Detecting HCl concentration


Thread Starter

Mike Boudreaux

Dear list,

Does anyone have any experience successfully detecting wt% HCl in a dilute
(0-10%) aqueous solution? The density of HCl solution does not change very
much in this concentration range - any suggestions for other methods?

Mike Boudreaux
Akzo Nobel Chemicals, Inc.

If the water you are making the HCl solution in has a fixed pH and fairly
constant chemistry or if you know the pH, you should then be able to relate
the pH of your HCl solution to wt%. pH can be easily measured using
inexpensive instrumentation. Be careful as changes in the water chemistry
can play havoc with the realtionship between pH and %wt HCl



R A Peterson

The concentration of HCl can be fairly accurately measured using conductivity
meters calibrated in percent HCl. Your favorite peddler of conductivity
instruments can no doubt sell you exactly what you need.

Luc Haverhals


I believe you can measure the conductivity of the HCl solution to have an idea of the concentration.
I think the conductivity changes approximately linear for low HCL concentrations.

Luc Haverhals

Reditech BVBA.
Tel : 0032 3 7113293
Fax : 0032 3 7113294
[email protected]

Steve Monnet


I'm sorry Mike but I dont think that the measurement of Ph is a good
solution for this range of concentration.

the pH is calculated (for acid) through the following formula :


H+ equal the molar concentration of the HCl

The molar concentration is calculated Mhcl= concentration[g/l] / molecular
weight of HCl

Based on this simple calculation you will found that the following result
(in case of the concentration is 0 I assume that the pH will be 7 -pure


pH(0)=7; pH(10)=0.56; pH(20)=0.26; pH(30)=0.08; pH(40)= -0,03;
pH(50)= -0.13; pH(60)= -0.21; pH(70)= -0.28; pH(80)= -0.34; pH(90)= -0.39;
pH(100)= -0,43

The conclusion is that the pH is a good possibility to analyse on-line the
concentration of HCl but in the range of 0-0.1 Mole/liter (0 - 0.365 %P).

I thing the solution is a simple chemical analysis but unfortunately this is
very difficult to implement on-line.

In my view, the first step is to look for the existence of ion selective sensors. Companies like Rosemont, Orion have (probably) been approached for that before.
In the time I was familiar with Orion, they had such a variety of ion selective probe.
At least, they would tell you about the chemstry of your project based on the constituants.
According to the "Handbook of Chemistry and Physics" (The Chemical Rubber Company, 51st Edition) the density change between 0% and 10% is actually very significant if you consider a vibrating tube or fork density meter (i can't answer for other techniques such as Nucleonic or ultrasonic but you should investigate these as well).
It quotes the relative density (20degC/20degC) as 1.0000 at 0.00% and 1.0488 at 10%. Since even a halfway decent density transducer will measure to +/- 0.001gm/cc or better accuracy (repeatability +-0.0001gm/cc typical so a good in-situ cal can get better accuracy), this gives resolution to 0.2%concentration (someone check my math please) or better.
But this is not all the solution.
Density varies with temperature so the device you chose should have good algorythms to relate density at line temperature to the density at 20degC and further algorythms to convert between the density at 20degC and concentration (%wt or %vol) which is not linear though it depends on what accuracy you need as to whether you treat it as linear.
The last problem is of materials compatability. If Nucleonic or Ultrasonic offer the accuracy you need then these are non-invasive and could be your choice. Most manufacturers of vibrational density measurement systems offer a choice of materials including 316 Stainless steel, Hastelloy (various grades inc. C22, B2), Monel 400 or Titanium and some will offer coated devices.
You could try also some coriolis mass flow manufacturers as they also have a range of "exotic" materials and offer density as an option.
These are representative of the density and massflow manufacturers but not all, you should be able to discover a range of other manufacturers of vibrational density meters, coriolis meters, ultrasonic density meters and nucleonic meters with a simple web search.
I have a PS. I have assumed a process measurement. The same arguments and resolutions apply to a lab device except you will probably have control of the temperature. Paar have a choice of lab instruments (with glass tubes) which are suitable.