Analytical Chemistry

Table 8.9 Precision of silicate analyses

Element Approx.

comp./wt %

Std.dev./wt %

X-ray Wet Arc/spark

O 48.0 0.4 0.3 —

Na 2.6 0.02 0.15 0.11

Mg 0.5 0.01 0.15 0.12

Al 9.3 0.04 0.19 0.53

Si 30.0 0.10 0.14 1.1

K 3.7 0.03 0.21 0.15

Ca 2.5 0.01 0.07 0.14

Ti 0.22 0.003 — —

Fe 3.1 0.02 0.21 0.14

Table 8.9 shows an analysis of a silicate rock and compares the precision of X-ray fluorescence analysis
with wet chemical methods and arc/spark emission spectrometry.

Problems

(1) List the advantages and disadvantages of dc-arc spectrometry and inductively coupled plasma
spectrometry, when used to determine trace metals in solid samples. Indicate the main reasons for
preferring ICP spectrometry, in most cases.

(2) Flame atomic absorption spectrometry has achieved very wide use as a routine method for the
determination of trace metals in solution. However, for alkali metals flame photometry has remained
popular. Why is this?

(3) A sample of stainless steel (0.320 g) was weighed out and dissolved in nitric acid. The resulting
solution was made up to 1 dm^3 with water. Five standards and the sample solution were analysed for
nickel consecutively on a flame atomic absorption spectrophotometer with the following results:

Solution Ni concentration/ppm Absorbance

1 2 0.126

2 4 0.250

3 6 0.374

4 8 0.500

5 10 0.626

Sample — 0.220

Calculate the amount of nickel in the steel.