Potassium Dichromate Reference

Potassium Dichromate solvated in dilute (0.001N) perchlorate acid has been found to be the best method for testing the absorbance scale and linearity of spectrophotometers in the UV.  Our quality vendors purchases the potassium dichromate crystals directly from NIST (SRM 935a). We then make up the solutions as per the instructions from NIST and permanently heat fuse seal the solutions in UV quartz spectrophotometer cells. The filled cells are tested and certified against the expected values from NIST and a certificate is provided with each set of cells which has the certified absorbance values for each cell and the confidence limit. The certificate supplied is suitable for use with inspecting agencies when used in the context of a standard operating procedure for the quality control that is established in your laboratory. Each set is supplied with a sealed blank cell that was used for the certified measurement. Below is a table showing approximate absorbances for each concentration: Concentration 235nm 257nm 313nm 350nm 430nm 20 mg/L 0.2430 0.2810 0.0950 0.2090 n/a 40 mg/L 0.4920 0.5720 0.1920 0.4260 n/a 60 mg/L 0.7410 0.8620 0.2890 0.6340 n/a 80 mg/L 0.9960 1.1590 0.3850 0.8530 n/a 100 mg/L 1.2430 1.4480 0.4800 1.0690 n/a 600mg/L n/a n/a n/a n/a 0.950 Suggestions for Use Measure the absorbance of the potassium dichromate cells at the four wavelengths specified by NIST. These wavelengths for the 20 to 100mg/L solutions are 235, 257, 313 and 350nm and the 600mg/L solution measurement is made at 430nm. The absorbance scale is verified by assuring that the absorbance readings on your instrument are within your instruments tolerance to the readings on the certificate supplied with the reference set. To evaluate linearity, plot a graph of concentration against absorbance for each of the four wavelengths. The 600mg/L solution is used for European Pharmacopoeia compliance with a single wavelength measurment. Scan of Potassium Dichromate Linearity Plot of Potassium Dichromate Interpertation of Results Consult your spectrophotometer's reference manual for the specification of your instrument's absorbance scale tolerance. This figure will be listed as a plus and minus (+/-) number in absorbance units. Add your instrument's absorbance tolerance to the confidence limit on the certificate supplied with the Potassium Dichromate set for your expected absorbance range. Compare the absorbance values measured on your instrument with the expected values calculated from your instrument manual and the NIST traceable certificate. If the readings are within the expected range for your instrument then your absorbance scale is correct. If not, consult your service techincian and have the instrument evaluated. Quite often if the results of your absorbance measurments are low, it shows that your lamp needs to be replaced. Quality program Each time that the analysis is repeated you can compare the new results with the previous results to check for any variance. If your instrument needs service because of a change in absorbance, the historical data will greatly assist the service technician.