precipitation titration Flashcards
define precipitation titration
is a titrimetric method which involves formation of precipitates and the volume of the precipitant which convert sample into precipitate is measured
define precipitate
precipitate is a compound of limited solubility
define argentiometric precipitation
is a type of precipitation titration in which silver nitrate(AgNO3) is used as the precipitant
state the four methods of ppt titration
mohr’s method
volhard method
modified volhard method
fazan method
name the indicator used in each method
mohr-potassium chromate(K2CrO4)
volhard-ferric alum(iron III ammonium sulphate)
fazan -adsorption indicator(flouroscein)
explain mohrs method
titration btn silver nitrate and halides where K2CrO4 is used as indicator.
at end point when all Cl- has precipitated the first drop of excess Ag+ react with dichromate indicator to form red-brown ppt of Ag2CrO4
state the two limitations of mohrs method
- its only used in neutral medium as in ACID would dissolve the Ag2CrO4 formed at endpoint hence causing errors and in BASE silver nitrate would precipitate into silver hydroxide
- it cant be used for iodides for both the endpoint indicator Ag2CrO4 and iodine have reddish brown color
explain volhard method
titration between silver nitrate and sodium or potassium or ammonium thiocyanate with the use of ferric alum as the indicator
state the 2 limitations of volhards method
it can only be done in acidic medium(HNO3)to coagulate the ppt formed and to prevent hydrolysisof the indicator into Fe(OH)3
cannot be used for analysis of halides directly
explain fazan method
involves titration btn silver nitrate and halides in the presence of an adsorption indicator(flourescein)
define adsorption indicator
is a chemical placed in a solution that will indicate when an excess of a substance or ion has been reached by coloring the ppt after the chemical itself has been totally adsorbed
explain adsorption
is the processa t which solid holds molecules of a gas or liquid as a thin film
state the role of acidic medium in volhard method
.prevent the hydrolysis of ferric alum
.coagulate the precipitate formed
how does acidity and alkalinity affect mohrs method
ACIDITY;dissolve Ag2CrO4 hence causing errors of identification at endpoint
BASIC;hydrolyses AgNO3 into Ag(OH)3
why is mohrs method unsuitable for iodides
because Ag2CrO4 which is the indication of end point in mohr titration is redish browh where as iodides also have reddish brown color so it will be dificcult to identify end point
