Task 1: Standardisation of EDTA

To accurately calculate the concentration of EDTA it is necessary to titrate it against a standard solution of calcium carbonate of a known concentration (to a high degree of accuracy). You will be using a solution of calcium carbonate of a concentration of 0.010 mol l^-1.

The concentration of the EDTA solution is precisely calculated by titration of a known volume (10.00 ml) of a primary standard calcium solution of a known concentration (0.010 mol l^–1). The molar concentration (or molarity M) of the EDTA solution by titrating 10.00 ml of a standard calcium solution of concentration 0.010 mol l ⁻¹ is determined as follows:

M x V₁= (0.010 mol l^-1 x 10.00 ml)

where V₁ is the volume (ml) of EDTA used in the standardisation titration.

You will use the Complexometric titration application to determine the precise concentration of the EDTA solution that you will later use to determine the hardness of your river water sample. Detailed instructions are provided within the experiment but the following is a summary of the steps you need to take:

1. Fill your burette with EDTA solution.
2. Measure as accurately as possible 10.00 ml of the standard calcium solution using the appropriate glassware and transfer into a conical flask.
3. Add deionised water by selecting appropriate glassware and dilute to 50 ml.
4. Add 2.00 ml ammonia buffer solution using the appropriate glassware.
5. Add one or two drops of ErioT indicator. The solution will turn pink.
6. Record the initial burette reading.
7. Titrate with the EDTA solution until the colour of the mixture turns from pink, through purple, to blue. While adding EDTA, make sure the solution in the conical flask is thoroughly mixed using a magnetic stirrer. As the end point approaches and the solution turns purple, the EDTA should be added very slowly – drop by drop. At the end point the last of the pinkish tinge disappears and a pure blue colour is left.
8. Record the volume of EDTA added.

Note that it is good practice to do a ‘rough’ titration first to estimate the approximate volume of EDTA required and then do the titration more carefully adding drop-by-drop as you approach the estimated end point. You should repeat this standardisation procedure at least twice. You may need a third replicate measurement if your analytical results lack precision. Although you should include the value of your ‘rough’ standardisation in your records, you should not use it in any subsequent calculations.

Table 1 shows a template you could use to record your experimental data.

Table 1 Volumetric data for the standardisation of EDTA solution

You are now ready to attempt Task 1 – the standardisation of EDTA solution.

Once you have completed Task 1, you can calculate the concentration of the EDTA solution supplied in this experiment.

Work through the example calculation below to familiarise yourself with the calculation required to complete Task 1.

You should now calculate the concentration of the EDTA solution supplied in this investigation using the average volume of EDTA used.

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