the 2^nd endpoint was found much later than expected which led to it not being recorded in the data. That problem was rectified in the 3^rd titration because 2^nd titration let us know a much closer range for when the 2^nd end point might be hit.

Data Analysis and Results

First a regular titration curve was made through excel. The x-axis is total volume of NaOH added and y-axis is the pH. When graphed it looked like this:

Then a close up of each of the endpoint was graphed to determine the endpoint:

The endpoints are 22.05mL

1^st derivative for the 1^st derivative titration curve was found using this formula

ΔpH/Δvol

Then an average volume was used for the x-axis using this formula

(current amount of NaOH added + past amount of NaOH added)/2

These two calculations were used to chart the first derivative curve:

The 2^nd derivative titration curve used Δ^2pH/ Δvol^2 for its y axis and Average volume of NaOH for x- axis :

The endpoints are 13.5mL and 20.76mL

Then the concentration of NaOH was determined by using the volume of NaOH needed for the first endpoint in the 2^nd derivative graph which was 13.5mL . The amount of mols of NaOH used is calculated using .0135L * ([NaOH]=0.0927M)= 0.00125mols of NaOH reacted with both acids in the mixture.

Then it was determined that the volume for the 2^nd endpoint would be 20.76mL according to the 2^nd derivative graph. So to determine the mols used to react to H2PO4- only in the

mixture the formula (2^nd endpoint volume-1^st endpoint volume)*[NaOH]=

(.00726L)*.0927M= 0.00067mols of H2PO4- reacted with the NaOH

The amount of HCl in the mixture can be found by subtracting the total mols of NaOH reacted by the amount of NaOH reacted with H2PO4- which is .00125mols-.00067mols= 0.00058mols of HCl was in the sample.

Then to determine the molarity of each acid the formula mols/volume= [conc.]

This led to .00067mols/.025L= 0.027M concentration of H2PO4-

This also led to .00058/.025L=0.023M concentration of HCl

Discussion

Since there are no questions this discussion will touch on the topics on how the endpoints for each graph was determined. The regular titration curve endpoints were determined by seeing where the inflection points were in the graph and it was a close estimate. To confirm if that as correct, close up of the endpoint ranges were taken to see if the titration curve endpoint was valid. Then the first derivative graph was made to see a better picture if the endpoints were valid. The endpoints for these graphs were determined by finding the maximums in the graph. Then the 2^nd derivative graph was made to confirm the endpoints since the endpoints found in the second derivative graphs are the most accurate endpoints according to the manual.

Conclusion

A potentiometric titration is a form of titration where no indicator is used but the potential across the analyte is measured. The 1^st 2^nd endpoint of a HCl H3PO4 compound was determined through both the use of titration and excel calculations. The results obtained were satisfactory because all the numbers made sense and fit well to make neat number such as the concentration of HCl and H2PO4- being .023 and .027 respectively adding to become .05M acidic compound. The only thing that was wrong in the analysis was the determined second endpoint in the regular titration curves and 1^st derivative graphs. The actual 2^nd endpoint was determined through the 2^nd derivative graph which is considered the most accurate measure of endpoint relative to the other graphs. Some error in this experiment came from the 2^nd endpoint in the 2^nd titration. It was skipped over since we thought the endpoint was much earlier than the actual endpoint. That is why the data looks skewed for the 2^nd endpoint. This could’ve been avoided if we had used smaller aliquots of NaOH, A way to make this lab run smoother would also be use better clamps for the potentiometer since it was not stable most of the time because we had to use a makeshift clamp for it.

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