24
6.
Advance the wavelength setting by 1nm and repeat steps 2 to 5.
7.
Repeat step 6 until the wavelength setting reaches 815nm.
8.
Look for the maximum absorbance reading obtained, and this should be found between 805
and 809nm.
9.
If a “middle” wavelength check is desired, set the wavelength to 522nm (optional)
10.
Make sure the cuvette holder is empty in the sample compartment. Close the sample lid.
11.
Set zero Abs by pressing the 0A/100%T key. The reading should then be 0.000A .If not,
press 0Abs/100%T again
12.
Remove the cuvette holder and insert the Didymium filter into it. Place it in the sample
compartment and close the lid.
13.
Record the absorbance reading on the LCD display.
14.
Advance the wavelength setting by 1nm and repeat steps 10 to 13.
15.
Repeat step 14 until the wavelength setting reaches 536nm. Again, look for the maximum
absorbance reading. It should be between 527 and 531nm.
Absorbance Accuracy Checks
Specification: +0.004A at 0.5A.
The absorbance accuracy should be checked against a set of neutral density filters accurately calibrated to
the NIST standards. Contact your UNICO representative for more information (800-588-9776).
An alternative method using potassium dichromate is described below. Due to the many factors that
might affect the results (i.e. temperature, bandpass, weighing and diluting errors), this method is less
accurate and should only be used as a guide.
Reference: Johnson E A
Potassium Dichromate as an absorbance standard
PSG Bulletin 1967, No. 17, page 505
1.
Make up N/100 sulfuric acid as the solvent and use part of it to make a solution containing 120
+0.5mg/litre of potassium dichromate.
2.
Wash out a square cuvette with solvent, and fill with solvent.
3.
Put the cuvette into the sample compartment and close the lid.
4.
Select
BASIC MODE
and Set the wavelength to 350nm.
5.
Set the reading to 0.000A using the 0Abs/100%T key.
6.
Empty the cell. Wash out with dichromate solution, and fill with dichromate solution.
7.
Put the cuvette into the sample compartment and close the lid.
8.
Read the absorbance of the standard from the LCD display. The value should be Calibrated
Value + 0.004A. Refer to the notes above when interpreting the result.
