Relative error distribution due to instruments

For many contemporary spectrophotometers it is possible to measure absorbance up to within 4 or 5 units. These high values correspond to transmitted intensities that are extremely weak (I/Ig = 10 for A = 5), giving results that are considered as less reliable. 

As these sources of error are additive, the total error in the concentration corresponds — in accepting the three expressions above — to a curve whose ordinate of each point is the sum of the ordinate of the three individual errors discussed above. This curve passes through a minimum generally located around A = 0.7. For quantitative measurements it is therefore advisable to adjust the dilutions in order that the absorbances are situated in this favourable domain. 

From the Lambert—Beer law it is also possible to link relative standard deviation (see Section 22.2) on the measurement of concentration C to the relative standard deviation made on the transmittance T (curve 4). 

---