ANALYTICAL CHEMISTRYERT 207

EXERCISES20 JAN 2011

Example 1

Find the mean and standard deviation of A’s results.

Student

A 10.08 10.11 10.09 10.10 10.12

B 9.88 10.14 10.02 9.80 10.21

C 10.19 9.79 9.69 10.05 9.78

D 10.04 9.98 10.02 9.97 10.04

Example 2:

In a new method for determining selenourea in water, the following values were obtained for taped water samples spkide with 50 ng/ml of selenourea:

50.4, 50.7, 49.1, 49.0, 51.1

Is there any evidence of systematic error?

Example 3:

In a comparison of two methods for the determination of chromium in rye grass, the following results were obtained

Method 1: mean=1.48; standard deviation= 0.28

Method 2: mean = 2.33; standard deviation= 0.31

For each method five determinations were made

Does these two methods gives results having means which differ significantly?

Example 4: In a series of experiments on the determination of tin

in food stuffs, samples were boiled with hydrochloric acid under reflux for different times. Some of the results are shown below:

Does the mean amount of tin found differ significantly for the two boiling times?

Refluxing time (min)

Tin found

30 55,57,59,56,56,59

75 57,55,58,59,59,59

Example5:

It is suspected that an acid-base titrimetric method has a significant indicator error and thus tends to give results with a positive systematic errors. To test this an exactly 0.1M solution of acid is used to titrate 25 ml of an exactly 0.1M solution of alkali, with the following results (ml):

25.06, 25.18, 24.87, 25.51 25.34, 26.41

Test for positive systematic errors in these results

Example 6:

Standard aqueous solutions of fluorescein are examined in a fluorescence spectrometer, and yiels the following fluorescence intensities:

Determine the correlation coefficient, rFluorescence intensities

2.1 5.0 9.0 12.6 17.3 21.0 24.7

Concentration 0 2 4 6 8 10 12

Example 7:

Calculate the slope and intercept of the regression line for the data given in the previous example.

Example 8:

Riboflavin (vitaminB2) is determined in a cereal sample by measuring its fluorescence intensity in 5% acetic solution. A calibration was prepared by measuring the fluorescence intensities of a series of standards of increasing concentrations. The following data were obtained. Use the method of least squares to obtain the best stright line for the calibration curve and to calculate the concentration of riboflavin. The sample fluorescence intensity was 15.4.

Riboflavin (µg/ml) Fluorescence intensity

0.000 0.0

0.100 5.8

0.200 12.2

0.400 22.3

0.800 43.3