Precipitation Titrations

What does the reaction of sodium chloride and silver nitrate form?

insoluble precipitate silver chloride

How does precipitation reaction occur?

cations and anions in aqueous solution combine to form an insoluble ionic solid called a precipitate

What is the ion-product constant?

the product of the molar concentrations of the ions in solution raised to their stoichiometric coefficients

What is the solubility-product constant?

The ion product constant when the solution becomes saturated

What are the degrees of saturation?

unsaturated vs saturated vs supersaturated

When will precipitation occur?

Qsp>Ksp

When will no precipitation occur?

Qsp<Ksp

When will reaction be at equilibrium?

Qsp=Ksp

What is degree of saturation?

The equilibrium that exists between a partially soluble ionic compound and its saturated solution

The value of Ksp of AgCl is 1.8× 10 ^-10. will precipitation occur if the concentration of Ag+ and Cl- added is 0.00002 M each?

Since Qsp is greater than the Ksp, precipitation will occur

True or False: The smaller the Ksp, the more soluble the salt

False, the smaller the Ksp, the less soluble the salt

What is used to determine whether the titration of multiple ions is possible in precipitation titrations?

Solubility differences between precipitates

What principle does the common ion effect follow?

Le Chetalier’s Principle

What is the Le Chetalier’s Principle?

followed for the shift in concentration of products and reactants upon addition of either products and reactants to a solution

True or False: The solubility of a slightly soluble ionic compound is lowered when a second solute that furnishes a common ion is added to the solution

True

What is the importance of Le Chetalier’s Principle?

important in titration to reduce possible interferences of other ions and salts in titration reaction

When is the reaction considered to have completeness of precipitation?

it is generally considered to be “complete“ if about 99.9% of the target ion is precipitated (0.1% or less left in solution)

What are the three conditions generally favor completeness of precipitation?

1. a very small value of Ksp (few ions in solution)

2. A high initial concentration of the target ion

3. A concentration of common ion that greatly exceeds that of the target ion (use soluble salts e.g. alkali metal salts)

Completeness of reaction is critical for ?

selective precipitation

The effect of pH on solubility is critical for?

setting titration conditions

True or False: If the anion of a precipitate is that of a weak acid, the precipitate will dissolve somewhat when the pH is highered.

False - lowered

Ex: CaF2 — Ca + 2F (added H+ reacts with F and removes F-; Le Chateliers principe says more F- forms)

True or False: If the anion of the precipitate is that of a strong acid, the precipitate will dissolve somewhat when the pH is lowered

False: lowering the pH will have no effect on the precipitate (H+ does not consume Cl- acid does nor affect the equilibrium)

Complex Ion Formation

some precipitates do dissolve if a soluble complex ion forms

Where does AgCl dissolve to form soluble complex ion

aqueous ammonia

What happens when AgNO3 is added to a mixture containing Cl and I

AgI (yellow) precipitates first , All AgI precipitates before any AgCl (white) precipitates - selective precipitation

Precipitation Titration

titration method based on the formation of precipitate

Ex: clorides can be quantitatively determined by reacting the sample with solver nitrate and using the volume of silver nitrate needed to reach equivalence

What are the common titrants used in precipitation titrations?

silver nitrate (argentometry) and potassium thiocyanate (back titrant)

silver nitrate (argentometry) forms?

forms insoluble precipitates with halides (Cl, Br, and I) and pseudohalides (S2, HS, CN, SCN)

the reaction ratio is 1:1 and slver salts formed are generally quite insoluble

Endpoint may be detected using:

• cessation of precipitation or the apperance of a turbidity

• formation of a colored ccomplex ion

• formation of a colored secondary precipitate

• formation of a colored adsorption compound

Mohr’s Method

use potassium chromate (chromate ions- CrO4 ^2-) as an indicator in the titration of Cl, Br, and CN ions with a silver nitrate standard solution as titrant

What precipitate is formed in Mohr’s Method

after all the chloride has been precipitated, the first excess of silver results in the formation of a silver chromate precipitate

what are the titration conditions in mohr’s method

performed only in neutral or slightly basic medium to prevent silver hydroxide formation at (ph>10) or the fomration of chromic acid at ph<7

Volhard Method

used in residual titrations where potassium thiocyanate is used as a back titrant and is suitable for the determination of chlorieds, bromides, and iodides in acidic solutions

Steps of Volhard Method

1. chlorides and halides are precipitated by excess AgNO3 : AgCl

2. excess Ag+ is titrated with KSCN in the presence of Fe3+ : AgSCN

3. when Ag+ had been consumed, a red complex forms as a result of FeSCN²⁺

Titration Conditions:

the solution must be acidic, with a concentration of about 1M in nitric acid to sensure complex formed is stable, and to prevenet the precipitation of iron as ferric hydroxide

In case of Br and I ans analyte, indicator should nor be added until all the I is precipitate with Ag since it would be oxidized by the Fe (III)

Ex: 2Fe³⁺ + 2I —> 2Fe²⁺ + I₂

Fajan Method

uses an adsorption indicators such as dichlorofluorescein and Eosin

the indicator absorb onto the surface of the silver salt precipitate at the endpoint

the adsorption process causes a change in the color of the indicator

Process of Adsorption Indicators

before equivalence point, there are still unreacted chlorides that form a primary adsorption laer with the precipitate

after equivalence, all chlorides have been consumed, and solvent are now in excess. the indicator forms a secondary adsorption layer

Potassium Bromide Monograph: what is the analyte

potassium bromide

Potassium Bromide Monograph: what is the titrant

0.1M silver nitrate VS

0.1 M ammonium thiocyanate VS

Potassium Bromide Monograph: what type of titration

residual precipitation titration

what is the indicator? Is the indicator used appropriate

ferric ammonium sulfate, yes because it is conducted in an acidic environment

Potassium Bromide Monograph: what significant steps are conducted during the titration?

addition of nitric acid: prevent formation of ferric hydroxide

addition of dibutyl phthalate: keeps the silver bromide in a coagulated state so that it will not interfere with reaction of silver, iron and thiocyanate ions