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Determination of acidity in pickling and phosphatizing

Hi.

I have known this home page and I think is very useful.

I am a student of chemical engineering and currently I am working in a company where motorcycles are assembled. I develop my work in the quality area. One of the stages of the process is the adequacy of metal parts before painting. I am doing analysis of the chemicals used at this stage (zinc phosphate, phosphate sealant, pickling product) to verify the specifications supplied to me by the producer. I need to know if I can replace the volumetric determination of total acidity of these products, for a determination of pH, or whether it must necessarily make the two analysis, and where can I find a paper that I support this change?.

The reason is that currently the company carries out quality control of these products through titration with phenolphthalein ⇦this on eBay & Amazon [affil links] and NaOH, but I think it could be more appropriate to change the method for determination of pH, as this would save us time and consumption reagents, and I think it might be more accurate (although I am concerned that the water which were prepared reagents probably have dissolved CO2, which would affect the determination of this parameter). I would like to know your thoughts.

Thank you for your cooperation.

Juliana Segura, Chemical Engineering Student
apprentice Motorcycle Assembly Industry - Cali, Colombia
July 11, 2008

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July 25, 2008

Juliana,
What you propose to do makes sense in principle but may be really hard to do right, for the following reasons:

1. You would need to take very good care of your pH meter and electrode (calibration, temperature compensation...), since even an error of 0.01 pH units translates into a concentration error of 2.3%, more than in any reasonable titration.

2. Even if your pH reading is perfectly accurate, changes in your solution will probably affect the relationship between the pH reading and the acid concentration. For instance, any less-than-strong acid will act as a buffer and change the straightforward relationship (this includes the CO2 that you mentioned, and also hydrolyzable metal ions such as zinc...). Also, the changes in ionic strength as your solution ages will affect your pH reading because they will affect the pKa's of not-strong acids such as phosphoric and (the second pKa of) sulfuric acids.

Just as an example to the second point: The first pKa of phosphoric acid changes from 2.148 at an ionic strength (mu)of 0.00 to 1.90 at mu=0.1, to 1.82 at mu=0.5.

So no, I can't recommend replacing the titration with a single pH measurement, unless you can show that you can guarantee high measurement accuracy and that the variability of the data over the bath life is realy small.
If you just want to use this as a screening method for fresh solutions that's more practical, but then I would personally prefer to see an additional piece of data such as accurate density.

Emanuel Cooper
- Danbury, Connecticut, USA

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August 10, 2008

Sir,
The pH meter will not give the required results. One or more titrations are required. The choice of indicators is very important. You must use an indicator which changes color around 3 to 5 to get "free" acid. The indicator that you now use does NOT for the required free acid determination.
Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA