Analytical Chemistry

Table 2.3 Values of t for various levels of probability Degrees of freedom Confidence level (%) 80 90 95 99 99.9 1 3.08 6.31 12. ...

5 5.41 5.19 5.05 4.95 4.68 4.56 4.36 6 4.76 4.53 4.39 4.28 4.00 3.87 3.67 12 3.49 3.26 3.11 3.00 2.69 2.54 2.30 20 3.10 2.87 2.7 ...

Having established that the standard deviations of two sets of data agree at a reasonable confidence level it is possible to pro ...

Table 2.5 Some practical problems with relevant statistical tests Practical problems Relevant tests One result in a replicate se ...

...

(c) Equation (2.9) is then applied to compare the means From t-criteria the confidence level for this difference to be significa ...

0.904 10.13 In order to decide whether the error is constant or proportional plot a graph ...

of results against sample weight. Take care in selecting the scale to ensure that the trends are not obscured. The graph (Figure ...

Table 2.6 Precision limits for laboratory equipment used in volumetric analysis burettes ± 0.02 cm^3 for each reading pipettes ( ...

The relative standard deviation for the weighing is then Similarly, the overall precision of the volume measurement is obtained. ...

Finally, the estimated precision for the determination of the concentration of hydrochloric acid is obtained using equation (2.1 ...

deviations of the blank and sample are the same, i.e. σB = σS = σ. In most cases, a 95% confidence level is a realistic basis fo ...

where a large number of blanks (preferably more than 20) have been measured or where a smaller number of paired measurements of ...

Figure 2.5 A typical pair of Shewhart charts. (a) Averages chart and (b) ranges chart. Point A shows a lack of control of averag ...

Cusum Charts An alternative method of monitoring the behaviour of a process is the cumulative sum technique (cusum). The cusum p ...

Figure 2.7 A typical calibration curve. that, to avoid confusion, the analytical signal is plotted on the y-axis and the amount ...

Figure 2.8 Calibration curve for standard addition; S = unspiked sample; SS1, SS2, SS3 = spiked samples. as possible. These ensu ...

the values of a, the intercept on the y-axis, and b, the slope, can be computed from equations (2.17) and (2.18) respectively. C ...

Details can be found in the references at the end of this chapter. Where relationships are not straight lines, polynomial curve ...

Figure 2.9 'Best fit' plot of data from Table 2.7 obtained by 'least squares' regression analysis. (Important note: This graph i ...