The Differences Between Strong Acid And Weak Acid Titration

The Differences between Strong acid and Weak Acid Titration

Introduction:

This experiment is to explore the certain properties of strong acid (HCl) and weak acid (HCOOH) and how these properties make effects on titrations. The first is to find out the properties and differences of strong acid using phenolphthalein measurement and curve measurement. The second step is to find out the properties and differences of weak acid using phenolphthalein measurement and curve. The final step is to compare the salts that produced.

For both acid-base titrations, the basic mechanisms are the same. The general procedure is adding 3-4 drops of phenolphthalein before adding NaOH into acids. Then start to add NaOH until the pH indicator turns into pink and keep recording the volume of NaOH addition all the time.

The choice of the pH indicator is very important in this experiment because it determines the experimental equivalence point and whether or not move on to next steps. Phenolphthalein is an appropriate indicator because the solution will change from acidic into basic and phenolphthalein shows pink between 8.2 and 9.8. (McMurry. Fay. 2012)

For the HCl and NaOH titration, we start with 50mL HCl (0.05M). Before the titration add 3-4 phenolphthalein into the HCl solution and record the pH meter and the burette. Add about 4 mL NaOH each time until the pH is about 2.5. Then change the addition of NaOH into 0.2mL or less for each time until the solution turns to faint pink that exists for few seconds. Then continue adding 0.2mL additions until there is a large change of pH value. Next, add four 0.4 mL additions and then keep adding large amount of NaOH until the base is consumed about 15 to 20 after faint pink.

For the HCOOH and NaOH titration, we...

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...n more than expected. Thirdly, the indicator also cause small errors because it is not neutral, instead, it is weakly basic so the original solution may be less acidic than we expected. Last but not least, the pH meter may not precise. For example, after we calibrating the pH meter and discard the buffer, the pH changes a little bit (6.98). Therefore, all the data collected afterwards may a little bit smaller than actual value.

Conclusion:

The properties of acid and base determine the pH a salt. Basic salt has the equivalence point above 7 (pH(HCOONa)=8.6) Neutral solution Acidic salt associates below 7 (pH (NH4Cl)=6.45). And neutral salt such as NaCl has equivalence point at 7.

Bibliography:

J.E.McMurry. R.C.Fay. Chemistry. 6E. Pearson Prentice Hall. 2012

Laboratory Manual for Chemistry 139. University of Toronto Chemistry Department. Winter 2015