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FAQ for colorimetry

Question - I have two Model 254 colorimeters and when I put the same solution in each, even after blanking, why do I get different results?
Answer - This is because each instrument and particularly each filter have slight differences in the way they pass the light through . You must therefore, for quantitative work , not only blank the colorimeter, but a standard of known concentration must be measured and the absorbance of the unknown sample be measured and compared with that of the standard.

( When blanking ( zeroing), the cuvette ,which will be used for measuring the standard and samples, should be filled with the solvent (usually water)that is to be used for the sample and standards (including all other reagents except the sample and standard substance). The cuvette is placed in the cuvette holder and the absorbance is set to 0.00 (0.000 for the Model 257) by adjusting the coarse and fine controls

Question - How do I choose the correct wavelength filter for my measurement.
Answer - If you are using a commercial kit of chemicals to perform an assay the kit manufacturer will give the wavelength in the kit "insert" or instructions.

If you are developing a new method and want to measure the concentration of a substance only at one wavelength then perform a spectral scan of the susbstance under investigation on a spectrophotometer to determine the wavelength of maximum absorbance.

Determine how broad the wavelength peak is at half peak height.

If it is less than 10nm then choose an interference filter with the wavelength the same as the peak

If it is > 20nm then you could choose a gelatin filter with a wavelength within 10nm of the peak wavelength.

If you have no spectrophotometer available then choose a filter of complementary wavelength.

For further information see our technical help page

Question - How long does a bulb last
Answer - About 1000 hrs.