How to Measure Sulfuric Acid Concentration ?

The on-line continuous measurement of sulphuric acid/oleum is based on the fact that a physical property will change as the concentration changes.  Properties such as conductivity, refractive index and sound velocity are typically used to measure sulphuric acid/oleum concentrations.  The recommended methods of measurement over the whole range of sulphuric acid and oleum are given below.

Electrical conductivity is a property of a fluid that can be used to determine its concentration.  The graph below shows the conductivity (micromhos) at 25ºC for several different fluids, including sulphuric acid from 0 to 100%wt.  The vertical axis ranges from 0 to 900,000 micromhos.For sulphuric acid, the conductivity varies such that for a given conductivity there could be more than one concentration.  This makes conductivity unsuitable for measurement over the full range of acid concentration (i.e. 0 to 100% H2SO4).  However, conductivity can be used where the concentration will vary over a small range and where the range will not cross over one of the peaks or valleys of the conductivity curve.Below shows the correlation between conductivity and concentration for four common solutions

Conductivity measurement is a reliable indicator of the concentration of most acid or base solutions. The desired chemical concentration is achieved using a two stage mixing procedure. During the first stage, the flow ratio control unit on the missing tank is set to provide (x) gallon per minute of the full strength solution and (y) gallons per minute of water. These values are adjusted to produce a concentration value which is slightly weaker than the desired value. This ratio control must include alarm capabilities to indicate "low flow" conditions for both the full strength solution and the water in order to prevent wasted chemicals or hazardous situations. 

SJG-2083C Acid and Alkali Concentration Meter

At the second stage, a conductivity sensor and analyzer function as a "trim control". This combination adds small amounts of full-strength solution to the mixing tank to produce the exact concentration desired. For example, to produce a 4% caustic solution from a large bulk caustic supply at 50%, the flow ratio controller is adjusted to make a 3% solution and the conductivity information is used to add additional caustic to achieve the 4% concentration.Conductivity is a very reliable index of the concentration for most acid and base (caustic) solutions. Because the DDG-2080C Acid and base Concentration Meter works with DDG-GY  Inductive conductivity sensor ,it is virtually maintenance free; it is the first choice for any application.

               

Acid and Alkali Concentration Meter

               

Inductive Conductivity Sensor with Flow-cell

               

Inductive Conductivity Sensor for Pipe install