I am trying to control a process with a very high NaOH content.  The pH is around 13 to 13.5, and I have learned that pH probes are prone to sodium error when the measured solution is above 12.  The electrode error for pH above 12 is in the order of 0.1 to 0.3 pH.  The error is always low.

My process is delivering NaOH (50%) to a tank of water.  To have an error of 0.1 at a pH of 13 results in a lot of extra NaOH.  Based on what I have read, it would seem that a direct pH measurement will not work.  What I need is to know is the NaOH concentration in my solution.  Does anyone have any experience in measuring this solution using conductivity?  And is it more accurate than a pH probe?

Measure density.  That would be simple and accurate
 

I've never measured density.  Can you explain how that would be done?  Any recommended density meters?

eeprom,

I don't have a suggestion like ash9144, but I have had customers measure conductivity in lieu of pH, with never a good result.  Too many other variables affect conductivity.

I'll be interested to see a good solution in this thread, I've been wondering a long time.

 

Good on ya,

Goober Dave

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Most mass flow meters (coriolis) measure density.

Good luck,
Latexman

I am looking into density and conductivity meters right now.  I think an inherent problem with density is that I am working with a 400 gallon tank which is 1 to 2% NaOH by weight.  And I'd like to be able to measure small fluctuations in NaOH.  So the density change of the entire fluid is going to be very small.

To add a little more information...There are only three inputs to the tank: raw water, blowdown water, and NaOH solution.  The blowdown water will have a pH of about 10 and will have some effect on conductivity.  But I believe that the NaOH will be the dominant influence on conductivity.   

I think that there are different sensor technologies around for higher pH measurements.  Mettler Toledo (for example) seems to offer probes that run to pH14 along with others that cap out at 12.

Take a look at the specs and see if they meet your needs.  

pH is inherently a high maintenance sensor and you need to be prepared for a maintenance schedule that matches the accuracy you want to achieve.  Is the NaOH going to cost more than the required maintenance?
  

KiwiMace,
I have read a few papers on sodium error, and it seems to be a problem in all pH electrodes.  I don't know about the Mettler Toledo electrode, but I will look into it.  One important note though, lots of manufacturers offer electrodes that go to 14. It's the ability to go to 14 with high concentrations of NaOH which is the problem.

Perhaps coriolis meters could measure density for liquid a bit better than for gas.

Refractive Index measures concentration in aqueous solutions.  K-Patents makes a continuous, on-line process refractometer.  http://www.kpatents.com/

 

Is there any other Na in the system?  Could you go with direct Na measurement?  Look at ion specific probes.

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Plymouth Tube

A high pH glass electrode should be used to allow pH measurement above 10 pH without sodium error.

Conductivity is best suited to measure solution concentration of caustic chemicals.

However, why bother with the pH probe?

You can do make this work with a simple mass balance, especially if the blowdown is boiler blowdown. Boiler blowdown should have very little alkalinity and will react somewhat similiar as demineralizied water will. The alkalinity content of the blowdown is actually more important than the pH.

You can analytically determine the amount of caustic necessary to raise the pH of the water. Then you can add the caustic in proportion to the raw water. The boiler blowdown that is added should have little effect on the pH.

oh my, you have to measure pH in this app, just get a good probe,

surprized that the process is only using a mix tank, more complex process, more commonly the problem is poor mixing and your controller setup at least once you've got the correct probe, forget density meas, or concentration, etc.

At 2% caustic concentration you are right in the best range for conductivity measurement.  Between 1 percent and 3 percent concentration, the conductivity will approximately triple from 50 to 150 ms/cm.

pH is best used for solutions less than 1 percent and density for solutions over 10 percent.

Inline caustic dilution systems use conductivity for this concentration range. See Lewa for example.