Page 209 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

OETERMINATION OF NICKEL AS THE OIMETHVLCLVOXIME COMPLEX 6.15

Tin(I1) chloride solution. Dissolve 10 g of tin(I1) chloride dihydrate in 100 mL
of 1M hydrochloric acid.
Potassium thiocyanate solution. Prepare a 10 per cent aqueous solution from
the pure salt.
Procedure. Construct a calibration curve by placing 1.0,2.0, 3.0,4.0, and 5.0 mL
of the 0.001 per cent Mo solution (containing 10 pg, 20 pg, 30 pg, 40 pg, and
50 pg Mo respectively) in 50 mL separatory funnels and diluting each with an
equal volume of water. Add to each funne12.0 mL of concentrated hydrochloric
acid, 1.0mL of the ammonium iron(I1) sulphate solution, and 3.0 mL of the
potassium thiocyanate solution; shake gently and then introduce 3.0 mL of the
tin(I1) chloride solution. Add water to bring the total volume in each separatory
funnel to 25 mL and mix. Pipette 10.0 mL of redistilled 3-methylbutanol into
each funnel and shake individually for 30 seconds. Allow the phases to separate,
and carefully run out the lower aqueous layer. Remove the glass stopper and pour
the alcoholic extract through a small plug of purified glass wool in a small
funnel and collect the organic extract in a 1.0 cm absorption cell. Measure the
absorbance at 465 nm in a spectrophotometer against a 3-methylbutanol blank.
Plot absorbance against pg of Mo. A straight line is obtained over the range
0-50 pg Mo: Beer's law is obeyed (Section 17.2).
Determine the concentration of Mo in unknown samples supplied and
containing less than 50 pg Mo per 10 mL: use the calibration curve, and subject
the unknown to the same treatment as the standard solutions.
The above procedure may be adapted to the determination of molybdenum
in steel. Dissolve a 1.00 g sample of the steel (accurately weighed) in 5 mL of
1 : 1 hydrochloric acid and 15 mL of 70 percent perchloric acid. Heat the solution
until dense fumes are evolved and then for 6-7 minutes longer. Cool, add 20 mL
of water, and warm to dissolve al1 salts. Dilute the resulting cooled solution to
volume in a 1 L flask. Pipette 10.0mL of the diluted solution into a 50mL
separatory funnel, add 3 mL of the tin(I1) chloride solution, and continue as
detailed above. Measure the absorbance of the extract at 465nm with a
spectrophotometer, and compare this value with that obtained with known
amounts of molybdenum. Use the calibration curve prepared with equal
amounts of iron and varying quantities of molybdenum. If preferred, a mixture
of 3-methylbutanol and carbon tetrachloride, which is heavier than water, can
be used as extractant.
Note. Under the above conditions of determination the following elements interfere in
the amount specified when the amount of Mo is 10pg (error greater than 3 per cent):
V, 0.4 mg, yellow colour [interference prevented by washing extract with tin(I1) chloride
solution]; Cr(VI), 2 mg, purple colour; W(VI), 0.15 mg, yellow colour; Co, 12 mg, slight
green colour; Cu, 5 mg; Pb, 10 mg; Ti(III), 30 mg (in presence of sodium fluoride).

6.15 DETERMINATION OF NICKEL AS THE DIMETHYLGLYOXIME COMPLEX
Discussion. Nickel (200-400 pg) forms the red dimethylglyoxime complex
in a slightly alkaline medium; it is only slightly soluble in chloroform
(35-50 pg Ni mL- '). The optimum pH range of extraction of the nickel complex
is 7-12 in the presence of citrate. The nickel complex absorbs at 366 nm and
also at 465-470 nm.