IOM 2017, Експеримент — Задача 1. Titrimetric determination of free iron in a water sample

Standardization of permanganate solution with potassium hexacyanoferrate

Dissolve the weighed amount of solid $\mathrm{K_4Fe(CN)_6}$ in the 50 mL graduated flask in ca. 25 mL of 3 M $\mathrm{H_2SO_4}$ solution. Bring up to the mark with water and mix thoroughly.

Fill the burette with $\mathrm{KMnO_4}$ solution (use the funnel). Note: you are provided with dry and clean burettes and pipettes. Do not spend solutions for rinsing the glassware.

Transfer with the volumetric pipette a 5.00 mL aliquot of the prepared hexacyanoferrate solution into the conical flask. Add with the cylinder 15 mL of 3 M $\mathrm{H_2SO_4}$ solution and 15 mL of water. Mix the solution and titrate it with $\mathrm{KMnO_4}$ solution. Add the titrant dropwise swirling the flask constantly. Repeat the titration until the results differ by less than 0.1 mL (the number of titrations is not graded).

1.1. Write down the volumes of permanganate solution used for hexacyanoferrate titration:

[13 т.]

Titration of the sample solution with potassium permanganate (without prior reduction)

Pipette 5.00 mL of the standardized potassium permanganate solution into a 100 mL volumetric flask and bring it up to the mark with distilled water. Mix the solution thoroughly. Wash the burette and fill it with the prepared potassium permanganate solution.

Pipette a 10.0 mL aliquot of the sample solution into a conical titration flask, add 5 mL of the Reinhardt-Zimmermann solution, 10 mL of 3M $\mathrm{H_2SO_4}$ solution, 10 mL of water, and 5 drops of the Phenylanthranilic acid solution. Titrate the prepared mixture with the $\mathrm{KMnO_4}$ solution till light-orange coloration appears. Repeat the titration as necessary.

Note: titrate the prepared mixtures immediately. Prepare the next portion of the sample just before the titration.

1.2. Write down the volumes of the permanganate solution used for titration:

[13 т.]

Titration of the sample solution with potassium permanganate with prior reduction of Fe(III)

Transfer with the 10.0 mL pipette 30.0 mL of the sample (from the 120 mL container) into the 150 mL glass beaker containing zinc granules. Add with a cylinder 10 mL of HCl (1:1 v/v). Cover the beaker with the watch glass and keep heating for 30 min. Cool the beaker under tap water down to nearly room temperature.

Transfer the solution from the beaker into the 50 mL volumetric flask (thoroughly wash the beaker containing the non-dissolved zinc granules with water). Bring the solution in the flask up to the mark and mix. Pipette a 10.0 mL aliquot of the prepared solution into the conical flask; add 5 mL of the Reinhardt-Zimmermann solution, 10 mL of 3M $\mathrm{H_2SO_4}$ solution, 10 mL of water and 5 drops of the Phenilanthranilic acid solution. Swirl the flask and titrate the mixture with the potassium permanganate solution until the color changes from lemon-yellow to light-orange. Repeat the titration as necessary.

Note: titrate the prepared mixtures immediately. Prepare the next portion of the sample just before the titration.

1.3. Write down the volumes of the permanganate solution used for titration of the sample (with reduction of Fe(III)):

[14 т.]

1.4. Calculate the concentration of $\mathrm{K_4Fe(CN)_6}$ solution in mol/L (mass of the trihydrate is 3.16 g).

$c$ = [1 т.]

1.5. Write down the ionic reaction equation occurring upon standardization of the permanganate solution: [1 т.]

1.6. Calculate the concentrations of non-diluted and diluted potassium permanganate (mol/L) based on the titration results:

$c$ (initial) =

$c$ (diluted) = [1 т.]

1.7. Write down two reactions occurring upon heating of the acidic sample solution with zinc:

2. [1 т.]

1.8. Calculate the concentration of iron(II) and iron(III) (mg/L) in the sample based on the titration results:

Calculations:

Concentration of iron(II) ___________ mg/L

Concentration of iron(III) ___________ mg/L [1 т.]

2. Answer the theoretical questions.

2.1. Natural water often contains a considerable amount of chloride ions. Write down the competing reaction equation that interferes with the titrimetric determination of Fe(II) with permanganate in the presence of chloride: [1 т.]

2.2. Free chlorine can be obtained by the reaction of manganese dioxide with hydrochloric acid. Write down the balanced equation of this reaction: [1 т.]

2.3. Permanganate reacts extremely slowly with diluted hydrochloric acid solutions in the absence of iron. However, the reaction accelerates upon addition of a Fe(II) salt, and the characteristic chlorine odor appears. Suggest a scheme of reactions explaining the catalytic action of Fe(II) in the system (use the properties of manganese compounds you have written in i. 2.2):

No catalyst:

In the presence of Fe(II): [2 т.]

2.4. To get rid of the interfering effect of chloride ions (i. 2.1) you have added the Reinhardt-Zimmermann solution containing Mn(II) ions to the titrated solution (the manganese action is due to decreasing the redox potential of $\mathrm{MnO_4^-/Mn^{2+}}$). Addition of fluoride ions is an alternative method to avoid the interfering action of chloride ions on the titration process. Write down the reaction equation explaining a possible mechanism of the protecting action of fluoride (take into account your scheme in i. 2.3): [1 т.]