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-----pH measurement in the range of 0.5 and below
Q. Hi all., I am endeavoring to locate a vendor who will supply pH equipment which will read accurately and is tolerant to extended exposure to an acidic medium. I am interested in the pH range of 0.5 and below. I have tried several types of meter/probe without success. The main source of the acidity is HCL. Can anyone help? Regards Sean.
Sean O' Reilly- Republic of Ireland
1999
A. Measuring pH below around 2.0 is of limited usefulness in the finishing of metals, and I have never seen anyone use electrodes successfully in low pH swills.
If we needed to control acidity very closely in strongly acid solutions, I suspect that one would try a continuous automatic titration, and the bigger supply houses, such as Cole Parmer or Fisher should have one. Some of the electroless nickel supply houses in the U.S.A. also sell monitoring equipment.
Best regards,
Tom Pullizzi
Falls Township, Pennsylvania
1999
A. Dear Sean,
I echo Tom's advice. You can try measuring very low pH's by adding DI water in fixed amounts. (like 2 to 3 parts water to 1 part acid) But, you'll need to see if this is meaningful as pH is not a simple measurement as the ionic (metal salts) concentration goes up.
Regards,
Fred Mueller, CEF- Royersford, Pennsylvania
1999
A. Only lab grade equipment can make a meaningful measurement of pH below one. Lab grade equipment will not survive long on the production floor.
A pH probe will rapidly die because of the large amount of acid that will diffuse thru the frit or even a membrane. A double junction probe will last longer, but has to be cleaned and refilled regularly, as in frequently. Msot probe use a silver chloride or a mercury chloride reference cell and you can see what any chloride getting into that area will do to the accuracy.
Probes will last in a 30 sec dip, but they will not last in a 30 minute insertion into a HCl strip tank.
Dilution would have to be on the order of 10 parts water to one part of a 0.1 pH HCl to get a repeatable value.
A couple of years ago, they started making a "solid" probe. This has a membrane covering a chip to measure pH like some of the selective ion electrodes. I doubt if it will work, but it might be worth checking on.
James Watts- Navarre, Florida
1999
A. Hey Sean,
In a large zinc plating jobshop (I know of) they use a conductivity probe to control the strength of the acid dip before plating. The system is computerized (in real time) and will add acid automatically. So, maybe your going after the wrong measure?
Hope this helps,
Fred Mueller, CEF- Royersford, Pennsylvania
1999
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