Colorimeters

There are two different types of instrument in the laboratory (Griffin and Jenway). The procedures are similar, but make sure you know which type of instrument you are using and always use the same machine for your measurements.

The filters are used to isolate a part of the visible light spectrum that is absorbed maximally by the sample. Different colourimeters use different sets of filters but typical wavelengths passed are red filter: 630-750nm, green filter: 510-570nm and blue filter: 360-480nm. Although you will normally be told which filter to use you should consider and understand the reason for this choice.

1. Switch on the instrument at least 5 minutes before use to allow it to stabilize.
2. Select the most appropriate filter for the analysis and insert it in the light path (Griffin) or dial it in with the selector (Jenway).
3. Place the reagent blank solution (or water) in the cuvette and zero the instrument (either using the ëset zeroí control (Jenway) or the ëcalibrateí switch (Griffin). Make sure the clear faces of the cuvette are in the light path (which runs from back to front in the Jenway but side to side in the Griffin).
4. Place the sample in the colorimeter and read the absorbance of the solution. If the absorbance is "over range" (usually > 2.0) then the sample must be diluted to yield a value within the limits of the instrument.
5. At intervals, recheck the reagent blank to ensure that there is no drift in the zero value.

Notes

• The machine must be re-zeroed (step 3) if a new filter is chosen.

• The sample cuvettes must be at least two-thirds full.

• The outside of the cuvette must be dry and clean.

• If an instrument appears to be faulty, do not abandon it but consult a demonstrator!

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