The Spectronic 21 Colorimeter is an update of the Specronic 20 Colorimeter and we have several of these in use. The basic operation is very similar to that of the Spec 20, but the zeroing has been automated and the controls are different. 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 lower front left is the knob that controls the 100% position on the %T scale. On the lower front right is the switch that turns on the power. (These features are also diagrammed in example 8 in your workbook.)
Here are the steps to follow when using a Spec 21.   Turn on the instrument using the switch at the lower right front of the instrument. Allow about 5 minutes for  warmup when first turned on.
Select the appropriate wavelength for the sample to be measured.
With the Spec 21 no adjustment is necessary to set 0%T, this is done automatically.
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 all the way to 100%T using the light control knob on the lower left front 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 21.
General:	Specifics:
Power	Turn on power.
Warmup	Allow about 5 minutes when first turned on.
Wavelength	Select appropriate wavelength.
Zero	This step is automatic on the Spectronic 21.
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