Surfactant concentration monitoring for industrial baths and electrolysis processes

Surfactants perform important functions in industrial baths. In cleaning solutions, they are the active ingredient, while in electroplating or etching baths, they act as wetting agents to ensure uniform wetting of the entire workpiece. When gases are released, as in electroplating or the electrolytic extraction and refining of metals such as copper, surfactants prevent the formation of toxic or corrosive spray mist (acid mist) by reducing the size of rising gas bubbles or by forming a foam blanket. To ensure that all these processes run optimally, the surfactant content must be checked at regular intervals.

Regular bath checks with a mobile bubble pressure tensiometer

Bath checks must be carried out frequently so that a quick response can be made if the surfactant concentration falls below a critical value. Otherwise, high costs can arise due to quality and time losses: the system must be shut down for a long period of time, foam or dirt must be laboriously skimmed off by hand and parts must be reprocessed, while orders pile up.

Efficient monitoring requires mobile solutions to measure the surfactant content directly in the bath or after sampling. This is exactly what bath monitoring based on dynamic surface tension (SFT) with a mobile bubble pressure tensiometer can do, determining the concentration of a surface-active substance in a matter of seconds.

Concentration measurement based on dynamic surface tension

The SFT of surfactant solutions depends on the concentration, which is why SFT can be used for indirect testing of surfactant content. However, classic tensiometry based on force measurement is not suitable for this purpose. It measures the static, time-independent SFT, which only reacts to changes in surfactant content up to a relatively low limit concentration, the critical micelle concentration (CMC). Bubble pressure tensiometers, on the other hand, measure the dynamic, time-dependent SFT, which is concentration-dependent in a wide range above the CMC that is relevant for industrial baths containing surfactants. With the help of this correlation, the concentration can be determined by measuring the dynamic surface tension.

In the bubble pressure method, new bubbles are cyclically generated in the solution, whose maximum pressure correlates with the SFT. The time from the start of bubble formation to the occurrence of maximum pressure is referred to as the surface age. By varying the bubble formation rate, this surface age can be specifically adjusted in order to record the temporal change in the SFT.

▶ Details on the measurement method can be found in our bubble pressure tensiometer glossary article.

How concentration determination works

▶ Our Solution Guide shows a clear diagram of the concentration determination process.

First, measurement curves are recorded over a wide range of surface ages using samples from a surfactant concentration series with the mobile bubble pressure tensiometer. The instrument now knows the concentration dependence of the SFT for this surfactant. It automatically determines the optimal surface age for subsequent individual measurements, at which the vertical distance between the measurement curves of the concentration series is greatest. The bath tests are then carried out using such individual measurements, in which the previously determined optimum surface age is specified.

The measured value is then compared with the stored curves, and the surfactant concentration is calculated and displayed together with the SFT.

Meaningful results

The mobile measuring instrument from KRÜSS for bath control has a range of options for outputting the measured values:

• Output of the result with a passed/failed evaluation based on previously defined limit values.
• Simplified measurement without stored concentration series. This option can be selected if the user does not need to know the exact concentration and SFT limit values are sufficient for well-functioning and under-dosed baths.
• Display of a trend line for the measurements performed so that re-dosing can be carried out in good time. For this purpose, several independent measurement series can be created for several baths.

The bubble pressure technique thus quickly and reliably provides all the information needed to prevent underdosing and ensure trouble-free operation of surfactant-containing baths.