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T-741.04.032 [2024-08] Alkaline cleaning agents containing carbonate in the presence of phosphates and soiling - sodium hydroxide and soda

Ready-made cleaning agents are composed of a large number of different components. The analysis of individual components is irrelevant in practice, and it is usually sufficient to determine the alkaline or acid-reacting components by summary analysis. The manufacturers of cleaning agents and disinfectants specify so-called titration factors in the operating instructions for the individualproducts, which are used to calculate the concentration in percent by volume or weight.

For products that are marketed as liquid concentrates, both factors - both for volume and weight percentages - are printed, as the products are both titrated and weighed by the consumer.

The titration factor (TF) is the reciprocal of the amount of standard solution (HCI or NaOH) in ml, which must be used to titrate a certain volume of exactly 1 % solution of a detergent.
TF × ml consumption = GG-% product in solution

In practice, there are various different templates of the cleaning solutions to be determined and normalities of the standard solutions in use: generally 10, 50 or 100 ml, which are diluted with 0.1 mol/l (0.1 N), 0.5 mol/l (0.5 N) or 1 mol/l (1.0 N) hydrochloric acid or sodium hydroxide solution against phenolphthalein or methyl orange. The resulting consumption of hydrochloric acid or sodium hydroxide solution in ml multiplied by the factor specified by the manufacturer gives the concentration of the cleaning solution in percent by volume or weight. For highly alkaline cleaning solutions, templates of 5 ml are also suggested. Some cleaning and disinfectant manufacturers also prescribe sulphuric acid or acetic acid instead of hydrochloric acid and recommend a certain normality, which makes further calculation unnecessary, so that the ml of acid used gives the concentration of the cleaning solution directly.

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When using alkaline cleaning solution, a reaction with carbonates from the air or from contamination can occur. This is particularly relevant (but not limited) for brewery and beverage plants due to the higher amount of CO₂ in the atmosphere. If an effect of carbonic acid on the caustic is expected or if the cleaning solution is used over a longer period of time, it is necessary to consider the degree of carbonation to determine the exact concentration.

This is because the cleaning effect of sodium carbonate (Na₂CO₃) is greatly reduced, but the titration simulates a high concentration of the active cleaning agent (NaOH).

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alkaline cleaners

carbonates

cleaning agents

cleaning concentrates

Application/Purpose

Consideration of phosphates and impurities, especially hydrolysis products of proteins in the concentration determination of alkaline cleaners on sodium hydroxide (NaOH) and soda (Na₂CO₃).

Scope of Application

All alkaline cleaning solutions or "batch solutions" that contain soda (Na₂CO₃) as a cleaning component in addition to sodium hydroxide (NaOH).

Principle

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.