Mohr Method: Colored Precipitate Formation

Formation of Colored Precipitate (Mohr Method)

• Precipitation titrations can use colored precipitate formation to indicate completion, similar to acid-base titrations with indicators
• Mohr's method uses this principle, titrating chloride against Ag+ with K2CrO4 as the indicator
• Titration endpoint marked by the first permanent appearance of reddish silver chromate precipitate

Solubility and Precipitation Sequence

• Ag+ forms salts with Cl- and CrO4-2, but silver chromate is more soluble (8.4 x 10^-5 mol/L) than silver chloride (1.0 x 10^-5 mol/L)
• Silver chloride precipitates first because silver ions added to a solution with chloride and chromate ions will precipitate silver chloride before silver chromate can form
• Silver chromate forms only when the silver ion concentration is high enough to exceed silver chromate's Ksp

Minimum Silver Ion Concentration for Precipitation

• During titration of 0.1M NaCl with 0.1M AgNO3, dilute potassium chromate is used as an indicator
• The equilibria expected is AgCl ⇌ Ag+ + Cl-
• Ksp=[Ag+][Cl-]=1.2*10^-10
• Minimum silver ion concentration = 1.2 x 10^-9 M
• Similarly, Ag2CrO4 ⇌ 2Ag+ + CrO42-
• Ksp=[Ag+]^2[CrO4^-2] = 1.7 x 10^-12
• Minimum silver ion concentration = 2.4 x 10^-5 M if 0.003 M K2CrO4 is added as an indicator
• Silver chloride precipitates before silver chromate since the minimum Ag+ concentration for silver chloride precipitation is less than that for silver chromate

Calculating Ion Concentrations for Precipitation

• Calculating minimum chloride and chromate concentrations:
• Ksp AgCl = [Ag+][Cl-] = 1.2 x 10^-10
• Ksp Ag2CrO4 = [Ag+]^2[CrO4^-2] = 1.7 x 10^-12
• [Ag+]=[Ksp AgCl]/[Cl-]=1.2*10^-10/Cl
• [Ag+]=[Ksp Ag2CrO4]/[CrO4^-2]=1.7*10^-12/CrO4^-2

Concentrations at Equivalence Point

• At equivalence point:
• Ksp AgCl = [Ag+][Cl-]
• [Cl-] = √Ksp AgCl = 1.1 x 10^-5 M

Precipitation of Silver Chromate

• The chloride concentration: [Cl-] / √[CrO4^-2] = 9.2 x 10^-5
• Concentration of chromate: [CrO4^{-2}]=\left(\frac{\left[Cl^-\right]}{9.2\times10^{-5}}\right)^2
• [CrO4^{-2}]=\left(\frac{1.1\times10^{-5}}{9.2\times10^{-5}}\right)^2
• [CrO4^{-2}]=1.4 * 10^-2 = 0.014M

Adjusting Chromate Indicator Concentration

• The calculated minimum chromate concentration should be 1.4 x 10^-2 M
• In practice, a more dilute K2CrO4 solution (0.003-0.005 M) is preferred so the original orange color of the concentrate doesn't interfere with detecting the red silver chromate endpoint

Addressing Titration Errors

• A slight excess of AgNO3 is added to precipitate Ag2CrO4 due to the use of dilute K2CrO4
• An indicator blank must be determined, by measuring Ag+ ion consumption in a Cl- free CaCO3 suspension, to correct for titration error

Optimal Titration Conditions

• Titration should occur in a neutral or slightly alkaline solution (pH 6.5-9.0)
• Silver hydroxide may precipitate in strong alkaline solutions.
• Acidic solutions cause the reaction: 2 CrO4-2 + 2 H+ = 2 HCrO4-2 = Cr2O7-2 + H2O, which reduces chromate concentration

Temperature Considerations

• Perform titration at room temperature because silver chromate solubility increases with temperature

Limitations

• Iodide and thiocyanate ions cannot be directly titrated due to adsorption effects
• Bromide and cyanide ions can be titrated with silver in slightly alkaline solutions

Volhard's Method

• An indirect (back) titration method
• Excess of standard AgNO3 solution is added to an acidic solution containing chloride, bromide, or iodide
• The excess AgNO3 is then back-titrated with a standard solution of potassium or ammonium thiocyanate (KSCN or NH4SCN)
• Fe+3 is used as an indicator
• Ag+ + Cl- ⇌ AgCl(s) Ksp AgCl = 1.2 x 10^-10
• Ag+ + SCN- ⇌ AgSCN(s) Ksp AgSCN = 7.1 x 10^-13

Detecting the Endpoint

• The thiocyanate solution added produces a silver thiocyanate precipitate
• Once complete, excess thiocyanate forms a reddish-brown complex with the Fe+3 indicator
• Fe+3 + SCN- ⇌ [FeSCN]+2 (Red color)

Potential Problems

• Issues arise if the anion's silver salt is more soluble than silver thiocyanate in this indirect method
• For instance, AgCl is more soluble than AgSCN, which leads to chloride redissolving:
• AgCl + SCN- ⇌ AgSCN(s) + Cl-
• Due to AgCl’s solubility, added thiocyanate gets consumed by excess Ag+ and silver chloride precipitates, increasing titration error.

Minimizing Errors in Volhard's Method

• Filter off the AgCl precipitate to prevent reaction with thiocyanate
• Add potassium nitrate as a coagulant after adding excess AgNO3 to prevent Ag+ ion readsorption
• Add an immiscible liquid like nitrobenzene to coat silver chloride particles and prevent reaction with thiocyanate
• Use a high Fe+3 concentration (about 0.2M) to reach the endpoint color at a lower thiocyanate concentration

Equilibria for Bromide Estimation

• Ksp(AgBr) / Ksp(AgSCN) = [Br-] / [SCN-] = (4.5 x 10^-13) / (7.1 x 10^-13) = 0.5

Titration Error Comparison

• Titration error is smaller for bromides than for chlorides
• Error is negligible for silver iodide as it is still less soluble
• Fe+3 ions indicator should be added after the addition of excess of AgNO3 solution
• The dissolved iodide reacts with Fe+3 ions: 2 Fe+3 + 2 I- = 2 Fe+2 + I2

Acidity

• Acidic conditions are required for Volhard method to prevent Fe+3 precipitation and avoid interference

K. Fajan's Method (Adsorption Indicators)

• Adsorption indicators are colored organic substances that adsorb onto a precipitate and change color
• Fajan's indicators include acid dyes like fluorescein and eosin (as sodium salts) and basic dyes from the rhodamine series (as halogen salts)

Titration with Silver Nitrate

• AgCl precipitates adsorb excess Cl- ions in solution, forming a primary layer and causing AgCl colloidal particles to be negatively charged
• These particles attract positive Na+ ions, creating a secondary layer of Na+ ions
• After the equivalence point, excess Ag+ ions are adsorbed by AgCl forming a primary layer
• The charged primary layer attracts negatively charged NO3-ions, forming a secondary adsorbed layer.

Fluorescein Indicator

• Fluorescein, added as an indicator, is strongly attracted by Ag+ ions, forming a secondary layer which immediately forms a pink complex with the first trace of excess Ag+ ions

Surface Phenomenon

• Color change occurs at the precipitate's surface
• If excess chloride ions are added to the solution, fluorescein ions pass back into the solution, restoring the original greenish-yellow color

Indicator Properties

• The precipitate must be colloidal because the precipitate's surface is the active agent
• Coagulation of AgCl is avoided for a true reading

Dextrin

• A substance like dextrin is required for AgCl to be dispersed
• At the equivalence point, coagulation would occur where neither Ag+ nor Cl- ions are in excess

Indicator and Precipitating Agent

• Must be oppositely charged

Indicator Precautions

• Indicator ion should only be adsorbed after complete precipitation and form the secondary layer
• Silver halides are light-sensitive, so titrations occur without sunlight
• Indicator concentrations should be between 2 x 10^-4 and 3 x 10^-4 moles per mole of silver halide

Importance of pH

• Fluorescein is a weak acid, thus a pH between 7 and 10 is used for chloride titrations
• Dichlorofluorescein is stronger and is used in solutions with a pH greater than 4.4
• Tetrabromofluorescein (eosin) is stronger and is used in the pH range of 1 to 2

Eosin

• Eosin cannot be used for chloride titrations because it forms a primary layer on AgCl, which causes premature endpoints

Solubility Factors

• Includes common ion effect, excess of precipitant, temp, salt effect, nature of solvent, pH etc

Common Ion Effect

• Salt solubility decreases when solutions contain common ions
• E.g., AgCl solubility in water is reduced when Cl- ions are present

Temperature Effect

• Solubility of precipitates generally increases with temp because dissolution is endothermic

Diverse Ion Impact

• Solubility of sparingly soluble salts often increases with foreign ions (diverse ion effect)
• E.g., Barium sulfate (BaSO4) solubility rises by about 70% in 0.01M KNO3, because electrolyte strength increases

Solvent Influence

• Inorganic salts usually dissolve more readily in polar solvents (like water) than in non-polar organic solvents

pH's Role

• Salt solubility depends on the pH of the solution
• Solubility of strong acids remains largely unaffected
• Solubility of weak acids may be vastly affected
• In some cases, solid precipitate may dissolve in excess acid

Hydrolysis Impact

• Hydrolysis takes place when a weak acid salt is dissolved in water
• Anion is completely hydrolyzed

Complex Reactions

• Slightly soluble salt solubility also relies on the substance concentration, forming complexes with the salt's cation
• AgCl precipitates in water with ammonia will form Ag(NH3)2+. This will increase the solubility of AgCl

Must Read Books

• Fundamentals of analytical chemistry by Skoog and West
• Textbook of quantitative chemical analysis by Vogel
• Analytical chemistry by Day and Underwood