Experiment: Mohr's Method for Chloride Ion Estimation
This is a classic experiment related to Precipitation Titrations in chemistry. The principle involved is the titration of chloride ions with a silver nitrate solution, followed by endpoint determination using potassium chromate as an indicator. The formation of a brick-red precipitate of silver chromate signals the endpoint.
Materials:
- Standard 0.1M Silver nitrate solution (AgNO3)
- Unknown Chloride Salt Solution (e.g., NaCl solution of unknown concentration)
- Potassium Chromate Indicator (K2CrO4) (approx. 5% w/v solution)
- Conical Flask (250 mL)
- Burette (50 mL)
- Pipette (25 mL)
- Wash bottle containing distilled water
Procedure:
- Pipette 25.0 mL of the unknown chloride solution into a clean, dry conical flask.
- Add approximately 1-2 mL of 5% potassium chromate indicator solution to the flask. The solution will turn a light yellow.
- Fill a burette with the 0.1M AgNO3 solution. Record the initial burette reading.
- Slowly add the AgNO3 solution from the burette to the conical flask, swirling the flask constantly to ensure thorough mixing.
- Continue the titration until the first persistent appearance of a brick-red precipitate of silver chromate (Ag2CrO4) is observed. This indicates the endpoint of the titration.
- Record the final burette reading. Subtract the initial burette reading from the final burette reading to determine the volume of AgNO3 solution used.
- Repeat the titration at least two more times to obtain consistent results. Calculate the average volume of AgNO3 used.
Observations:
- The color change from yellow to brick-red signals the endpoint of the titration.
- This color change occurs because silver ions (Ag+) first react with chloride ions (Cl-) to form a white precipitate of silver chloride (AgCl). Once all the chloride ions have reacted, excess silver ions react with the chromate ions (CrO42-) to form the brick-red silver chromate precipitate.
Chemical Reactions:
- Ag+(aq) + Cl-(aq) → AgCl(s) (white precipitate)
- 2Ag+(aq) + CrO42-(aq) → Ag2CrO4(s) (brick-red precipitate)
Significance:
This experiment demonstrates a precipitation titration, where the reaction involves the formation of a precipitate. It's widely used to estimate halides (chloride, bromide, iodide) and pseudohalides (thiocyanates). The AgNO3 solution is a standard solution, meaning its concentration is accurately known. This method is significant in various applications where precise determination of halide concentrations is crucial.
Calculations:
The concentration of chloride ions in the unknown sample can be calculated using the following formula (derived from stoichiometry):
Moles of AgNO3 = Moles of Cl-
(Molarity of AgNO3) x (Volume of AgNO3 used in Liters) = (Molarity of Cl-) x (Volume of Cl- solution in Liters)
Where:
- Molarity of AgNO3 = 0.1 M
- Volume of AgNO3 used = Average volume from titration (in Liters)
- Volume of Cl- solution = 0.025 L (25 mL)
- Molarity of Cl- = Concentration of chloride ions in the unknown sample (to be calculated).
Remember to convert volumes from mL to L before calculation.