1. Precipitimetry

2. Precipitimetry
It is a type of quantitative analysis in which the compound to be analyzed is able to form insoluble product precipitate (ppt) when treated with certain reagents known as precipitating agents. The precipitate must be sparingly soluble and formed quickly. -To understand precipitation reactions the following topics should be studied. Solubility product (Ksp). Precipitate formation. Fractional Precipitation. Common ion effect.

3. Solubility Product
Solubility product (ksp) is the product of the molar concentrations of the ions, each concentration being raised to a power equal to the number of ions derived from the ionization of one molecule of the original substance. -Examples: AgCl = Ag+ + Cl- Solubility product constant,Ksp (AgCl ) = [Ag+] [Cl-] -Calculate the Ksp of AgCl if you know that the solubility of it in water is g/l and its M.wt. = g/mol Molar concentration = / = 1.05 x 10-5 (Ksp) AgCl = [Ag+] [Cl-] = (1.05 x 10-5 ) (1.05 x 10-5 ) = 1.1 x 10-10

4. Precipitate Formation
The ppt. is formed when the product of ions concentrations exceed the solubility product at given temperature. -Dissolving the formed ppt.: The ppt. is dissolved by making the product of ions concentration below the solubility product of the precipitate. Fractional Precipitation -When a precipitating agent is added slowly to a solution containing two ions capable of being precipitated by the agent, the substance with the lower Ksp will precipitate first followed by the one having higher Ksp. Example: Solution of Ba+2 and Sr+2 ions, upon the addition of H2SO4, BaSO4 of (lower Ksp) will precipitate first, then SrSO4 (higher ksp)

5. Common Ion Effect
- Common ion is an ion similar to one of the ions of weak electrolyte present in solution. - Common ion has a pronounced effect on the weak electrolytes. A -Effect on ionization: Ionization of weak electrolyte may be strongly reduced by the presence of a common ion. -Example: CH3COOH = H+ + CH3COO- HCl → H+ + Cl- Ionization of CH3COOH decreases by adding H+ or CH3COO- and the equilibrium shifted to left. B -Effect on Precipitation: It is clear that the addition of common ion will decrease the solubility of the salt and increase its precipitation. e.g. Addition of Ag+ or Cl- to a saturated solution of AgCl AgCl = Ag+ + Cl- ppt ions (soluble) -The presence of Ag+ or Cl- in the solution as common ion will shift the reaction to left and accordingly the concentration of ionized part of AgCl decreased.

6. Detection of End Point in Precipitimetry
I. No indicator method A- Disappearance of ppt. Principle :- The ppt. formed during titration is redissolved by the titrant. The end point is detected by disappearance of turbidity. Example: Det. of Ag+ by standard CN- The first drop of addition of st. CN- titrant  AgCN is formed. Precipitation is continued until all Ag+ is ppted. as AgCN. Adding more CN- will form the soluble argento cyanide complex. Ag+ + CN-  AgCN AgCN + CN- = [Ag(CN)2]- The process is continued until all AgCN is dissolved as soluble complex. The end point is the disappearance of last trace of turbidity.

7. Detection of End Point in Precipitimetry
B- Appearance of ppt. (Liebeg’s method) Principle :- This method depends on formation of complex. The end point is detected by the appearance of the first turbidity. Example: Det. of CN- by st. Ag+ 1) st. Ag+ titrant is added to CN- soln.  AgCN ppt. 2) The ppt. then dissolve as soluble complex. 3) Titration is continued till all CN- form soluble argentocyanide. AgCN + KCN = K[Ag(CN)2] 4) The first xss. of Ag+ added will form silver argentocyanide. K[Ag(CN)2] + Ag+ = Ag [Ag(CN)2] + K+ Insoluble or 2 AgCN

8. Liebeg's Method
KCN + AgNO3  AgCN + KNO3 AgCN + KCN = K[Ag(CN)2] KCN + AgNO3 = K[Ag(CN)2]+ KNO3 K[Ag(CN)2 ]+ AgNO3  2 AgCN + KNO3 5) The end point is detected by the appearance of the first turbidity. To make end point more clear we add I- which forms with precipitating agent more insoluble salt. This modification is known as Denige’s Modification. Add I- in ammoniacal medium to detect the end point as yellow turbidity of AgI because AgI is insoluble in ammonia and has lower Ksp than AgCN i.e. AgI is ppted. at the end point in place of AgCN

9. Detection of End Point in Precipitimetry
II –Using indicator methods: -Formation of colored ppt.: e.g. Mohr’s method -Formation of colored soln.: e.g. Volhard's method -Mohr’s method -It is a direct method used for determination of soluble chloride and bromide salts by titration with standard solution of AgNO3 , in neutral or slightly alkaline medium at pH 7-9, using K2CrO4 solution as indicator. -End point After complete precipitation of halide with AgNO3, the first excess drop of AgNo3 will precipitate CrO4-as brick red ppt. of Ag2CrO4 NaCl + AgNO3 → AgCl (ppt.) +NaNO3 K2CrO4 + 2AgNO3 → Ag2CrO4 +NaNO3

10. Mohr's Method
The medium must be neutral or slightly alkaline (pH 7-9).
In acidic medium: formation of soluble acid chromate and dichromate
2 CrO H+  2 HCrO4- = Cr2O72- + H2O
In strongly alkaline medium: silver ppted. as its oxide
2 Ag OH-  2 AgOH = Ag2O + H2O

11. Detection of End Point in Precipitimetry
Volhard’s method
It is an indirect method for determination of halides in an acidic medium (HNO3 ).
A known excess of standard AgNO3 solution is added to the soluble halides, filter and acidify the filtrate with HNO3 .
Residual AgNO3 is back titrated with standard SCN- in presence of (Fe+3) ferric alum as indicator.
End point is detected by the appearance of red color of Fe(SCN)3 or [Fe(SCN)6]-3
NaCl AgNO → AgCl (ppt.) NaNO3
AgNO3 (xss) + KSCN → AgSCN (ppt.) KNO3
At end point : Fe SCN- → [Fe(SCN)6] (red color)

12. Volhard's Method
The titration must be carried out in acidic medium (HNO3)
(i) the formed ppt. is insoluble in HNO3.
(ii) prevent pptn. of Ag+ and Fe3+ as AgOH or Fe(OH)3 in alkaline medium.
Filtration is very necessary to prevent attacking the formed AgCl ppt. by CNS- due to the Ksp of AgCNS (Ksp = 7x10-13) is less than that of AgCl (Ksp = 1.2x10-10). Therefore after the CNS- reacts with all Ag+, it will displace the Cl- from AgCl to form the more insoluble AgCNS.
AgCl + CNS  AgCNS + Cl-
This problem can also be avoided by adding nitrobenzene which surround the particles of ppt. by a thin film which protect the ppt. from attacking by CNS-