Concentration Sensing for H2O Sodium Acetate Solution


Thread Starter

Sheldon Roberts

I'm looking for a possible real time method for measuring the concentration level of a water/sodium acetate solution. One sensor supplier mentioned using near IR spectrometry but that doesn't really lend itself to a realtime sensing. My solution is heated to 190F and any extraneous dust particle, etc may cause the solution to start to crystalize again, thus the need for in tank sensing. If there is any other method that people have used, ie, ph, conductiivty, etc that has seemed to work for them, please let me know.

Sheldon Roberts

Vince Dooley

Electrical conductivity is clean, easy and reliable to use and will probably work.
However a check on the sensitivity at the operating concentration and temperature
needs to be done because most solutions have a maxima in the conductivity curve. I
probably have data at work. I will check today.

What is the concentration of sodium acetate in your solution?

Vince Dooley.

vince dooley

OK... Sheldon. I checked my files today. I have data at 25degC only. The maximum conductivity at 25degC is 78mS/cm at a concentration of about 22% by weight. At your temperature conductivites are going to be much higher than they are at 25degC. The maximum at your temperature will be at a higher concentration but not necessarily much higher. I assume that if you are concerned about precipitation you are operating at high concentrations. So conductivity will do your job
provided you are not operating too close to the maximum conductivity for the measurement to be insensitive. You can be on the other side of the maximum and still have sensitivity..... just not too close to it.

At the sort of conductivities that you are probably at it would be best to use an electrodeless system. Rosemount (Emerson), Foxboro (Who are they now???) and Yokogawa make good systems...... but the possibility of being too close to the maximum needs to be sorted out first. There are two things you need to know.
First is the sensitivity of conductivity to a concentration change in your solution and second is the temperature coefficient of conductivity at operating conditions. One of the suppliers may have data or may be prepared to help you do a test.... one sample at typical plant concentration... heat in a beaker on a hot plate to around plant temperature.... readings at three different temperatures... dilute... three
more readings at different temperatures.... dilute... ditto temperatures.... done.

Alternatively, there is a US company, OLI who have a modelling software package for physiochemical properties of electrolytes. The properties include conductivity. Sodium acetate may be in their data base. I'd be surprised if it
isn't. Their business is selling the package but it would be worth asking if they can run the package for your application for a small fee.

Vince Dooley
Concentration can be determined from the density of the solution. But you can't just measure the density if your temperature changes. What you do
is measure the density and temperature. Then use an algorithm to find the density at a reference temperature. Now you use an equation to model the
relationship between density at 15degC and concentration (you may find a simpler relationship will be suitable if your range is limited. Now you need to do this in real time and in a tank. One solution is a digital density sensor which uses resonance to determine density to an accuracy of 1kg/m3 or better and which will do the calculations for you. To do the calculations you may need to do some in-situ calibration if you can't source any published data on the temperature density relationship for different concentrations.
One supplier is Solartron ( "": and follow the links to the 7828 )
but try a search for density transmitters to find a number of such supplier. Many will be inline devices so you may have to sift a few to find a tank measurement system. You should find some others like "": and then take a little time to enquire of the companies for the accuracy you can expect. This will depend on the degree to which density varies with concentration and not just how accurate the sensor is at measuring density.

Willem de Jong

The relationship between sodium acetate and the sound velocity is a fairly straight line, even at very high concentrations and temperatures. Our website "": gives you some information about the capabilities of ultrasonic in-line analyzers. If you wish, I can send you some application notes and brochures.::

Best regards,
Willem de Jong
[email protected]