Search: alkaline capacity

Analogous to the p and m values obtained in the determination of acid capacity (pH 8.2 and 4.3), this analysis is performed according to W-000.13.031 Acid Consumption (Alkalinity, p-Value and m-Value)/Acid Capacity to pH of 8.2 and/or 4.3 for Water. The alkaline capacity of the boiler water is determined through titration of the sample with 0.1 N sodium hydroxide (instead of hydrochloric acid) to a pH of 4.3 and/or 8.2.

Potassium permanganate oxidizes many organic and certain inorganic substances more or less completely in acidic, neutral or alkaline solutions. The volume of potassium permanganate required in the analysis is determined potentiometrically. Since oxidation depends on the type of solution, on its temperature and on the reaction time, the procedure described below must be followed precisely.

In acidic solutions, permanganate ions are typically reduced to manganese(II) ions:

MnO₄^- + 5 e- + 8 H₃O⁺ → Mn²⁺ + 12 H₂O

In alkaline solutions, the reduction results in tetravalent manganese only:

MnO₄^- + 3 e- + 4 H₃O⁺ → MnO₂ + 6 H₂O

Since in both cases the titration takes place in an acidic solution, this is irrelevant for the calculation. By adding oxalic acid, both the excess permanganate ions as well as the tetravalent manganese are reduced to manganese(II) ions:

2 MnO₄^- + 5 C₂O₄^2- + 16 H₃O⁺ → 2 Mn²⁺ + 24 H₂O + 10 CO₂

MnO₂ + C₂O₄²- + 4 H₃O⁺ → Mn²⁺ + 6 H₂O + 2 CO₂  

The sodium hydroxide bound as aluminate and titrated in the p-value determination is calculated by subsequent determination of the aluminum and deducted, as it is no longer available for purification.

The aluminum determination itself is based on the reaction between aluminum hydroxide and sodium fluoride, in which sodium hydroxide is released and thus serves as a measure for calculating the aluminum.

Carbonates interfering with the titration are precipitated with Ba²⁺ ions.

The sodium hydroxide bound as aluminate and titrated in the p-value determination is calculated by subsequent determination of the aluminum and deducted, as it is no longer available for purification.

The aluminum determination itself is based on the reaction between aluminum hydroxide and sodium fluoride, in which sodium hydroxide is released and thus serves as a measure for calculating the aluminum.

Titration against methyl orange instead of the Tashiro indicator leads to incorrect results. When sodium fluorif (NaF) is added to the alkaline solution, hydrofluoric acid (HF) is formed. The hydrofluoric acid destroys the methyl orange indicator.

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.