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Friday, October 24, 2008

Chemistry Bath-life extension and control

Question:
How can bath-life be monitored?

Answer:
Some very simple, yet effective, techniques can be used to monitor bath life, including:
  • Conductivity—When you are using detergents with high ionic content, measurement of the ionic content of the cleaning solution, using electrodes connected to a conductivity meter, helps determine when to add more detergent to the cleaning solution. Detergents high in salt content, for example, potassium hydroxide or sodium metasilicate, generally, have high ionic content. Conductivity will drop as soils react with the salts. This is not a useful technique for monitoring high emulsifying cleaners that rely heavily on surfactants.
  • Refractometry—This is the indirect measure of the concentration of dissolved components which influence the refractive index of a sample of solution. The measurement may be taken using a simple handheld refractometer. Refractometry can also be used to monitor the build up of soils and concentration of a solution as a result of water evaporation. Empirical observations of cleaning solutions may be compared with recorded measurements to determine appropriate times to recharge or discard solutions.
  • Foam Height—The foam height and foam stability, in a sample of cleaning solution in a vigorously-agitated, stoppered test tube will decrease as oils accumulate. When foam height decreases, recharge or discard cleaning solutions. Observations must be made at consistent temperatures.
  • pH—This is the measurement of the acidity or alkalinity of a solution on a scale of 0–14. It is expressed as the negative log of the hydrogen ion concentration, which is measured using electrodes dipped in the solution connected to a pH meter. Note that pH paper should not be used with surfactant-containing cleaners, because they commonly hamper accurate reading. A given brand of detergent will have a typical pH. If the soils are acidic, inorganic, or saponifiable natural oils, the pH will drop as the cleaning solution is used up. Typically, when pH drops 0.5 pH units, the detergent should be recharged. Then the solution can be used to exhaustion as it drops one full pH unit. The technique chosen to monitor bath life will depend on the both the type of detergent you are using and the soil or residue you are removing. For example, when cleaning with an ionic detergent, use conductivity to monitor dilution, dragout, and loss of detergent. Whereas, when using a highemulsifying or dispersing detergent to remove oils or particulate soils, refractometry is an effective means of measurement and control. Foam height observation is most effective when cleaners contain foaming surfactants that rely on emulsification to remove oily soils. When using an alkaline cleaner to remove a soil that is either acidic or neutralizing in character (as most soils are), pH can be used as a control measure.

- The Aqueous Cleaning Handbook, 4th Ed. p 127-128

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