48. Precipitation titration

Question 1

What is the purpose of PRECIPITATION TITRATION (ARGENTOMETRY)?

Answer 1

for determining the concentrations of such ion pairs with each other, which form an ionic compound barely soluble in water, but the individual ions can be taken into solution from other soluble salt

Question 2

Principle of PRECIPITATION TITRATION (ARGENTOMETRY)

Answer 2

The method builds on the solubility equilibrium and common ion effect

Question 3

What is common ion effect?

Answer 3

the solubility of a hardly soluble ionic compound is decreased by another, more soluble ionic compound if the latter adds an ion in common with the former.

→ a special case of the Le Chatelier principle

Question 4

Determination of chloride ion concentration of an unknown solution

What are the materials?

Answer 4

• factorized 0.05 N AgNO3 solution (the titrant, it is photosensitive, therefore stored in a brown bottle),
• 5% K2CrO4 solution (the indicator, yellow, stored in a bottle with a dropper),
• solution of unknown chloride ion concentration (analyte in clear bottle).

Question 5

Determination of chloride ion concentration of an unknown solution

The chloride ion concentration can be determined with __

Answer 5

• factorized 0.05 N AgNO3 solution (the titrant, it is photosensitive, therefore stored in a brown bottle),
• 5% K2CrO4 solution (the indicator, yellow, stored in a bottle with a dropper),
• solution of unknown chloride ion concentration (analyte in clear bottle).

Question 6

Determination of chloride ion concentration of an unknown solution

The method is based on the fact that silver ions (Ag+) and chloride ions (Cl─) form (1)____, a white compound barely (2)____ in water.

Answer 6

1. silver chloride (AgCl)
2. soluble

Question 7

Determination of chloride ion concentration of an unknown solution

Combining solutions of AgNO3 and a chloride yields a mixture which is a ___ solution as well.

Answer 7

1. silver chloride (AgCl)

Question 8

Determination of chloride ion concentration of an unknown solution

AgCl is barely soluble in water, so thus it is ___.

Answer 8

precipitated

Question 9

Determination of chloride ion concentration of an unknown solution

The end-point of the titration is detected by __

Answer 9

potassium chromate (K2CrO4)

Question 10

Determination of chloride ion concentration of an unknown solution

The end-point of the titration is detected by potassium chromate (K2CrO4).

Why is this solution suitable comfortable to be an indicator?

Answer 10

The soluble, yellow potassium chromate is suitable as an indicator, because it adds chromate ions (CrO42─) which with silver ions form the terra cotta silver chromate (Ag2CrO4),

Question 11

Determination of chloride ion concentration of an unknown solution

During the titration, potassium chromate is added as an indicator to a solution containing chloride ions of unknown concentration, followed by ___ from the burette.

Answer 11

titration with a silver nitrate solution of known concentration