Argentometry
Encyclopedia
In analytical chemistry
, argentometry is a type of titration
involving the silver
(I) ion. Typically, it is used to determine the amount of chloride
present in a sample. The sample solution is titrated against a solution of silver nitrate
of known concentration. Chloride ions react with silver(I) ions to give the insoluble silver chloride
:
, 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 [Fe(OH2)5(SCN)]2+ at the end point:
, potassium chromate
is an indicator, giving red silver chromate
after all chloride ions have reacted:
The solution needs to be near neutral, because silver hydroxide forms at high pH, while the chromate forms H2CrO4 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
, 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.
, typically dichlorofluorescein
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 end-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 color change upon adsorption, representing the end-point. Eosin
(tetrabromofluorescein) is suitable for titrating against bromide
, iodide
, and thiocyanate
anions, giving a sharper end-point than dichlorofluorescein. It is not suitable for titrating against chloride
anions because it binds to AgCl more strongly than chloride
does.
Analytical chemistry
Analytical chemistry is the study of the separation, identification, and quantification of the chemical components of natural and artificial materials. Qualitative analysis gives an indication of the identity of the chemical species in the sample and quantitative analysis determines the amount of...
, argentometry is a type of titration
Titration
Titration, also known as titrimetry, is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of an identified analyte. Because volume measurements play a key role in titration, it is also known as volumetric analysis. A reagent, called the...
involving the silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
(I) ion. Typically, it is used to determine the amount of chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
present in a sample. The sample solution is titrated against a solution of silver nitrate
Silver nitrate
Silver nitrate is an inorganic compound with chemical formula . This compound 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...
of known concentration. Chloride ions react with silver(I) ions to give the insoluble silver chloride
Silver chloride
Silver chloride is a chemical compound with the chemical formula AgCl. This white crystalline solid is well known for its low solubility in water . Upon illumination or heating, silver chloride converts to silver , which is signalled by greyish or purplish coloration to some samples...
:
- Cl− (aq) + Ag+ (aq) → AgCl (s) (Ksp = 1.70 × 10−10)
Volhard method
An example of back titration, the Volhard method, named after Jacob VolhardJacob Volhard
Jacob Volhard was the German chemist who discovered, together with his student Hugo Erdmann, the Volhard-Erdmann cyclization reaction. He was also responsible for the improvement of the Hell-Volhard-Zelinsky halogenation....
, 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
Ammonium thiocyanate
Ammonium thiocyanate is an inorganic compound with the formula NH4SCN. It is the salt of the ammonium cation and the thiocyanate anion.-Uses:...
, with ferric ammonium sulfate as an indicator which forms blood-red [Fe(OH2)5(SCN)]2+ at the end point:
- Ag+ (aq) + SCN− (aq) → AgSCN (s) (Ksp = 1.16 × 10−12)
- Fe(OH)(OH2)52+ (aq) + SCN− (aq)→ [Fe(OH2)5(SCN)]2+ + OH−
Mohr method
In the Mohr method, named after Karl Friedrich MohrKarl Friedrich Mohr
Karl Friedrich Mohr was a German chemist famous for his early statement of the principle of the conservation of energy. Ammonium iron sulfate, 2Fe2.6H2O, is named Mohr's salt after him.-Life:...
, potassium chromate
Potassium chromate
Potassium chromate is a yellow chemical indicator used for identifying concentrations of chloride ions in a salt solution with silver nitrate...
is an indicator, giving red silver chromate
Silver chromate
Silver chromate is a brown-red monoclinic crystal and is a chemical precursor to modern photography. It can be formed by combining silver nitrate and potassium chromate...
after all chloride ions have reacted:
- 2Ag+ (aq) + CrO42- (aq) → Ag2CrO4 (s) (Ksp = 9 × 10−12)
The solution needs to be near neutral, because silver hydroxide forms at high pH, while the chromate forms H2CrO4 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 acetate
The chemical compound calcium acetate is the calcium salt of acetic acid. It has the formula Ca2. Its standard name is calcium acetate, while calcium ethanoate is the systematic IUPAC name. An older name is acetate of lime...
, 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 method
In the Fajans method, named after Kazimierz FajansKazimierz Fajans
-External links:*...
, typically dichlorofluorescein
Dichlorofluorescein
Dichlorofluorescein 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....
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 end-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 color change upon adsorption, representing the end-point. Eosin
Eosin
Eosin is a fluorescent red dye resulting from the action of bromine on fluorescein. It can be used to stain cytoplasm, collagen and muscle fibers for examination under the microscope. Structures that stain readily with eosin are termed eosinophilic....
(tetrabromofluorescein) is suitable for titrating against bromide
Bromide
A bromide is a chemical compound containing bromide ion, that is bromine atom with effective charge of −1. The class name can include ionic compounds such as caesium bromide or covalent compounds such as sulfur dibromide.-Natural occurrence:...
, iodide
Iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
, and thiocyanate
Thiocyanate
Thiocyanate is the anion [SCN]−. It is the conjugate base of thiocyanic acid. Common derivatives include the colourless salts potassium thiocyanate and sodium thiocyanate. Organic compounds containing the functional group SCN are also called thiocyanates...
anions, giving a sharper end-point than dichlorofluorescein. It is not suitable for titrating against chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
anions because it binds to AgCl more strongly than chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
does.