Experiment 1: Preparation of a standard solution

To prepare a standard solution of potassium hydrogen phthalate and use it to standardise a sodium hydroxide solution

Notes

Potassium hydrogen phthalate is an acid because it contains a hydrogen atom which is released into the solution as a hydrogen ion when the compound is dissolved in water. It is a monobasic acid and reacts with sodium hydroxide in a ratio 1:1.

Potassium hydrogen phthalate is a suitable standard as:

1. it has a high relative molecular mass
2. it is soluble
3. it is stable in the atmosphere (non-hygroscopic and unreactive)

Instructions

Measure accurately about 5 g of the acid and dissolve it in a 250ml volumetric flask.

Use a pipette to deliver 25cm3 of the unknown sodium hydroxide solution into a 250ml conical flask.

Titrate against the standard potassium hydrogen phthalate using phenolphthalein indicator.

Repeat the titration until concordant results are obtained.

Calculate the average (of the concordant results) titre and use this to calculate the concentration of the unknown sodium hydroxide solution.

Experimental report

The following factors must be considered:

• Accurate recording of raw data
• Presentation of raw data
• Data analysis and presentation
• Evaluation of errors both qualitative and numerical

Assessment

• Data collection
• Data analysis
• Evaluation