Methods for Determining Residual Alkalinity in Rinse Water and on Equipment Surfaces: Advantages and Disadvantages

Currently, cleaning and disinfecting agents based on alkaline components are widely used. The final stage of cleaning and disinfection processes involves monitoring the thorough rinsing of residual cleaning agents by determining a parameter called residual alkalinity. Various methods exist for determining residual alkalinity in rinse water and on equipment surfaces.

To determine residual alkalinity in rinse water, the phenolphthalein test can be used. For the analysis, 10-15 ml of the sample is placed in a flask, and 2-3 drops of a 1% alcoholic solution of phenolphthalein are added. If no alkaline components of the cleaning agents are present, the rinse water remains colorless; if they are present, it turns shades ranging from pink to crimson. To determine residual alkalinity on surfaces, a cotton swab is moistened with a phenolphthalein solution and pressed firmly against the tested surface for 1-2 seconds, after which its appearance is evaluated. If the swab’s color does not change, there is no residual alkalinity. A color change to pink or crimson indicates inadequate surface rinsing. The phenolphthalein test can also be performed using indicator strips coated with phenolphthalein. To analyze rinse water, the test strip is immersed in the sample for 1-2 seconds, and its coloration is then assessed. If no alkaline detergent components are present, the strip remains white; otherwise, it turns pink to crimson. For surface testing, the strip is pressed firmly against the surface for 1-2 seconds, and its appearance is evaluated. If the strip remains white, there is no residual alkalinity. A color change to pink or crimson indicates the need for additional rinsing. The phenolphthalein method for determining residual alkalinity has several drawbacks. It is known that the phenolphthalein indicator (4,4′-dihydroxyphthalophenone) changes color in the pH range of 8.2 to 10, from colorless to crimson. However, it should also be noted that phenolphthalein becomes colorless in concentrated alkaline solutions and forms a pink cation in concentrated sulfuric acid. These characteristics of phenolphthalein make it difficult to ensure the reliability of the analysis and guarantee proper equipment rinsing.

Another method for determining residual alkalinity is the use of universal indicator paper or pH test strips. To analyze rinse water, the indicator strip is immersed in the sample for 1-2 seconds, and its color is compared to a reference scale. For surface analysis, immediately after cleaning and rinsing, the strip is pressed against the wet surface of the sanitized equipment for 1-2 seconds, and its color is then compared to the scale. The disadvantage of this method is its low sensitivity and poor color contrast.

Delta-CT LLC has developed “Residual Alkalinity” test strips, manufactured according to TU 2642-008-66948373-2010, designed for rapid testing of residual alkaline components of cleaning and disinfecting agents in rinse water and on surfaces. The method for determining residual alkalinity using these strips has been tested at numerous food industry enterprises in Russia and abroad. The advantages of these strips include high-contrast color transition (the indicator zone changes from yellow to crimson), high sensitivity, and, consequently, the ability to rinse equipment to permissible limits of harmful substance concentrations. The “Residual Alkalinity” test strips produced by Delta-CT LLC do not have the drawbacks associated with phenolphthalein. The indicator zones are yellow in strongly acidic, acidic, and neutral solutions and turn orange-red to crimson in weakly alkaline and strongly alkaline solutions. These strips can be used to detect traces of hydroxides of alkali and alkaline earth metals (sodium, potassium, calcium, barium, etc.), soda, and tertiary amines. The testing procedure is detailed in the product instructions.