Page 204 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

6 SOLVENT EXTRACTION

may, however, be utilised for the joint determination of boron and anionic
detergents by the one procedure. The basis of this joint determination is that
the ferroin-anionic detergent complex may be immediately extracted into
chloroform, whereas the formation of the borodisalicylate anion from boric acid
and salicylate requires a reaction time of one hour prior to extraction using
ferroin. The absorbance of the chloroform extract obtained after zero minutes
thus gives a measure of the anionic detergent concentration, whereas the
absorbance of the extract after a one-hour reaction period corresponds to the
amount of boron plus anionic detergent present. Interference due to copper(I1)
ions may be eliminated by masking with EDTA.

Reagents. Sulphuric acid solution, 0.05M.
Sodium hydroxide solution, 0.1 M.
Sodium salicylate solution, 10 per cent wlu.
EDTA solution, 1 per cent wlu. Use the disodium salt of EDTA.
Ferroin solution, 2.5 x IO-' M. Dissolve 0.695 g of iron(I1) sulphate
heptahydrate and 1.485 g of 1,lO-phenanthroline hydrate in 100 mL of distilled
water.
Boric acid solution, 2.5 x M. Dissolve 61.8 mg of boric acid in 1 L of
distilled water; dilute 250 mL of this solution to 1 L to give the standard boric
acid solution.
Use analytical reagent grade materials whenever possible and store the
solutions in polythene bottles.
Procedure. (a) Zero-minutes veaction time. Neutralise a measured volume of
the sample containing 1-2 mg L- ' of boron with sodium hydroxide or sulphuric
acid (0.05M) to a pH of 5.5 (use a pH meter). Note the change in volume and
hence calculate the volume correction factor to be applied to the final result.
Measure 100 mL of the neutralised sample solution into a flask, add 10 mL of
10 per cent sodium salicylate solution and 17.5 mL of 0.05M sulphuric acid,
and mix the solutions thoroughly. Adjust the pH of the solution to pH 6 to 7
with 0.1M sodium hydroxide and transfer the solution immediately to a
separatory funnel; wash the flask with 20 mL of distilled water and add the
washings to the rest of the solution. Add by pipette 1 mL of 1 per cent EDTA
solution and 1 mL of 2.5 x 10-'M ferroin solution and again throughly mix
the solution. Add 50 mL of chloroform and shake the funnel for 30 seconds to
mix the phases thoroughly. Allow the layers to separate and transfer the
chloroform layer to another separatory funnel. Wash the chloroform by shaking
it vigorously for 30 seconds with 100 mL of water and repeat this process with
a second lOOmL of water. Filter the chloroform phase through cotton-wool
and measure the absorbance, A,, against pure chloroform at 516 nm in a 1 cm
ce11 [this 1 cm ce11 reading is used to calculate the boron concentration on the
basis of equation (1) (see below), but if the zero-minutes reading is to be used for
determination of anionic detergent concentration a 2 cm ce11 reading is more
suitable].
(6) One-hour veaction time. Measure a second 100 mL of neutralised sample
solution into a flask, add 10mL of 10 per cent sodium salicylate solution and
17.5 mL of 0.05 M sulphuric acid solution. Mix the solutions thoroughly, allow
the mixture to stand for one hour and adjust the pH of the solution to 6-7
with 0.1 M sodium hydroxide. Now proceed as previously described, under (a),