Question

To test a spectrophotometer’s accuracy a solution of 60.06 ppm K2Cr2O7 in 5.0 mM H2SO4 is prepared and analyzed. This solution has an expected absorbance of 0.640 at 350.0 nm in a 1.0-cm cell when using 5.0 mM H2SO4 as a reagent blank. Several aliquots of the solu- tion produce the following absorbance values.

0.639 0.638 0.640 0.639 0.640 0.639 0.638 Determine whether there is a significant difference between the experi-

mental mean and the expected value at a = 0.01.

Answer 1

Mean value =(sum of the values)/(no of values taken)

Here, mean =(0.639+0.638+0.640+0.639+0.640+0.639+0.638)/7

= 4.473/7

= 0.639

According to the question, the expected value is 0.640

The difference between two absorption values = 0.640-0.639

= 0.001

Percentage error = difference between two values * 100/ actual value

= 0.001*100/0.640

= 0.156%

So, the difference between expected value and measured value is very less. Hence it is acceptable.

[ In practical situation, this deviation is not a big deal for UV-visible instrument. Sometimes instrumental error comes in this range or fluctuations is observed. ]