Measuring colorimeter precision and accuracy

Jo Long — Tue, 2012-05-15 16:40

Methods

To calculate the accuracy of the colorimeter we have used the ammonia colorimetric assay. A freshly opened 12.2 ppm ammonia standard solution (Ricca Chemical Company from Scientific Stategies Cat # 5450-4RC) was carefully diluted with distilled water as follows:

0.61 ppm - 5 mL in 100 mL volumetric flask
1.22 ppm - 10 mL in 100 mL volumetric flask
2.44 ppm - 20 mL in 100 mL volumetric flask

Each dilution was carried out in duplicate and the API ammonia test kit was used to develop 1 mL of each solution in triplicate. Absorbance was measured with the colorimeter red LED and data is shown in the Table below.

Results

Resolution	0.01 Absorbance Units
Accuracy	0.02 Absorbance Units (Up to 0.06 ppm ammonia)
Precision within cuvette	0.01 Absorbance Units
Precision between cuvettes	0.03 Absorbance Units

Sample Data

Ammonia (ppm)	Absorbance #1	Absorbance #2	Absorbance #3	Absorbance Mean	SD	*Colorimeter Reading	Accuracy (ppm)
0.61	0.20	0.18	0.19	0.19	0.010	0.60	-0.021
0.61	0.19	0.20	0.21	0.20	0.010	0.63	0.031
1.22	0.39	0.36	0.39	0.38	0.017	1.19	-0.021
1.22	0.40	0.38	0.41	0.40	0.015	1.25	0.022
2.44	0.85	0.82	0.80	0.82	0.025	2.59	0.061
2.44	0.81	0.81	0.81	0.81	0.000	2.55	0.044

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