PRECIPITATION, COMPLEXATION AND REDOX TITRATIONS - PART 1

precipitimetry

based on the formation of relatively insoluble substances to cause the reaction to go to completion quantitatively

silver nitrate

• most widely used precipitating agent / standard solution

• determination of halides

argentometry

if the titrant is AgNO₃ standard solution

end point of precipitimetry

• cessation of precipitation or appearance of turbidity

• use of internal indicators

• instrumental methods (potentiometry / amperometry)

ferric ammonium sulfate (ferric alum)

• direct and residual titration

• use NH₄SCN as a standard solution

• end point: white (AgSCN) → reddish brown complex (FeSCN²⁺)

potassium chromate

end point: red precipitate (silver chromate) against the white background of AgCl

adsorption indicators

examples:

• dichlorofluorescein (DCF)

• Eosin Y

• Tetrabromophenolphthalein ethyl ester (TEE)

adsorption indicators

• used in analysis of halides by direct titration with AgNO₃ as standard solution

• weak acids

• end point best seen in diffused-light condition

fajans method

• direct titration using AgNO₃ as titrant

• uses an adsorption indicator, an organic compound that adsorbs onto or desorbs from the surface of the solid

• example: standardization of silver nitrate

volhard method

• residual or back titration

• complete precipitation of insoluble silver salts with the addition of excess silver nitrate to precipitate

• followed by titration of the unreacted silver nitrate with ammonium thiocyanate

• ferric alum as an indicator

reddish-brown

end point of ferric alum

mohr method

• potassium chromate serves as the indicator for the argentometric titration of neutral halides

• rarely used because Cr(Vl) is a carcinogen

Ag₂CrO₄ brick red

2Ag⁺ + CrO₄^-2 → ______

sodium tetraphenylboron titrations

• uses Na(C₆H₅)₄B to precipitate

  • organic nitrogen compounds (alkaloids, amines and quaternary salts)

  • ammonium

  • potassium

  • silver ions

• uses chloroform as extraction indicator

  • end point: colorless

ligand

a molecule which provides groups for attachments to metal ions

complex

coordination compounds

chelate

a complex that contains two or more groups that can donate electrons

coordination number

the number of covalent bods that a cation tends to form with electron donors is its ________

positive, neutral or negative

the species formed as a result of coordination can be electrically _______

unidentate

a ligand that has a single donor group is called _____

bidentate

two groups available for covalent bonding is called ______

complexation reaction

metal + ligand → ______

compleximetry

metal ion reacts with a suitable ligand to form a ____, and the end point is determined by an indicator or an appropriate instrumental method

metal

titrand in compleximetry

ligand

titrant in compleximetry

EDTA (ethylenediaminetetraacetic acid)

• most widely used titrant

• available as dihydrate of sodium salt

• 0.05M

• MW = 372

• a hexadentate ligand

EDTA titrations

• forms 1:1 ratio with metals, regardless of charge

• applied to the determination of virtually every metal cation except alkali metals

EDTA (ethylenediaminetetraacetic acid)

• a very unselective agent

• control over interferences can be done by masking

masking

the determination of a metal in the presence of another metal

pH 13

Ca titration at ___

pH 2

Bi will complex with EDTA at ___ even in the presence of other metal

pH 10

Zn will complex with EDTA at _____

addition of masking agent

• an auxiliary ligand that preferentially forms highly stable complexes with the potential interfering ion

• example: potassium cyanide solution

Mg and Ca

titration of ______ in the presence of ions such as Cd, Co, Cu, Ni, Zn and Pd ions

triethanolamine

suppress AI EDTA complex in the presence of a Mg EDTA complex

thioglycols

inactive metals such as Hg and Cu

ammonium fluoride

mask Ca, Mg, and Al to allow titration of Zn

masking agents

• triethanolamine

• thioglycols

• ammonium fluoride

• ascorbic acid

• citrates

• tartrates

EDTA indicators

• HNB

• eriochrome black T

• azo dyes

• phthaleins

qualities of good quality indicators

• sharpness of color change at the end point

• stability of the indicator for the metal ion

• stability constant smaller than that of the metal - EDTA complex

ferric chloride titration

for sodium fluoride determination

water hardness

• the capacity of cations in water to form sparingly soluble products that cause “scum” in the sink or bathtub

• expressed in terms of CaCO₃ or Ca content

temporary water hardness

due to the presence of bicarbonates

temporary

permanent

types of water hardness