upon reactions that yield ionic compounds of limited solubility a A + b T c Products (s)
one of the oldest analytical techniques
the slow rate of formation of most precipitates, limits the number of precipitating agents that can be used for titrim titrimetr etric ic analys analysis is
available in primarystandard grade purity, but it is expensive
used for determining halide and thiocyanate ions
titrants are prepared from less pure AgNO 3 can be standardized with primary-standard KCl
the stability of atomic Ag makes Ag ion a good reducing agent
certain proteins in skin can reduce the Ag ion to atomic Ag, leaving a dark brown stain that is unsightly and will remain until replaced by new skin
in cases where the amount of silver ion must be determined, KCl is usually the titrant
Titrants
(C6H5)4B-Na+
KSCN
obtainable in primary standard quality but somewhat hygroscopic ordinarily standardized against AgNO3 stable indefinitely
organic precipitating that forms salt-like precipitate
a near-specific precipitating agent for K + and NH4+
Indicators usually specific compound formers, they react selectively with the titrant to form a colored substance
Indicator (yields a ___ at or near the theoretical end point)
Example
colored complex ion
Fe
3+
Colored Substance at End Point
FeSCN
2+
colored 2o precipitate
K 2CrO4
Ag2CrO4
colored adsorption product
HDCF
. +:DCFAgCl Ag
Analytes
determination of halides, halide-like anions (SCN1- , CN1- , CNO1- ), mercaptans, fatty acids and several divalent inorganic anions
determination of
analytes involves one or a combination of titration methods
Applications 1.
Mohr’s Method
Titrant Analyte/s Indicator
AgNO3 Cl1- , Br1- , CN1Na2CrO4 / K 2CrO4
Argentometric Method by Direct Titration
Applications 2. Fajan’s Method
Titrant Analyte/s Indicator
AgNO3 Cl1- , Br1- , CN1HDCF / HEs
Argentometric Method by Direct Titration
Applications 3. Volhard’s Method
Titrants Analyte/s Indicator
AgNO3 and KSCN Cl1- , Br1- , CN1Fe 3+
Argentometric Method by Back-Titration
Exercise 13 A 100-mL sample of brackish water was made ammoniacal and the sulfide it contained titrated with 8.47-mL of 0.01310-M AgNO3. The net reaction is: 2 Ag 1+ + S 2- Ag2S (s) Calculate the parts per million of H 2S (mg H2S /L) in the water. Molar Masses: AgNO3 = 169.9 H2S = 34.08
Exercise 13 The phosphate in a 4.258-g sample of plant food was precipitated as Ag3PO4 through the addition of 50.00-mL of 0.0820-M AgNO 3: 3 Ag 1+ + HPO4 2-
Ag3PO4 (s) + H 1+
The solid was filtered and washed, following which the filtrate and washings were diluted to exactly 250.0-mL. Titration of a 50.00-mL aliquot of this solution required a 4.64-mL back-titration with 0.0625-M KSCN. Express the results of this analysis in terms of the percentage of P 2O5. Molar Mass: P2O5 = 141.9
Exercise 13 After pretreatment to convert the arsenic to the +5 state, a 0.821-g sample of a pesticide was treated with 25.00-mL of 0.0800-M AgNO3. The Ag3 AsO4 was filtered, washed free of excess silver ion, and then redissolved by treatment with nitric acid. The resulting solution required a 7.40-mL titration with 0.0865-M KSCN. (a) Express the results of this analysis in terms of percentage As2O3 in the sample. (b) If the analysis had been completed by titrating the excess silver ion in the filtrate and washings, what volume of 0.0865-M KSCN would have been used? Molar Mass: As2O3 = 197.9
