Practical investigations
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I am doing a practical to find the concentration of iron(III) ions in a sample of river water using sodium thiocyanate. I do not know the standard procedure for this experiment. Thanks.
Igloo writes ...
The conventional technique to use is colorimetry. Iron(III) ions form an intensely red coloured complex with thiocyanate ions, and even tiny concentrations of iron(III) will give some sort of colouration in the mixture.
First you will need to calibrate the colorimeter. Make up a set of solutions of iron(III) ions, using, say, iron(III) chloride. These solutions should be extremely dilute, matching the value of the tiny concentrations of these ions in river water. Next, take a measured volume of each solution, add an equal volume of sodium thiocyanate solution and measure its absorbance or transmittance in a colorimeter, using a suitable filter. I’ll leave you to decide how to choose this. Once this has been done for all the solutions, you will be able to plot your colorimeter readings against the concentration of iron(III). Don’t forget to halve the original concentrations of iron(III) in each case, since you have diluted the mixture by two when you added an equal volume of sodium thiocyanate. This plot is the calibration curve.
Next, take a measured volume of river water, add an equal volume of sodium thiocyanate and use the colorimeter to measure the absorbance or transmittance of the mixture. Use the calibration curve to read off the concentration of iron(III) ions. Don’t forget to double this value, since the reading represents the diluted solution as referred to for the calibrations readings.
Given that the red coloration of the mixture might be extremely faint, it is probably important to remove as much insoluble (muddy) matter from the river water as possible. A centrifuge might be useful here.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 20 January 2006
