Page 277 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

ANALYSIS OF A MIXTURE USINC THE INTERNA1 NORMALISATION METHOO 9.7

graphic analysis to the determination of trace amounts of metals as their chelate
complexes. The procedure described for the determination of aluminium may
be adapted for the separation and determination of aluminium and chromium(111)
as their acetyla~etonates.~~
Sample. The solvent extraction of aluminium from aqueous solution using
acetylacetone can provide a suitable sample solution for gas chromatographic
analysis.
Take 5 mL of a solution containing about 15 mg of aluminium and adjust
the pH to between 4 and 6. Equilibrate the solution for 10 minutes with two
successive 5 mL portions of a solution made up of equal volumes of acetylacetone
(pure, redistilled) and chloroform. Combine the organic extracts. Fluoride ion
causes serious interference to the extraction and must be previously removed.
Introduce a 0.30 pL portion of the solvent extract into the gas chromatograph.
It is found that solutions of concentrations greater than 0.3 M are unsuitable
as they deposit solid and thus cause a blockage of the 1 pL microsyringe used
for the injection of the sample. The syringe is flushed several times with the
sample solution, filled with the sample to the required volume, excess liquid
wiped from the tip of the needle and the sample injected into the chromatograph.
Apparatus. A gas chromatograph equipped with a flame-ionisation detector
and data-handling system. The use of a digital integrator is particularly
convenient for quantitative determinations, but other methods of measuring
peak area may be used (Section 9.4).
Pure nitrogen (oxygen-free), at a flow rate of 40 mL min-', is used as carrier
gas. The dimensions of the glass column are 1.6m length and 6mm o.d., and
it is packed with 5 percent by weight SE-30 on Chromosorb W as the stationary
phase. The column is maintained at a temperature of 165 OC.
Procedure. Extract a series of aqueous aluminium solutions containing 5-25 mg
aluminium in 5mL, using the procedure described above under Sample.
Calibrate the apparatus by injecting 0.30 pL of each extract into the column
and recording the peak area on the chromatogram. Plot a graph of peak area
against concentration.
Determine aluminium (present as its acetylacetonate) in the sample solution
by injecting 0.30 pL into the column. Record the peak area obtained and read
off the aluminium concentration from the calibration graph (see Note).
Note. The calibration procedure is, however, of limited accuracy and a more accurate
result may be obtained using the method of standard addition (Section 9.4)

9.7 ANALYSIS OF A MIXTURE USlNG THE INTERNA1 NORMALISATION METHOD
To obtain an accurate quantitative analysis of the composition of a mixture, a
knowledge of the response of the detector to each component is required. If the
detector response is not the same for each component, then the areas under the
peaks clearly cannot be used as a direct measure of the proportion of the
wmponents in the mixture. The experiment described illustrates the use of an
interna1 normalisation method for the quantitative analysis of a mixture of the
following three components: ethyl acetate (ethanoate), octane, and ethyl
n-propyl ketone (hexan-3-one).