Determination of total chromium by ammonium ferrous sulfate titration

In acidic solution, with silver salt as catalyst, ammonium persulfate oxidizes trivalent chromium into hexavalent chromium. Add a small amount of sodium chloride and boil it to remove excess persulfate and ammonia produced in the reaction. Using phenyl o-aminobenzoic acid as indicator, hexavalent chromium was reduced to trivalent chromium by titration with ammonium ferrous sulfate solution, and the end point was that the solution was green. According to the dosage of ammonium ferrous sulfate solution, the total chromium content in water sample is calculated.

2. Interference

Vanadium interferes with the determination, but the content of vanadium in chromium-containing wastewater is generally below the allowable limit.

3. Scope of application of the method

This method is suitable for the determination of total chromium in high concentration wastewater (>: 1 mg/l).

4. Reagent

1) (1+ 19) sulfuric acid solution: take 50m 1 sulfuric acid, slowly add it into 950ml water, and mix well.

2) sulfuric acid-phosphoric acid mixed solution: slowly add 150ml sulfuric acid into 700m 1 water, cool, add 150ml phosphoric acid and mix well.

3) 25% ammonium persulfate solution: weigh 25g ammonium persulfate, dissolve it in water, dilute it to l00ml, and prepare it in time.

4) Potassium dichromate standard solution: weigh 0.4903g of potassium dichromate (K2Cr2O7, excellent grade) dried at 65438 020℃ for 2h, dissolve it in water, transfer it to a l000ml volumetric flask, and add water to dilute it to scale. Shake well. The concentration of this solution ((1/6k2cr2o7) = 0.01000 mol/l.

5) Ammonium ferrous sulfate sub-standard titration solution: weigh 3.95g of ammonium ferrous sulfate ((NH4) 2fe (SO4) 2.6H2O), dissolve it in 500ml( 1+ 19) sulfuric acid solution, filter it into a 2000ml volumetric flask, and use (1+) to dissolve it. When in use, it is calibrated with potassium dichromate standard solution.

① Calibration: Absorb 25.00m 1 potassium dichromate standard solution, put it in a 500ml conical flask and dilute it to about 200ml with water. Add 20ml of mixed solution of sulfuric acid and phosphoric acid, and titrate with ammonium ferrous sulfate standard titration solution to light yellow. Add 3 drops of phenyl o-aminobenzoic acid indicator and continue titration until the solution changes from red to bright green.

② Record the dose (Y0m 1) and calculate it as follows:

c[(NH4)2fe(SO4)2·6h2o]= 0.0 1000×25.00ml/v 0

Where: c refers to the concentration of ammonium ferrous sulfate standard titration solution (mol/L).

6) 1% manganese sulfate solution: dissolve 1g manganese sulfate (MRSO4 2H2O) in water and dilute it to 100m 1.

7) 0.5% silver nitrate solution: dissolve 0.5g silver nitrate in water and dilute it to 100m 1.

8) 5% sodium carbonate solution: dissolve 5g anhydrous sodium carbonate in water and dilute it to 100m 1.

9) (1+ 1) ammonia water: add ammonia water (ρ=0.90g/ml) to the same volume of water and shake well.

10) 1% sodium chloride solution: dissolve 1g sodium chloride in water and dilute it to 100m 1.

1 1) indicating solution of phenyl o-aminobenzoic acid: weigh 0.27g of phenyl o-aminobenzoic acid, dissolve it in 5m 1 5% sodium carbonate solution, and dilute it to 250m 1 with water.

Step five

① Absorb a proper amount of water sample in the beaker of 150m 1, and transfer it to a 500ml conical flask after acid digestion (if the water sample is clear and colorless, you can directly take a proper amount of water sample in a 500m 1 conical flask). Neutralize the pH value of the solution with ammonia water to 1 2. Add 20 ml of mixed solution of sulfuric acid and phosphoric acid, 1-3 drops of silver nitrate solution, 0.5 ml of 1 manganese sulfate solution and 25 ml of ammonium persulfate solution, and shake well. Add a few glass beads, heat until permanganate appears purple, and boil for 10min.

(2) Take it out and cool it slightly, add 5m 1 sodium chloride solution, and heat it slightly 10- 15min to remove all chlorine. Take it off, cool it quickly, rinse the bottle wall with water and dilute it to about 250ml. Add 3 drops of phenyl o-aminobenzoic acid indicating solution, titrate with ammonium ferrous sulfate standard titration solution until the solution changes from red to green, and record the dosage (V 1). At the same time, take the same volume of pure water instead of water samples for determination, and record the dosage (V2).

calculate

Total chromium (Cr, mg/l) = (v1-v2)/v3× c×17.332×1000.

Where: V 1—— the dosage of ammonium ferrous sulfate standard titration solution when titrating water samples (ml);

V2—— the dosage of ammonium ferrous sulfate standard titration solution when titrating blank samples (ml);

V3-volume of water sample (ml);

C-the concentration of ammonium ferrous sulfate standard titration solution (mol/l);

17.332—— 1/3Cr molar mass (g/mol).

7. Preventive measures

① Pay attention to the time of heating and boiling. If the heating and boiling time is not enough and the excess ammonium persulfate and chlorine gas are not removed, the result will be high; If the boiling time is too long, the solution volume is small and the acidity is high, hexavalent chromium may be reduced to trivalent chromium, and the result is low.

(2) when determining water samples and blank solutions, the dosage of phenyl o-aminobenzoic acid indicating solution should be consistent.