Standardisation of Mercuric Nitrate

1. Principle

Chloride can be titrated with mercuric nitrate, because of the formation of soluble, slightly dissolved mercuric chloride. In the pH range 2.3 to 2.8, diphenyl – carbazone indicates the titration end-point by the formation of a purple complex with the excess mercuric ions. Xylene cyanol FF serves as a pH indicator and end-point enhancer. By increasing the titrant strength and modifying the indicator mixtures, the range of measureable chloride concentration can be extended.



2. Reagents

2.1 Standard Sodium Chloride

2.2 Sodium hydrogen carbonate (NaHCO3), AR

2.3 Ethanol, AR

2.4 1-5 diphenylcarbazone, AR

2.5 Nitric acid (HNO3), 70%, AR

2.6 Xylene cyanol FF, AR

2.7 Diphenylcarbazone

2.8 Bromophenol blue, AR

2.9 Indicator – acidifier reagent: Into 100 mL 95% ethanol dissolve in the following order: 0.250 g 1-5 diphenylcarbazone, 4 mL HNO3 and 0.030 g xylene cyanol FF. Mix well, after the addition of each reagent. Store in a dark bottle in the refrigerator.

2.10 Mixed indicator reagent: Dissolve 0.5 g diphenylcarbazone powder and 0.05 g bromophenol blue powder in 75 mL of Ethanol. Dilute to 100 mL with ethanol.

2.11 Mercuric nitrate (Hg(NO3)2.H2O), AR. Extremely toxic – read MSDS before use. Extremely hygroscopic. Keep jar well sealed.

2.12 Standard mercuric nitrate solution, ~0.0141 N: Dissolve 2.5 g Hg(NO3)2.H2O in 100 mL reagent grade water containing 0.25 mL HNO3. Dilute to 1 L. Standardise by procedure

3.1. Store away from light in a dark bottle.

2.13 Strong Standard mercuric nitrate titrant, ~0.141 N: Dissolve 25 g Hg(NO3)2.H2O in 900 mL of reagent grade water containing 5.0 mL of HNO3. Dilute to 1 L. Standardise by procedure 3.2. Store away from light in a dark bottle.

2.14 Nitric acid, 0.1 N: Carefully add 6.4 mL of nitric acid to 800mL of reagent grade water. Make to 1 L with water.



3. Procedure:

3.1 Standardisation of ~0.0141 N Mercury Nitrate

Titrate an aliquot of standard sodium chloride (e.g. 5.0 mL standard NaCl solution and 0.01 g NaHCO3 diluted to 100 mL) with mercuric nitrate titrant, adding 1.0 mL of indicator – acidifier reagent and 1.0 mL of mixed indicator reagent as the indicators. The solution turns from green-blue to blue a few drops before the ‘definite’ purple end-point. Determine a blank by titrating 100 mL reagent grade water containing 0.01 g NaHCO3. Record the results. Carry out standardization in triplicate. Calculate normality of Hg(NO3)2 as per section 5.

3.2 Standardisation of ~0.0141 N Mercuric Nitrate

Aliquot 25 mL of standard sodium chloride and 25 mL of reagent grade water, into a conical flask adding 0.5 mL of mixed indicator reagent and mix well. The colour should be purple. Add 0.1 N HNO3 dropwise until the colour just turns yellow. Titrate with strong mercuric nitrate titrant to the first permanent dark purple. Titrate a reagent grade water blank as well. Record the results. Carry out standardization in triplicate. Calculate normality of strong Hg(NO3)2 as per section 5.



4 Frequency of Standardisation

Restandardise every 6 months.



5. Calculation:



Where
B = Blank Titre (mL)



6. Reference:

APHA 20th Edition; 4500 - Cl – C; pp.-4-68-69.