The Spectronic 20 Colorimeter has been around for many years and we still have a few in use. It is often simply called a Spec 20. At the top of the instrument is a meter that displays both absorbance and percent transmittance of the sample being measured. On the left side is the compartment for holding the sample to be measured. On the right are the wavelength selector and indicator. On the front left is the knob that turns on the power and controls the zero position on the %T scale. On the front right is the knob that controls the 100% position on the %T scale. (These features are also diagrammed in example 8 in your workbook.)
Here are the steps to follow when using a Spec 20.   Turn on the instrument using the front left knob. Allow about 5 minutes for  warmup when first turned on.
Select the appropriate wavelength for the sample to be measured.
With the sample holder empty and closed, adjust meter needle to 0%T  using the zero control knob.
Fill a tube half full with water. This is called a blank. Place it in the sample holder and close the cover.
With the blank in the sample holder and the cover closed, adjust the meter needle to 100%T using the light control knob on the right side of the instrument.
Remove the blank and place the sample to be measured in the sample holder and close the cover.
Read absorbance value (or %T) from meter. In this case the readings are 20%T and 0.70 absorbance units. Repeat this step with additional known samples if making a calibration curve or verifying proportionality (Beer's Law). Repeat this step with a solution of unknown concentration, so that its absorbance can be compared to the absorbance of a known solution.

Here is a summary of the steps of operation of a Spec 20.
General:	Specifics:
Power	Turn on power.
Warmup	Allow about 5 minutes when first turned on.
Wavelength	Select appropriate wavelength.
Zero	With sample holder empty and closed, adjust meter needle  to 0%T (or infinite A) using zero control knob.
Blank	Fill tube half full with water. Place in sample holder and close cover. Adjust meter needle to 100%T (or 0 A) using light control knob.
Standard	Measure absorbance (or %T) of known solution. Fill tube half full with sample of known concentration. Place in sample holder and close cover. Read absorbance value (or %T) from meter. Repeat this step if making a calibration curve or verifying proportionality (Beer's Law).
Sample	Measure absorbance (or %T) of solution with unknown concentration as in previous step.

Clackamas Community College
©1998, 2003 Clackamas Community College, Hal Bender