Argentometric Analysis

Precipitation is a very valuable method of separating a sample into its components. The process involved is the process by which substances to be separated is used to form a solid precipitate. Precipitation reaction has been used extensively in analytical chemistry, especially in argentometry and gravimetric methods. Argentometry is a quantitative volumetric analysis with standard AgNO3 solution by precipitation. Argentometry is used to determine levels of an element in titrations involving silver salts with the corresponding indicator. The usefulness of this analysis is to determine the levels argentometry halogenida, such as Cl-, which is contained in the sample that is useful for oceanography, food, and industrial.

Argentometry is a quantitative volumetric analysis to determine halogen levels in the samples by using a standard solution of AgNO3. At argentometry titration end point is determined by the formation of colored solutions or first onset of turbidity.

1. MOHR METHOD
Used to establish levels of halogen ions are conducted in a neutral atmosphere with K2CrO4 indicators and standard solutions of AgNO3. Chromate ions will react with silver ions form a red-brown precipitate of silver chromate.

Reaction:

Ag⁺ + Cl^- → AgCl(s) (white precipitate)

2Ag⁺ + CrO₄^2- → Ag₂CrO₄(s) (red-brown precipitate)

Basic titration with this method is a multilevel precipitation of AgCl and after all just happened sediment settles Ag2CrO4. For more details, we can see the following example.
For example in a solution of 0.1 M NaCl contained a K2CrO4 indicators that have a concentration of 0.01 M, the concentration of Ag + ions to precipitate the Cl-and CrO42-can be calculated.

A. Cl-ions to precipitate
At this place the point of equality. Both the silver ions and chloride ions no excess, and each concentration is the square (of) Ksp. At this point the titration curve is called the equivalence point , ie a point on the curve showing the number of gram equivalents of titrant is equal to an equivalent number of grams of substance titrated.
                   Ksp AgCl = 1,0 x 10^-10

[Ag^+] = [Cl^-]

[Ag^+]2 = 1,0 x 10^-10

[Ag⁺] = 1,0 x 10^-5

B.  to precipitate CrO42- ions
Ksp Ag₂CrO₄ = 2 x 10^-12

[Ag⁺]² [CrO₄^2-] = 2 x 10^-12

[Ag⁺]² [10^-2] = 2 x 10^-12

[Ag⁺]² = 2 x 10^-10

[Ag⁺] = 1,4 x 10^-5

From the above examples can be seen that the number of ions required to precipitate silver chromate ions is greater than that required to precipitate the chloride ion. So when end point of titration occurs, chloride ions have been precipitated practically all, so the new silver chromate precipitate after all chloride ions to form silver chloride precipitate.

The things to consider in the use of the Mohr method:

1.       Good for determining chloride and bromide ions but not suitable for iodide ions and tiocianide.

2.       Titration in an atmosphere of neutral or slightly alkaline, pH 7 - 10.5.

3.       Not suitable for titration of colored solutions, such as CuCl2 (blue), CaCl 2 (silver), NiCl (green) because it would complicate observations at TAT.

4.       Unable to salts of hydrolyzed Cl and Br, because the precipitate formed which was expected. Cl or Br salts with cations of Al, Fe, Bi, Sn, Sb, and Mg.

5.       The solution must not contain CO 3 2 -, SO 4 2 -, PO43-, C2O42-because it will precipitate with Mg.

6.       The solution should not contain ions Pb2 + and Ba2 + as it will precipitate as chromate salts are colored. Eliminated by the addition of saturated Na2CO3.

2. VOLHARD METHOD
                This method uses the principle of back-to-Titration, ion halogenide samples are added by a standard solution of AgNO3 in excess, then all of AgNO3 solution titrated back with standard NH4CNS. The indicator which is used to this method is Ferric Ammonium Sulfate. In the process the solution must be acid in order to prevent hydrolysis ferric salts into ferric hydroxide which color is disturbing observation end point of titration. Acidic conditions can be made by adding concentrated HNO3. But do not be too concentrated HNO3 use because it will cause NH4CNS be oxidized to NO and CO2.

3NH₄CNS + 13HNO₃ → 16NO + 3CO₂ + NH₄HSO₄ + 5H₂O

In this method the reaction mechanism will form silver chloride and silver thiocyanate.

Aqueous AgNO3 + Cl-→ AgCl (s)
NH4CNS → NaNO3 + residual AgCNS (s)
CNS-rest +  Fe 3 + →Fe (CNS) 3 blood red (when the end point titration)

Silver chloride is more soluble than silver thiocyanate, and chloride tend to dissolve back according to the reaction.

AgCl(s) + SCN^-    → AgSCN(s)  + Cl^-

Equilibrium constant of this reaction is determined by the rate constants versus the solubility product of silver chloride to silver thiocyanate. Since the first constant is greater than the second, then the above-mentioned reactions are very likely to run from left to right. So thiocyanate can be spent not only by the excess silver ions, but also by the precipitation of silver chloride itself. If this happens, it will obtain results that are too low in chloride analysis. But this reaction can be prevented by filtering the silver chloride. For the assay Br-filtering unnecessary because AgBr solubility product constants smaller than AgCNS, whereas for I-addition of the indicator after approaching the TAT because when I-see indicator Fe3 + I2 happen that often lead to errors titration.

Things that must be considered in the method of  Volhard:
1. The solution should be acid, in order to avoid hydrolysis ferric salt into ferric hydroxide which color is disturbing observation at endpoint of titration.
2. HNO3 is used to provide acidic conditions would not be too concentrated for oxidize NH4CNS into NO and CO2. Where is the CO2 that is formed can react with H2O to form H2CO3 which can react with Ag + and produces a white Ag2CO3 making it difficult observations during end point of tiration. In addition levels Fe3 + is reduced, so the chances endpoint of titration will happen much.
3. AgCl precipitate first formed should be filtered, washed with water and washing water to be made one with the new filtrate titrated with NH4CNS.

3. FAJANS METHOD
                In this method is used indicator adsorption. If a colored organic compounds adsorbed on the surface of a precipitate, the organic structure modification can occur, and that color can be greatly altered and can be older. These symptoms can be used to detect the titration end point of deposition of silver salts.
                The mechanism of operation of such indicators is different from any mechanism that has been discussed so far. Fajans found that fluorescein and some fluorescein substituted can act as an indicator for the titration of silver. When silver nitrate is added to a solution of sodium chloride, silver chloride particles are very fine it tends to hold on its surface (adsorb) a number of excess chloride ions present in solution. Chloride ions are said to form the primary adsorbed layer and thus causes the colloidal particles of silver chloride is negatively charged. Negative particles are then likely to attract positive ions in solution to form a secondary adsorption layer is more loosely bound.