What are the recommended methods for measuring pH of the preflux solution?

The pH of any process tank solution can be easily measured at the galvanizing plant by sampling and the use of a universal indicator solution, portable pH meter, or pH indicator paper. However, for preflux solutions, the recorded pH value using standard methods is only an approximate number and should be considered accurate within one unit on the pH scale. This is because the presence of iron or other salts can interfere with indicators, the pH probe itself, or the color formed on the paper strips, can lead to inaccurate results at pH values typical of preflux solutions. 

As a result, pH measurements by standard methods should only be utilized for monitoring and making adjustments once a sample has been sent to an accredited laboratory yielding an accurate pH value.  A laboratory test value should be acquired each time a new flux solution is generated. Once the true pH is provided by the laboratory, compared with the results taken at the galvanizing plant to determine the amount to adjust all pH readings for use in monitoring and adjusting the preflux solution.

For an explanation on adjusting pH readings, let us look at an example where a true pH value of 4.0 was obtained by the accredited laboratory, but a sampling of that same solution at the galvanizing facility resulted in a reading of 3.5 (i.e. an erroneous reference pH).  In this example, all measurements for this preflux solution performed by the galvanizer should, therefore, be adjusted by +0.5 to achieve true pH readings. Furthermore, if the galvanizer samples the same preflux solution at a later date and records a reference pH of 2.5, this means the true pH value is 3.0 and adjustments can be made accordingly based on this adjusted true pH value.

When sampling the bath for pH monitoring purposes, be sure to agitate the preflux to ensure samples are taken from a uniform solution and from several different bath locations.  The pH of the preflux solution will vary with temperature, so all measurements must be performed at the same temperature to ensure consistent results (preferably at room temperature).

For more detailed information, be on the lookout for a new AGA Troubleshooting Guideline on Measuring & Adjusting pH of the Preflux Solution which will be distributed via the TechNotes distribution list, and posted on the Troubleshooting Guidelines page within the members-only section of the AGA website. If you would like to subscribe to TechNotes, please send an email request to Alana Hochstein.