Need of a spectrophotometer:
The specific instructions will differ with other models, but the principles remain.
1. The instrument must have been warm for at least 15 min. prior to use. The power switch doubles as the zeroing control.
2. Use the wavelength knob to set the desired wavelength. Extreme wavelengths, in the ultraviolet or infrared ranges, require special filters, light sources, and/or sample holders (cuvettes).
3. With the sample cover closed, use the zero control to adjust the meter needle to "0" on the % transmittance scale (with no sample in the instrument the light path is blocked, so the photometer reads no light at all).
4. Wipe the tube containing the reference solution with a lab wipe and place it into the sample holder. Close the cover and use the light control knob to set the meter needle to "0" on the absorbance scale.
5. Remove the reference tube, wipe off the first sample or standard tube, insert it and close the cover. Read and record the absorbance, not the transmittance.
6. Remove the sample tube, readjust the zero, and recalibrate if necessary before checking the next sample.