Instant Notes: Analytical Chemistry

Determinate errors can be proportionalto the size of sample taken for

analysis. If so, they will have the same effect on the magnitude of a result

regardless of the size of the sample, and their presence can thus be difficult to

detect. For example, copper(II) can be determined by titration after reaction with

potassium iodide to release iodine according to the equation

2Cu^2 ++4I-Æ2CuI +I 2

However, the reaction is not specific to copper(II), and any iron(III) present in

the sample will react in the same way. Results for the determination of copper in

an alloy containing 20%, but which also contained 0.2% of iron are shown in

Figure 2for a range of sample sizes. The same absolute error of +0.2% or relative

error of 1% (i.e. a positive bias) occurs regardless of sample size, due to the

presence of the iron. This type of error may go undetected unless the

constituents of the sample and the chemistry of the method are known.

B1 – Errors in analytical measurements 23

Sample size (g)

0

21

20

19

0.1 0.2 0.3 0.4 0.5 0.6 0.7

Copper found (%)

True value

Positive

bias

Fig. 2. Effect of a proportional error on the determination of copper by titration in the

presence of iron.

Constantdeterminate errors are independent of sample size, and therefore

become less significant as the sample size is increased. For example, where a

visual indicator is employed in a volumetric procedure, a small amount of

titrant is required to change the color at the end-point, even in a blanksolution

(i.e. when the solution contains none of the species to be determined). This

indicator blank(Topic C5) is the same regardless of the size of the titer when

the species being determined is present. The relative error, therefore, decreases

with the magnitude of the titer, as shown graphically in Figure 3. Thus, for an

indicator blank of 0.02 cm^3 , the relative error for a 1 cm^3 titer is 2%, but this falls

to only 0.08% for a 25 cm^3 titer.

Known also as random errors, these arise from random fluctuations in

measured quantities, which always occur even under closely controlled condi-

tions. It is impossible to eliminate them entirely, but they can be minimized by

careful experimental design and control. Environmental factors such as temper-

ature, pressure and humidity, and electrical properties such as current, voltage

and resistance are all susceptible to small continuous and random variations

described as noise. These contribute to the overall indeterminate error in any

Indeterminate
errors