analytical chemistry Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separati ...

, argentometry is a type of

titration Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte (a substance to be analyzed). A reagent, termed the ''titrant'' ...

involving the

silver Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...

(I) ion. Typically, it is used to determine the amount of

chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride s ...

present in a sample. The sample solution is titrated against a solution of

silver nitrate Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar causti ...

of known concentration. Chloride ions react with silver(I) ions to give the insoluble

silver chloride Silver chloride is a chemical compound with the chemical formula Ag Cl. This white crystalline solid is well known for its low solubility in water (this behavior being reminiscent of the chlorides of Tl+ and Pb2+). Upon illumination or heat ...

: : Ag⁺ (aq) + Cl⁻ (aq) → AgCl (s) (''K'' = 5.88 × 10⁹)

Methods

Volhard

An example of back titration, the Volhard method, named after Jacob Volhard, involves the addition of excess silver nitrate to the analyte; the silver chloride is filtered, and the remaining silver nitrate is titrated against ammonium thiocyanate, with ferric ammonium sulfate as an indicator which forms blood-red e(OH₂)₅(SCN)sup>2+ at the end point: : Ag⁺ (aq) + SCN⁻ (aq) → AgSCN (s) (''K''_sp = 1.16 × 10⁻¹²) : Fe(OH)(OH₂) (aq) + SCN⁻ (aq)→ e(OH₂)₅(SCN)sup>2+ + OH⁻

Mohr

In the Mohr method, named after Karl Friedrich Mohr, potassium chromate is an indicator, giving red

silver chromate Silver chromate is an inorganic compound with formula Ag2CrO4 which appears as distinctively coloured brown-red crystals. The compound is insoluble and its precipitation is indicative of the reaction between soluble chromate and silver precursor sa ...

after all chloride ions have reacted: : 2Ag⁺ (aq) + CrO (aq) → Ag₂CrO₄ (s) (''K''_sp = 1.1 × 10⁻¹²) The solution needs to be near neutral, because

silver hydroxide Silver oxide is the chemical compound with the formula Ag2O. It is a fine black or dark brown powder that is used to prepare other silver compounds. Preparation Silver oxide can be prepared by combining aqueous solutions of silver nitrate and a ...

forms at high pH, while the chromate forms Ag₂Cr₂O₇ at low pH, reducing the concentration of chromate ions, and delaying the formation of the precipitate. Carbonates and phosphates precipitate with silver, and need to be absent to prevent inaccurate results. The Mohr method may be adapted to determine the total chlorine content of a sample by igniting the sample with

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...

, then ferric acetate. Calcium acetate "fixes" free chlorine, precipitates carbonates, and neutralizes the resultant solution. Ferric acetate removes phosphates. All chlorides are dissolved out of the residue, and titrated.

Fajans

In the Fajans method, named after Kazimierz Fajans, typically

dichlorofluorescein Dichlorofluorescein (DCF) is an organic dye of the fluorescein family, being substituted at the 2 and 7 positions by chloride. It is used as an indicator for argentometry by Fajans method. When used as an indicator, upon reaching the equivalence ...

is used as an indicator; the end-point is marked by the green suspension turning pink. Prior to the end-point of the titration, chloride ions remain in excess. They adsorb on the AgCl surface, imparting a negative charge to the particles. Past the equivalence point, excess silver(I) ions adsorb on the AgCl surface, imparting a positive charge. Anionic dyes such as dichlorofluorescein are attracted to the particles, and undergo a colour change upon adsorption, representing the end-point.

Eosin Eosin is the name of several fluorescent acidic compounds which bind to and form salts with basic, or eosinophilic, compounds like proteins containing amino acid residues such as arginine and lysine, and stains them dark red or pink as a resul ...

(tetrabromofluorescein) is suitable for titrating against

bromide A bromide ion is the negatively charged form (Br−) of the element bromine, a member of the halogens group on the periodic table. Most bromides are colorless. Bromides have many practical roles, being found in anticonvulsants, flame-retardant ...

iodide An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. In everyday life, iodide is most commonly encountered as a component of iodized salt, which many governments mandate. Worldwide, iodine de ...

, and

thiocyanate Thiocyanate (also known as rhodanide) is the anion . It is the conjugate base of thiocyanic acid. Common derivatives include the colourless salts potassium thiocyanate and sodium thiocyanate. Mercury(II) thiocyanate was formerly used in pyr ...

anions, giving a sharper end-point than dichlorofluorescein. It is not suitable for titrating against

anions because it binds to AgCl more strongly than

does.{{cite book , author=Harris, Daniel Charles , title=Quantitative chemical analysis , publisher=W.H. Freeman , location=San Francisco , year=2003 , pages=142–143 , isbn=0-7167-4464-3 , edition = 6th

References

Analytical chemistry Titration

Methods

Volhard

Mohr

Fajans

See also

References