Quantitative Determination of Quantitative Determination of Potassium Acid Phthalate (KHP)Potassium Acid Phthalate (KHP)

Erick N. ZorrillaMichelle Cruz

Abstract The experiment focused on the Standardization by titration of a weak acid, potassium acid phthalate (KHP), with a strong base, sodium hydroxide (NaOH) in order to determine the concentration of the NaOH solution and by quantitative analysis of potentiometric data. From the titration of the three trials of the standard KHP solution we obtained an average molarity of 0.12182M. From the three trials of the KHP unknown the average % KHPunknown was 40.80 % and the deviation error (RSD ppth) was 12.653. In the potentiometric titration the pKa average was 8.855 and the RSD ppth 7.4808.

Introduction• Direct TitrationDirect Titration• End PointEnd Point• Primary Standard Primary Standard

– High Purity – Stability – Non hygroscopic – Low cost – Reasonable solubility in

the titration medium – High formula weight to

minimize weighing errors– We used: KHP

• Equivalence PointEquivalence Point

KHP in the real worldKHP in the real world

• KHP can be used as a buffering agent. • Used in Industry as a Secondary Standard Solution.  

The Calcium Phosphate-Calcium Carbonate System: “Growth of Octacalcium Phosphate on Calcium Growth of Octacalcium Phosphate on Calcium CarbonatesCarbonates”. Stamatia Rokidi, Christele Combes, and Petros G. Koutsoukos. Department of Chemical Engineering and FORTH-ICEHT, University of Patras, GR 26 500, Patras, Greece.

Experimental• APPARATUS AND MATERIALS APPARATUS AND MATERIALS

– Class A Burets, 50.00 ± (0.05 mL) – KHP standard– Unknown KHP sample 99.97 % pure– NaOH 50% w/v – Phenolphthalein– Parafilm

Experimental

• Sample PreparationSample Preparation• ProcedureProcedure

A class A Buret was filled with the titrant 50 % w/v NaOH solution and used to titrate both the standard and unknown samples until a clear to pink color change was observed that persisted for more than 15 seconds.

KHP standard KHP Unkwown

DataTable 1. General specifications of KHP standard

% Purity FW: KHP Ratio Rx. Volume   (g/mol) (mol/mol) KHP (mL)

99.97 204.227 1  

Table 2. Gravimetric data for manual titrationsStandards Unknowns

Sample Mass (g) NaOH (mL) Sample Mass (g) NaOH (mL)S1 0.6816 27.97 D1 0.6959 11.57S2 0.6657 26.57 D2 0.6914 11.31S3 0.6453 25.57 D3 0.6530 10.59

Table 3. Titrimetric ResultsStandards

Sample KHP (g) Mol KHP M NaOH1 0.6814 3.336E-03 1.1929E-012 0.6655 3.259E-03 1.2264E-013 0.6451 3.159E-03 1.2353E-01

Averages 0.6640 0.0033 1.2182E-01Unknowns

Sample mol NaOH M KHP KHP (g) % KHP1 1.409E-03 1.218E-01 0.2879 41.362 1.378E-03 1.218E-01 0.2814 40.703 1.290E-03 1.218E-01 0.2635 40.35

Averages 1.359E-03 1.218E-01 2.776E-01 4.080E+01RSDppth       12.653

Calculations

Potentiometric Data

Potentiometric Data

Potentiometric Data ResultsTable 4. Potentiometric Data

YSample Final V Final V. (mL) Tit. Error pKa Avg pKaTrial 1 9.90 9.85 0.05 8.59 8.8550Trial 2 9.90 9.80 0.10 9.12

Y'Sample Final V Final V. (mL) Tit. Error    Trial 1 9.90 9.88 0.02    Trial 2 9.90 9.85 0.05  

Y"Sample Final V Final V. (mL) Tit. Error    Trial 1 9.90 9.91 0.01    Trial 2 9.90 9.90 0.00  

Gran PlotSample Final V Final V. (mL) Tit. Error    Trial 1 9.90 9.81 0.09    Trial 2 9.90 9.73 0.17  

Conclusions

From the titration of the three trials of the standard KHP solution we obtained an average molarity of 0.12182M. From the three trials of the KHP unknown the average % KHPunknown was 40.80 and the deviation error (RSD ppth) was 12.653. A source of error could be in the weighting of the KHP unknown sample that was measured to be approximately 0.6500-0.7000g yet the procedure called for 0.7500-0.8000g. Other errors could be reading the volumes on the buret incorrectly or that the measuring equipment wasn’t well calibrated. In the potentiometric titration the pKa average was 8.855 and the RSD ppth 7.4808. Both results are precise but it does not know if are accurate because the concentration of the KHP is unknown.

Additional Applications

Questions1. Which are the possible sources of error in your experiment and how can you correct them?

A possible error might be introduced in the weighing of the unknown KHP sample or the regular KHP sample. Titrating past the equivalence can introduce serious systematical errors.

2. What would happen if you do not boil the water used to prepare your titrant and sample solutions?

Impurities in the water such as high concentrations of Mg+ or Cu+ might interfere with the titration reaction taking place. Also water dissociation effects must be insignificant in order to assume that the neutralization of KHP with NaOH be considered a complete reaction. These results can also be affected by the presence of CO2 in the solution because this compound forms HCO3 which can be neutralized by the titrant NaOH.

3. Why does the typical acid/base indicator exhibit its color change over a range of about 2 pH units?

The theoretical useful range of an indicator is:

1/10≥[In^- ]/[HIn] ≥10/1 . Since pH = pKa + log (In-/HIn-) then pH =pKa – 1 for a change in acid color; pH = pKa + 1 for the basic color. So we can conclude the the pH range = pKa ± 1.

4. Why the standard reagents used in neutralization titrations are generally strong acids and bases rather than weak acids and bases.

The standard reagents used in neutralization titration are strong acids or bases because the strong acids and strong bases tend to react completely with all the reactive present. With the aid of indicators, Chemists can find the equivalence point where the stoichiometric analytical amount of titrant is chemically equivalent to the amount of the analyte reacted.

Questions

5. Explain in your own words how to calibrate a pH electrode. Using a 7.00 pH and a 4.01 buffer solution:Rinse the electrode using distilled water. Gently blot the electrode on a soft tissue to remove

the excess rinse water, not rubbing the bulb since it can cause a static charge build-up.Insert the electrode and place a thermometer along with the electrode in the 7.00 pH buffer

solution. Allow 30 seconds for the pair to reach thermal equilibrium with the buffer. Adjust the temperature dial on the meter to correspond with the thermometer reading. Then adjust the pH meter with the standardize/zero control for a pH indication equal to 7.00.

Repeat Step a, and insert the electrode and the ATC in a 4.01 buffer solution. Allow 30 seconds before adjusting the pH meter with the slope/span control for a pH indication equal to 4.01.

Repeat Steps b and c to maximize the precision of the calibration.

6. Do you think that the titration error of your experiment has a significant effect in your analysis? Justify your answer.

The titration error does have a significant effect in the analysis because if an error is made during the titration not all the results obtained are going to be accurate.

References

• De Jesus, M.A., M. Vera, and J.I. Padovani. "Quantitative Determination of Potassium Quantitative Determination of Potassium Acid PhthalateAcid Phthalate." Analytical Chemistry for Engineers Laboratory Manual. (2007): 67-73.