Precipitation titration involves the reaction between an analyte and titrant that form an insoluble precipitate. This can be used as the basis for a titration. Some key points about precipitation titration include: - Silver nitrate is commonly used as the titrant due to its ability to form precipitates with halide ions like chloride. - Endpoint detection methods include visual color changes of indicators like potassium chromate or potentiometric detection of the equivalence point. - Specific methods like Mohr's, Volhard, and Fajans titrations are used to determine halide concentrations in pharmaceutical products through precipitation reactions with silver nitrate and indicator detection of the endpoint.

1. 3.4 Precipitation Titration
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2. Objective
 At the end of discussion, the students are able to know
– What is precipitation titration?
– Describe the reaction involved in precipitation titration
– Explain the conditions required in precipitation titration
– Method of end point detection
– Application of precipitation titration for pharmaceutical products
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3. Precipitation titration
Introduction:
 Precipitation
• It occurs when two or more soluble species combine to form an insoluble
product that we call a precipitate. Or
– The formation of a slightly soluble solid from solution (the reverse of
dissolution).
• A reaction in which the analyte and titrant form an insoluble precipitate also
can serve as the basis for a titration. This type of titration is a precipitation
titration.
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4. Precipitation….
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5. Precipitation titration …
Precipitation titrations
 Are based upon reactions that yield ionic compounds of limited solubility.
 The most important precipitating reagent is silver nitrate.
 Titrimetric methods based upon silver nitrate are sometimes termed
argentometric methods.
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6. Precipitation titration cont...
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Illustration: In the determination of chloride, 50 ml of a 0.1M AgNO3 solution
would be required to completely react with 0.005 moles of chloride present in
solution.
Analyte Titrant
Produc
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7. Example
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8. Precipitation titration cont...
 Limitation of precipitation titration
 Slow rate of reaction b/n the titrant & analyte
 Lack of suitable indicator for many pptn titration
 Difficult to obtain an accurate precipitate composition, the co-precipitation
effect is often occurred.
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9. Endpoint detection
The end-point may be determined by:-
1. Visually by colour change of indicators
2.Potentiometrically;-
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10. Methods in precipitatometric titration
• Argentometric method (Ag)
• ƒ
Mercurimetric method (Hg)
• ƒ
Kolthoff titration(Zn)
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11. Argentometric titration
• Titrations with silver nitrate are sometimes called argentometric titration
• On the bases of the method used to identify the equivalent point,
Or to detect the end point argentometric titration can be classified as:
1.method of Mohr (indicator : chromic potassium)-direct titration
2. Method of Volhard (indicator : ferric salt)-back titration
3.Method of Fajans (indicator:fluorosceince)
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12. Example
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13. Mohr’s Method
Direct titration of chlorides with silver nitrate
Uses K2CrO4 indicator: - appearance of reddish brown Ag2CrO4
NaCl + AgNO3 AgCl (white ppt) + NaNO3
at eq point = 2Ag+ + CrO4 2- Ag2CrO4 (red ppt)
– Carried out at pH b/n 7-10 or at pH close to neutrality.
If pH < 7:- CrO4
2- + 2H+ Cr2 O7
2- + H2O so Ag2CrO4 precipitate
formation decreased
 If pH >10 :- 2Ag+ + 2OH- ↔ Ag2O + H2O
Ag2O formation increased which is a black precipitate
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14. Mohr’s Method cont....
Disadvantage:-
 Sulfides, phosphates and arsenates will also be precipitated
 The end point is less sensitive with dilute solution
 Adsorbed ions of the sample become trapped & cause low results.
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15. Mohr’s Method cont....
Application:
 NaCl given through IV infusion.
 in determination of the % w/v NaCl.
 Potassium iodide
 KI can be determined by direct titration with AgNO3 using iodine &
starch as indictor.
KI + AgNO3 KNO3 + AgI(ppt)
 At the end-point excess silver nitrate reduce the indicator, iodine, to
iodide ion, destroyed the purple starch-iodine complex & leaving a
pale yellow solution.
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16. Volhard Method
• Most common method used to determine halides or halogen contents.
• Involves a residual titration of excess AgNO3 with the second standard
thiocynate so/n.
• So/n of ferric ammonium sulfate or ferric nitrate serve as indicator & end
point is appearance of red ferric thoicynate complex.
• When excess of std. AgNO3 so/n is added in to so/n of halide preparation
of silver halide occurs:-
– Ag+ (excess) + X- (analyte) ↔ AgX (ppt) + excess Ag+
• the excess AgNO3 is then back titrated with ammonium thiocynate
– Ag+ (remaining excess) + SCN- ↔ AgSCN (ppt)
• At the end point excess thiocynate react with Fe3+ and form ferric
thiocynate red complex.
– Fe3+ + SCN ↔ [FeSCN]2+ (red soluble complex)
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17. Volhard Method cont...
AgX must be removed before residual titration if it is more soluble
than AgSCN(ppt)
E.g.AgCl is more soluble than AgSCN.
AgCl + SCN- → AgSCN + Cl-
Result:
 end point fades & over consumption of SCN-
low value for the chloride analysis
 to over come this source of error:-
The siliver chloride is filtered off before back titration.
Coating the silver chloride ppt with nitrobenzene. 17
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18. Volhard Method cont...
Titration carried out in acidic so/n to prevent hydrolysis of the
ferric ion.
Advantage
Ion w/c would normally interfere with mohr’s titration will not
ppt in the acidic media used in volhards method.
Care should be taken when iodide are being determined.
Fe3+ + I- → Fe2+ + I0
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19. Volhard Method cont...
Application
• Sodium chloride:-
• determination of the percentage of NaCl.
• Chlorbutol:-
 In the determination of the % of chlorbutol. Organically combined chloride is
converted by hydrolysis with NaOH to ionic chloride w/c can be determine by
volhards method in the presence of nitrobenzene.
CCl3.C(CH3)2 .OH + 3NaOH → C(OH3)2 .C(CH3)2 .OH + 3NaCl
3NaCl + 3AgNO3 → 3NaNO3 + 3AgCl
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20. Volhard Method cont...
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Product Heat Filtration Nitrobenzene
Aminophylline injection + + -
Ammonium chloride - - +
Dextrose & sodium Chloride injection - - +
Ringer solution - - +
Lactated ringer injection - - +
Ammonium bromide - - -
Potassium bromide - - -
Tetramethyl ammonium Bromide - - -
Tetramethyl ammonium Chloride - - +
Zinc chloride - + -
Sodium chloride - - +
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21. Fajans Method
 Makes use of adsorption indicators.
 Adsorption indicators are organic dyes, typically weak organic acids, which
adsorb onto the positively charged counter-ion layer of the solid particles
formed during the titration.
 In the ideal precipitation titration, the adsorption of the dye occurs near the
equivalence point and results in a color change for the dye.
 Fajans’ Method for the titration of chloride ion with silver nitrate involved,
originally, the use of dichlorofluorescein as an adsorption indicator.
 In an aqueous solution at pH 7, dichlorofluorescein will partially dissociate to
form negatively-charged yellow-green dichlorofluoresceinate Ions (Ind-).
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22. Fajans Method cont...
HInd(aq) → H+ (aq) + Ind- (aq)
 Since this indicator is weakly acidic, the indicator works best above pH 5.
However, the pH must be below ≈ 9 to prevent precipitation of AgOH.
Ag+ (aq) + OH- (aq) → AgOH(s)
 Other indicators:-dichlorofluorescin, tetra bromofluorescin,
tetraiodofluorescin
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O
OH
COOH
HO
Fluorsceine
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23. Fajans Method cont...
Consider titration of chloride ion with silver nitrate:-
• In the early stage:- the colloidal AgCl particles are –very charged, resulting from
surface adsorpition of excess Cl- ions. The dye anions are repelled from the
surface & impart yellow green color to the so/n.
• Beyond the equivalent point:- the AgCl particles strongly adsorb Ag+ & there by
acquiring +ve charge. Dichlorofluoresceinate anion are now attracting in to the
counter ion layer that surrounded each colloidal AgCl particles.
• Net results:- appearance of red color of silver dichlorofluoresceinate in the
surface layer of the so/n surrounding the solid.
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First step of titration Final step of titration : Ag+Ind-
Fajans Method cont...
yellow green red
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25. Fajans Method cont...
Precaution:
• Titration performed by withholding addition of the indicator till the end-point
approached
• To decrease the possibility of premature adsorption of the indicator ions , the
amount of the indicator used must be kept as minimum
• To avoid the adsorption of indicator ion & formation of premature end-point the
pH of the media should be adjusting depending up on the indicator used to keep
its dissociated form.
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26. Fajans Method cont...
Pharmaceutical application
 Determination of Cl content
◦ HCl, methoxamine HCl, phenylepiphrine HCl, pyridoxine HCl, succinylcholin
chloride.
 Determination of Br content
◦ Hydroxyamphetamine bromide , propantheline bromide
 Determination of I content
◦ Meglumineiodipamide inj. , sodium iodipamide inj. Sodium methiodal inj.
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