Page 208 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

6 SOLVENT EXTRACTION
reacting heavy metals by extracting them with a fairly strong solution of
dithizone in chloroform until a green extract is obtained. Vessels (of Pyrex)
should be rinsed with dilute acid before use. Blanks must always be run.
Only one example of the use of dithizone in solvent extraction will be given
in order to illustrate the general technique involved.
Procedure. Dissolve 0.0079 g of pure lead nitrate in 1 L of water in a graduated
flask. To 10.0 mL of this solution (containing about 50 pg of lead) contained in
a 250 mL separatory funnel, add 75 mL of ammonia-cyanide-sulphite mixture
(Note l), adjust the pH of the solution to 9.5 (pH meter) by the cautious addition
of hydrochloric acid (CARE!),* then add 7.5 mL of a 0.005 per cent solution
of dithizone in chloroform (Note 2), followed by 17.5 mL of chloroform. Shake
for 1 minute, and allow the phases to separate. Determine the absorbance at
510 nm against a blank solution in a 1.0 cm absorption cell. A further extraction
of the same solution gives zero absorption indicative of the complete extraction
of the lead. Almost the same absorbance is obtained in the presence of 100 pg
of copper ion and 100 pg of zinc ion.

Notes. ( 1) This solution is prepared by diluting 35 mL of concentrated ammonia solution
(sp. gr. 0.88) and 3.0 mL of 10 per cent potassium cyanide solution (caution) to 100 mL,
and then dissolving 0.15 g of sodium sulphite in the solution.
(2) One millilitre of this solution is equivalent to about 20pg of lead. The solution
should be freshly prepared using the analytical-grade reagent, ideally taken from a new
or recently opened reagent bottle.

6.14 DETERMINATION OF MOLYBDENUM BY THE THIOCYANATE METHOD
Discussion. Molybdenum(V1) in acid solution when treated with tin(I1)
chloride [best in the presence of a little iron(I1) ion] is converted largely into
molybdenum(V): this forms a complex with thiocyanate ion, probably largely
Mo(SCN),, which is red in colour. The latter may be extracted with solvents
possessing donor oxygen atoms (3-methylbutanol is preferred). The colour
depends upon the acid concentration (optimum concentration 1M) and the
concentration of the thiocyanate ion (4 1 per cent, but colour intensity is
constant in the range 2-10 per cent); it is little influenced by excess of tin(I1)
chloride. The molybdenum complex has maximum absorption at 465 nm.
Reagents. Standard molybdenum solution. Dissolve 0.184 g of ammonium
molybdate (NH4),[Mo,024]4H20 in 1 L of distilled water in a graduated
flask: this gives a 0.01 per cent Mo solution containing 100 pg Mo mL-'.
Alternatively, dissolve 0.150 g of molybdenum trioxide in a few millilitres of
dilute sodium hydroxide solution, dilute with water to about 100mL, render
slightly acidic with dilute hydrochloric acid, and then dilute to 1 L with water
in a graduated flask: this is a 0.0100 per cent solution. It can be diluted to
0.001 per cent with 0.1M hydrochloric acid.
Ammonium iron(I1) sulphate solution. Dissolve 10 g of the salt in 100 mL of very
dilute sulphuric acid.

* It is essential that the pH of the mixture does not fa11 below 9.5, even temporarily, as there is
always the possibility that HCN could be liberated.