QC LAB

• Complexometric titrations/

  Complexation

  Based in the formation of complex between the

  analyte and the titrant.
• Complexometric titrations/

  Complexation

  The chelating agent, EDTA is very commonly used

  to titrate metal ions in solutions.
• gravimetric method

  is used to determine inorganic

  ions and polyvalent ions such as, Al, Zn, Ca, Cu, Bi and Hg.
• With the introduction of a new analytical agent, called disodium

  ethylenediamine tetraacetate (EDTA), C10H4N2Na2O2 . 2H2O.
• EDTA

  will react with metal ions to form water soluble, stable complex or

  chelate compounds.
• Complex

  It is when a metal ion combines with a molecule which can

  donate electrons.
• Chelate

  It is when the combining molecule contains two or more

  groups of that donates electron
• The four oxygen and two nitrogen of EDTA is capable of entering a

  complexation process forming a hexadentate molecule. The molecule of EDTA which provides group of attachment to metal ions is called a ligand
• A complex ion forms from a metal ion and a

  ligand from one or more lone pairs of electrons because of a Lewis acid-base

  interaction.
• The positively charged metal ions acts as a

  Lewis acid, and the ligand, with one or more

  lone pairs of electrons, acts as a Lewis base.
• An effective masking agent is the cyanide ion, this forms stable cyanide

  complexes with the cations of Cd, Zn, Hg (II), Cu, Co, Ni, Ag and the

  platinum metals, but not with the alkaline earths, manganese and lead.
• Masking

  A term used to indicate the determination of a metal in the presence of

  another metal.
• Precipitimetric Method

  Example methods:

  ● Fajan

  ● Mohr

  ● Volhard

  ● Liebig

  ● Gay-lussac
• Assays are based on formation of insoluble substances or precipitates to cause the reaction to go in sufficient completion to be quantitative in

  nature.
• The indicators used for volumetric precipitation methods:

  ● Ferric ammonium sulfate

  ● Potassium chromate TS

  ● Adsorption indicators:

  ○ Dichloro fluorescein (DCF)

  ○ Eosin Y

  ○ Tetra bromo phenolphthalein ethyl ester (TEE)
• FAJAN METHOD

  Involves a change of color on the surface of the

  precipitate
• FAJAN METHOD

  ● Standard solution used: Silver Nitrate

  ● Positive indicators: Eosin, TEE, DCF

  ● Negative indicators: Rhodamine
• FAJAN METHOD

  Sample Assays: Phenylephrine HCL, Tubocurarine

  HCL, Meperidine HCL
• VOLHARD METHOD

  Involves the formation of a soluble red complex.
• VOLHARD METHOD

  Ex. At the end point, the excess thiocyanate will react with FAS

  forming a red complex (ferric thiocyanate)
• VOLHARD METHOD

  Standard solution: Potassium thiocyanate

  ● Indicator used: Ferric ammonium sulfate (FAS)

  ● Sample assays: Aminophylline for Theophylline. Sodium

  chloride, Dextrose and NaCl injection.
• MOHR METHOD

  Involves the formation of a second precipitate which is of a different color

  from the primary precipitate, but this second precipitate must be more soluble than the first.
• MOHR METHOD

  Ex. When a chloride content will react to silver nitrate to form a white

  precipitate, when the chloride is completely precipitated a drop of silver

  nitrate in excess will react with the indicator (chromate ion), forming a

  brownish red ppt.
• MOHR METHOD

  Standard solution: Silver Nitrate

  ● Indicator used: potassium chromate

  ● Sample assays: Sodium lauryl for chloride content.
• LIEBIG METHOD

  Involves formation of a blue precipitate
• LIEBIG METHOD

  Standard solution: Sodium tetraphenyl boron

  ● Indicator used: Bromophenol blue

  ● Sample Assay: Assay of Organic Nitrogen

  Compounds
• GAY-LUSSAC METHOD (EQUAL TURBIDITY METHOD)

  ● This method is a tedious method since visual perception is the

  tool used in determining the end point.

  ● This method is prone to error.
• GAY-LUSSAC METHOD (EQUAL TURBIDITY METHOD)

  Involves cessation of precipitate or turbidity.
• Preparation of Starch TS

  Triturate 1g of arrowroot starch with 10ml of cold water.

  Triturate until a pasty consistency occurs. Boil 200ml of

  hot water and add to the starch paste. Boil the solution

  for 30 mins or until a thin translucent solution is formed.

  Since a solution of starch deteriorates rapidly, it should

  be prepared freshly each day.
• Upon heating, starch granules burst and form colloidal

  dispersion, composed of beta amylose.
• Interaction of Iodine and iodide ion to beta-amylose results

  in the formation of a distinct blue color.