Experiment 19 pH Titration of Unknown Soda Ash

The unknown soda ash is titrated with standard HCl using a potentiometric (pH) end point measured with a pH meter using a pH glass electrode-saturated calomel reference electrode combination. The end-point breaks are compared with indicator color changes. 

Note that between the first and second end points, a gradual decrease in pH due to the HC03 /C02 buffer system will occur. This will give a poor visual end point, unless the buffer couple is destroyed. In practice, the visual titration used for standardization is continued until the methyl purple end point is reached, at which time the solution is gently boiled to remove the CO2, leaving only the remaining HCOs , which is then titrated to completion (see Chapter 8 for a more detailed discussion). 

Provided. 0.2% phenolphthalein in 95% ethanol, 0.1% methyl purple in water, primary standard Na2C03, standard pH 7 buffer. 

To prepare. Standard 0.1 M HCl solution. Use the solution prepared in Experiment 7. If this solution is not available, prepare and standardize 500 mL as described in Experiment 7. Alternatively, the acid may be standardized against the primary standard Na2C03 by pH titration as described below for the unknown soda ash. 

Prepare and standardize the HCl solution. This will require prior drying of primary standard Na2C03. 

---

If you are to do Experiment 19, pH Titration of Unknown Soda Ash, prepare twice this amount and use this standardization for that experiment. 

---