Investigation 15.3 Demonstration of pH Curves

Problem

What are the shapes of the pH curves for the continuous addition of hydrochloric acid to a sample of sodium hydroxide solution?

Design

Small volumes of HCl are added to a measured volume of base. After each addition, the pH of the mixture is measured. The volume of HCl  is the manipulated variable and the pH of the mixture is the responding variable. 

Spreadsheet Lab



Flash object created by Chemical Education Research Group, Department of Chemistry, Iowa State University

Procedure

1. Follow the numbered steps. First select strong acid strong base.
2. Fill the buret with acid.
3. Select the acid (HCl) and the base (NaOH).
4. Select the indicator (Bromothymol blue).
5. Push the slider up to add a volume of acid. Start with 16 or 17mL.
6. Record the volume and molarity of the base in the flask.
7. Insert the temperature and pH probes.
8. Record the starting pH.
9. Use the Dropwise button beside the buret to add titrant to the sample solution.
10. When a drop of titrant changes the color of the sample to pale yellow take a final buret reading.
11. Record the pH of the solution in the beaker at each 0.1 mL interval. 
12. Record the pH of the solution when the color change takes place.
13. Continue adding titrant and recording pH until at least 3 more mL have been added.
12. After titration calculate the molarity of the acid and enter it in the table. Click OK to see if your calculation is correct.
13. Use your data to draw a titration curve.
14. Indicate the endpoint pH and the equivalence point on the curve.

Evidence