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topic 773

Want reliable pH measurement of plating baths


(1998)

I have to measure pH of plating solutions within a wide range of pH with accuracy (+ - 0.05 pH unit); the range of measure is from pH 0.9(passivations) to pH 8.0. .

I used a lot of pH meters and a lot of probes, but the results are always unsatisfactory: the probes worked well only for a short period of time or the pH measure did not give assurance of the measure.

I would like to know a supplier of pH meters with experience of pH measurement of plating solutions, or the best type of probes to be used.

Can anyone help me ?

Thanks in advance

Francesca Assone
electroplating shop - Torino, Italy


(1997)

Francesca,
Most people have problems with pH meters because they treat them like cars (put gas in it and go)

The best pH probe is a dual junction. Nearly anybody's. Next is a unit that you can change out the liquid frequently such as the one Orion makes. Hach in Colorado makes a unique unit where you click it a couple of times with every use and it replenishes or replaces some of the liquid in a long tube that comes down and J hooks around the bulb.

Average longevity for a good quality probe is about one year. If it is in nasty solutions, at may be a few months.

To get good reliable readings, calibrate with the proper standard for the pH you are measuring, such as a 1 for the low reading. Anything below a pH of 1 is going to be suspect. Repeatability will be low.
Take care of the probe. Store it in a pH 4 or 7 solution. DI water is not good for storage, tap water is better. Leave the probe in the solution only long enough to get a stable reading, the remove it and rinse it. One place that I worked, the floor always had bad readings for the nickel tank. Turned out that they were leaving the probe in the nickel solution for hours. Osmosis allowed nickel ions to back up thru the frit and contaminate the reference solution which gave erroneous mV and made the unit worthless.
Orion has an excellent reference book "handbook of electrode technology" that I would strongly recommend that you buy.

I hope that you are not trying to use hand held meters. They just do not cut it for .05 reliability.

James Watts
- Navarre, Florida


(1997)

Francesca, our preferred pH electrode is the Ross Sure-Flow model sold by Orion. It's a combination "sleeve-type" pH electrode that can be flushed by depressing the top, in the same way a person "clicks" a ball-point pen. We use them in our environmental, oil, and coatings laboratories for testing wastewater, industrial wastes, and oily sludges. They are long-lived, and we consider them worth the premium price.

Michael W. Woods
technical service lab


(1997)

Dear Franca,

I have had no luck at all with combination electrodes, although I have heard many people say that they work.

I would use a calomel or silver/silver chloride reference electrode like Fisher catalog no. 13-620-51 or 13-620-53, and use it with Fisher 13-620-284, glass body pH electrode. Hook them up to a pH meter good to 0.05 units (0.01 is better); as you know these are not cheap in price.

Keep in 4 or 7 buffer, leave in the solution only long enough to stabilize temperature (better to cool the sample to room temperature) and take reading. Uncover the filling hole only when taking a reading, and use good lab practice otherwise, which is a problem, unless the operator has been well trained, or has a good background in chemistry.

tom pullizzi monitor
Tom Pullizzi
Falls Township,
   Pennsylvania 


(1997)

Now I wonder what you would need 0.05 accuracy in pH measurement in plating solutions

I would titrate any plating solution, below a pH of 2.5, with NaOH, and control it that way.

I don't know of any solution in the 2.5 to 8.0 range that can't run within 0.1 or 0.2 pH units. Can you tell us what solutions you want to control by pH measurement?

regards,


Tom Pullizzi
Falls Township, Pennsylvania

(1997)

To Tom Pullizzi

The biggest problem is related to pH of black passivation of Zn-Fe The pH specification for this bath is 0.85 - 0.95

We saw variations of shade and brightness related to variations of 0.1 pH unit and we need to fix them.

Regards

Francesca Assone
electroplating shop - Torino, Italy


(1998)

hi,

if your pH is fluctuating, you are dragging in too much of some processing solution. You have to find how this is happening and solve that problem. Is the pH dropping or rising, or moving around?

I spoke to the Technical Director at Gumm Chemical. It is difficult to use pH to control the bath.

Whose chemicals are you using? What additives do you use to replenish the bath?


Tom Pullizzi
Falls Township, Pennsylvania

(1997)

To Tom Pullizzi

Dear Tom,

1) we have an increase of pH related to dragging (I think)

2) the passivation bath is supplied by Schloetter and the composition is proprietary. We perform the pH corrections with Schloetter additives.

Francesca Assone
electroplating shop - Torino, Italy



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