Analysis of Acid by Titration with Sodium Hydroxide

Introduction

The purpose of this experiment is to demonstrate an example of how to determine the unknown molarity of hydrochloric acid by titration with a base (sodium hydroxide). Titration is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of an identified analyte (Wikipedia). The first step will be measuring and combining water and acid (Hydrochloric acid).

An indicator anthocyanin will be added to the solution to change the color to pink. Anthocyanin is a water-soluble vacuolar pigment that may appear red, purple, or blue depending on the pH (Wikipedia). This pink color acts as a pH meter and will show a change in color to represent a change in the pH. As the base (sodium hydroxide) is added, the solution will then change color (this is referred to as the “stoichiometric endpoint”). This color change represents the increase of pH, as well as showing that the moles of the acid in the solution is equal to the moles of the base (Survey of Chemistry lab manual).

Using a dilution equation of M1xV1=M2xV2, the concentration of the hydrochloric acid can be calculated. M1=NaOH(molarity), V1=NaOH(volume), M2=HCl(molarity), V2=HCl(volume).

Procedure

Instructions for the laboratory were found on page 91 of the Survey of Chemistry lab manual. All calculations were rounded off by 2 decimal places for accuracy. Preparation of the indicator required a combination of 50 grams of cabbage with 50ml of ethanol and 20mL of DI water to extract the anthocyanin indicator. This procedure was completed by the instructor and the anthocyanin indicator (“cabbage extract”) was provided.

Then 10mL of hydrochloric acid and 20mL in of deionized water was measured in a graduated cylinder and combined into an Erlenmeyer flask. Pipette 5 to 10 drops of anthocyanin indicator was added to the solution in Erlenmeyer flask until the solution change to pink color. Then the solution was titrated with sodium hydroxide until the solution changed color from pink to blue. The titration of the acid was repeated in three trials. All data and calculations were recorded.

Data Tables

Calculations

As previously stated in the introduction, formula M1xV1=M2xV2 found on page 93 of the Survey of Chemistry laboratory manual was used to calculate the concentration of hydrochloric acid (HCl).

• Trial 1 M1 = 0. 1 (NaOH) V1 = 18. 39mL (NaOH) M2=Unknown (HCl) V2=9. mL (HCl) 0. 1 x 18. 39 = M2 x 9. 1 1. 839/9. 1 = M2 M2 = 0. 202M
• Trial 2 M1 = 0. 1 (NaOH) V1 = 18. 40mL (NaOH) M2=Unknown (HCl) V2=9. 2mL (HCl) 0. 1 x 18. 40 = M2 x 9. 2 1. 839/9. 2 = M2 M2 = 0. 199M
• Trial 3 M1 = 0. 1 (NaOH) V1 = 19. 00mL (NaOH) M2=Unknown (HCl) V2=10. 10mL (HCl) 0. 1 x 18. 39=M2 x 10. 10 1. 839/10. 10 = M2 M2 = 0. 182M

Average of acid 0. 202 + 0. 199 + 0. 182/3 = 0. 194M = M average Discussion and conclusion: This lab was successful in the proper color change needed to represent an endpoint when acid (hydrochloric acid) and indicator anthocyanin is titrated with a base (sodium hydroxide).

As stated in the lab manual anthocyanin will go from a color of pink to blue, a pH of seven to eight, where the blue is an indication that acid has been neutralized by the base (Survey of Chemistry lab manual). During this experiment after adding enough sodium hydroxide to the indicator solution; the solution changed color to blue, an indication of the stoichiometric endpoint. All three trials showed proper color change. This proved that the color change indicated that the pH level had changed in solution and the amount of base added is chemically equivalent to the acid in the flask.

The formula M1xV1=M2xV2 was used to calculate the unknown concentration of hydrochloric acid. The results from the trials showed that the concentration of hydrochloric acid was 0. 2M. The concentration of the two acids should be relatively close in order to cancel each other out. It was complicated to compare the two concentrations since the given molarity of sodium hydroxide is in the one decimal place holder, whereas the calculated concentration for hydrochloric acid is in a two decimal place. From the lab, it can be concluded that pH indicators can be an imprecise method to calculate the concentration of an unknown concentration.

Critical Thinking Questions

1. • Trial 1 M1 = 0. 1 (NaOH) V1 = 18. 39mL (NaOH) M2=Unknown (HCl) V2=9. 1mL (HCl) 0. 1 x 18. 39 = M2 x 9. 1 1. 839/9. 1 = M2 M2 = 0. 202M
   • Trial 2 M1 = 0. 1 (NaOH) V1 = 18. 40mL (NaOH) M2=Unknown (HCl) V2=9. 2mL (HCl) 0. 1 x 18. 40 = M2 x 9. 2 1. 839/9. 2 = M2 M2 = 0. 199M
   • Trial 3 M1 = 0. 1 (NaOH) V1 = 19. 00mL (NaOH) M2=Unknown (HCl) V2=10. 10mL (HCl) 0. 1 x 18. 39=M2 x 9. 1 1. 839/10. 10 = M2 M2 = 0. 182M
2. 0. 202 + 0. 199 + 0. 182/3 = 0. 194 = M average.
3. 0. 08233 – 0. 194 x 100/0. 08233 = 135. 63 percentage error.
4. The main concept of this lab was to calculate the unknown molarity of acid through titration.
5. Some of the experiment performed was to determine the concentration of an unknown concentration of acid through titration by adding a second known concentration acid that will react with and cancel the first one. The endpoint of the titration will allow you to calculate the original concentration. 6. The dilution equation of M1xV1=M2xV2 was required to solve for the molarity of the hydrochloric acid where M1=NaOH(molarity), V1=NaOH(volume), M2=HCl(molarity), V2=HCl(volume).
6. The lab was successful in the properly changing color to represent a stoichiometric endpoint through titration. The dilution equation was then used to calculate the unknown molarity of one of the acid. The calculated results weren’t easily compared due to their decimal placing.

References:

1. Survey of Chemistry lab manual, “Chem 1151 Laboratory Experiments 3rd edition 2011-2012”
2. http://en.wikipedia.org/wiki/Anthocyanin
3. http://en. wikipedia.org/wiki/Titration
4. http://en.wikipedia.org/wiki/Endpoint_%28chemistry%29