Online acid concentration is not one measurement problem. Titration, conductivity, density and refractive index respond to different properties, so the best method depends on what the process result must mean and how the sample composition changes.
In brief: choose the measurand and matrix first; then compare direct endpoint titration with any viable physical-property correlation.
Why acid concentration needs a method decision
A process may ask for acid strength, free acid, total acidity or a titratable endpoint. Those are not always the same result, especially when dissolved salts, metals, mixed acids or reaction products are present. The specification must state the analyte, reporting units and endpoint that the plant uses.
What automated titration measures
The Galvanic method sheets for hydrochloric, hydrofluoric and sulfuric acid describe potentiometric acid-base titration with sodium hydroxide. A pH electrode and temperature measurement follow the titration until the programmed endpoint is detected from the change in pH with titrant volume.
The same sheets also describe an optical colour-titration route using an indicator and an optical probe. Both routes determine a configured titration endpoint; they are not generic density or conductivity measurements.
Titration compared with indirect methods
Potentiometric or colour titration
Titration can distinguish the neutralisation demand represented by the configured endpoint and can be useful where the process matrix makes a simple physical-property correlation unreliable. It needs titrant, endpoint control, cleaning and verification.
Conductivity
Conductivity responds to the total ionic behaviour of the sample. It can be fast and simple in a stable single-component mixture, but temperature and other dissolved ions can change the relationship between conductivity and acid concentration.
Density
Density can correlate with concentration when the composition and temperature are controlled. Mixed acids, dissolved solids or reaction products may change density independently of the target acid.
Refractive index
Refractometry also relies on a composition-specific correlation. It can be effective for a stable matrix, but additional dissolved components and temperature compensation must be understood.
Questions that decide the method
- Is the result free acid, total acidity, a specific acid or a process endpoint?
- Is the sample a single acid in water or a changing multicomponent matrix?
- Which salts, metals or reaction products vary independently?
- What normal, minimum, maximum and upset range is credible?
- What laboratory method currently defines the accepted result?
- What cycle time, reagent duty and maintenance plan are acceptable?
Sample handling and materials
Acid samples require compatible tubing, valves, wetted parts, drainage and containment. Temperature, pressure and crystallisation or precipitation behaviour may determine whether dilution, cooling or other conditioning is required. Hydrofluoric acid demands particular materials and safety review.
The analyser method should be tested against representative normal and upset samples. A calibration made on clean standards alone may not demonstrate performance in the real process matrix.
Use the result in control
Define whether the measurement adjusts make-up, controls an endpoint, detects contamination or triggers investigation. The useful cycle time follows from that decision and the process residence time, not from a generic preference for the fastest instrument.
Specification checklist
- Target acid, chemical form and reporting units.
- Agreed laboratory endpoint or reference method.
- Normal and upset composition, including other acids and salts.
- Temperature, pressure, solids and sample availability.
- Required response, outputs and control action.
- Reagents, waste, calibration and maintenance ownership.
How PMA can help
PMA can review the analytical duty, sample matrix, method choice and installation requirements before an AccuSeries configuration is proposed.
Source basis
This guide uses the Galvanic Applied Sciences AccuSeries method sheets for hydrochloric, hydrofluoric and sulfuric acid, which describe potentiometric and optical colour titration routes. Conductivity, density and refractive-index comparisons are selection principles; suitability requires application testing.
