determination of metals dr. mohammad a. khanfar. metals metals can be either impurities or the...
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Determination of Metals
Dr. Mohammad A. Khanfar
Metals
• Metals can be either impurities or the active ingredient in pharmaceutical preparations.
• If present as impurities (traces), then physical methods are used for their determination. Such as:
• 1) Visible absorption spectroscopy of metal-ligand complexes.• 2) Atomic absorption spectroscopy.• 3) Emission spectroscopy. • If the metal is the active ingredient it is usually present in high
level, so it’s determined by:• First the metal needs to be freed from the organic or inorganic
(salt) form.• Determined by certain elemental analysis procedures.
Determination of Mercury
• Organic and inorganic mercury may be used in pharmacy:– Mercuric chloride, phenyl mercuric benzoate are antiseptic.– Meralluride and mersalyl are diuretics.
• Sample preparation:• In pharmaceutical samples mercury is present as R-Hg-R`• First step is to free mercury by either:• 1) Dissolving inorganic Hg in HNO3 to give Hg2+.
• 2) For organic mercury (where Hg is bound to carbon) mercury is freed either by oxidation to Hg2+ or reduction to Hg0 metal.
Determination of Mercury
• Oxidation: the most common method is wet oxidation: mercury compound is refluxed with oxidizing mixture to give Hg2+.
• Oxidizing agents such as: H2SO4/HNO3, or H2SO4/H2O2, or HNO3/KMnO4
• Reduction: commonly done using Zinc (Zn) with acetic acid or in hydroxide solution (NaOH).
• The product of reduction is excess Zn and Hg0 metal(known as zing amalgam).
• After reducing Hg, the resulting amalgam is dissolved in HNO3/H2SO4 to fully oxidize Hg to Hg2+
Determination of Mercury
• Determination of Hg2+
• 1) Thiocyanate titration: most popular method. • Titrant: SCN-• Analyste: Hg2+ in HNO3/H2SO4 (or HNO3 for inorganic)• Indicator: Ferric ion (Fe3+)• End point: brownish-red due to the formation of (FeSCN)2+
• The reaction:
• The reaction must be carried at 15Co to minimize the dissociation of Hg(SCN)2
Determination of Mercury
• 2) Colorimetric determination of Hg2+:• Under acidic condition, Hg2+ forms complex with dithiazone
(colorful):
• Acetic acid is added to stabilize the color. • The complex is extracted with chloroform and measured at
500nm.
Iron determination
• 1) Redox titration• Fe2+ ion is titrated with oxidizing agent (KMnO4, K2Cr2O7, Ce4+).
• Mostly titration is direct. • If the sample contain both Fe2+ and Fe3+, the sample is first
reduced by Zn to convert all Fe3+ to Fe2+ then determined as usual .
• Fe3+ can be determined with I-:
• I2 is then titrated with Na2S2O3.
Iron determination
• 2) Complexometric titration• Fe3+ form stable complexes with EDTA and thus can be
determined compleximetrically.• 3) Fe3+ can be determined gravimetrically by precipitation as
Ferric oxinate.
Iron determination
• 4) Low concentration of Fe3+ are determined spectrophotometrically:
• Fe3+ + HClO4, absorbance at 240nm.
• Fe3+ + SCN-, brown color. • Fe2+ can be determined by 1,10-phenanthrolin:
Ferroin, red color
Calcium Determination
• Two common methods:• 1) ppt Ca2+ by oxalate, then Ca2+-oxalate is removed by filtration,
dissolved in acidic medium and titrated with KMnO4.
• Mg-oxalate is water soluble
Calcium Determination
• 2) The preferred assay of Ca2+ with EDTA• Indicator: best choice is hydroxynaphthol blue.
Lead determination
• The test identifies the substance to be examined as a salt of lead, Pb2+
• Lead will form a yellow insoluble salt with chromate
• The ppt is then filtered and titrated with measured excess of NaOH.
• And then back titrated with strong acid
Cupper determination
• Assayed by iodometric method.
• Then back titrated with sodium thiosulfate
Zinc determination
• In the first step of Zn determination, the test solution is added to strong NaOH solution. The zinc hydro complex is formed:
• Then ammonium chloride solution is added to change hydro complex into a soluble ammonia complex, the tetramminezincate(II) ion
• The last step is to add sodium sulfide solution, which causes the formation of a white flocculent precipitate of zinc sulfide, which determined gravimetrically
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