Search: stack solutions

Additives containing phosphoric acid in particular - but also other types of additive - can simulate a more or less significant Na₂CO₃ content due to their buffering effect. Firstly, phosphoric acid significantly blunts the p-value by forming sodium phosphate (Na₃PO₄) and secondly, the difference between the m-value and the p-value is increased because sodium dihydrogen phosphate (NaH₂PO₄) is largely formed from disodium hydrogen phosphate (Na₂HPO₄) in the pH range 8.2 to 4.3. The same applies - but to a lesser extent due to the molecular sizes - to typical phosphonic acids or their salts, but not, for example, to most surfactants.

To determine the exact concentration or pH value, the content of buffering substances - phosphates and impurities - must be taken into account in addition to the degree of carbonation, as otherwise the concentration of the active cleaning agent will be too high.

Determination of the p-value of the detergent solution with an acid solution (HCl or H₂SO₄) with corresponding normality up to the color change of phenolphthalein (pH 8.2).

   NaOH + HCl           →     NaCl     +    H₂O              colorless against phenolphthalein

2 NaOH + H₂SO₄      →     Na₂SO₄ + 2 H₂O

Determination of the soda ash (Na₂CO₃) content of the detergent solution with an acid solution (HCl or H₂SO₄) with corresponding normality up to the color change from methyl orange (pH 4.3).

  Na₂CO₃    + HCl       →     NaHCO₃  + NaCl

  NaHCO₃   + HCl      →      NaCl       + H₂O + CO₂   yellowish brown against methyl orange

2 Na₂CO₃   + H₂SO₄  →  2 NaHCO₃  + Na₂SO₄

2 NaHCO₃  + H₂SO₄  →  2 Na₂SO₄   + H₂O + 2 CO₂

The most important step is the expulsion of CO₂. This is to ensure that no more hydrogen carbonates or carbonates can form during the subsequent back titration with NaOH. For this purpose, a significant acidification below pH 4.3 must be carried out with acid in order to be able to drive out CO₂ using an inert gas.

The proportion of buffering substances can then be determined by back titration with NaOH to pH 4.3 or 8.2.

Disinfectants containing active chlorine, e.g. hypochlorites, oxidize iodide to iodine, which is titrated with sodium thiosulfate (Na₂S₂O₃) and converted to active chlorine.

Sodium thiosulphate reacts to form sodium tetrathionate (Na₂S₄O₆)

For the determination of low concentrations (up to 2.5 mg/l) of chlorine dioxide (CIO₂) and chlorite the photometric or colorimetric DPD methods are well established. Higher concentrations of ClO₂ that are present in the concentrates have to be determined by titration with sodium thiosulphate or after strong dilution.

Chlorine dioxide oxidizes iodide to iodine, which is then reduced by sodium thiosulphate. The sodium thiosulphate consumed is recalculated as ClO₂.

The quantitative determination is carried out in a sulphuric acid solution by titration with potassium permanganate:

5 H₂0₂ +  2  KMnO₄ + 3  H₂S0₄  →  2 MnSO₄ + H₂S0₄ + 8 H₂0 + 5 0₂

If the sample contains other peroxide-containing disinfectants in addition to hydrogen peroxide (H₂O₂), the method T-754-01-032 Peracetic-acid-based-disinfectants must be used.

Hydrogen peroxide oxidizes iodide to iodine, which is reduced by sodium thiosulfate (Na₂S₂O₃) and recalculated as hydrogen peroxide.

If the sample contains other peroxide-containing disinfectants in addition to hydrogen peroxide (H₂O₂), the method T-757-01-032 Peracetic-acid-based-disinfectants must be used.

In addition to peracetic acid (C₂H₄O₅), hydrogen peroxide (H₂O₂) is always present in disinfectants, so that both components must be determined together. The quantitative determination of hydrogen peroxide is carried out as described under T-753.01.032 Disinfectants-containing-hydrogen-peroxide-potassium-permanganate-method or T-753.02.032Disinfectants containing hydrogen peroxide - sodium thiosulphate method. The peracetic acid is determined iodometrically: