Chemical Strong Acid / Alkali Density Meter Selection Guide

Steps

1. Concentrated H2SO4 (98%)

   Recommended: PS7400 with PTFE coating and a titanium fork, or PS7110 whose sapphire prism resists concentrated sulphuric acid. The PS7110's advantage is that no metal contacts the acid at all, which avoids any catalytic interaction. An important caution on range: the sulphuric-acid property curves are not monotonic across the full 0–100 % span, so a meter that works in the 92–98 % band is not automatically valid if the process also runs dilute. State the actual working band at enquiry. Note too that with strong sulphuric acid the harder engineering problem is usually temperature compensation over a wide range rather than raw resolution.

2. NaOH (32% / 48%)

   Recommended: PS7400 with PTFE coating on a 316L body, or PS7100 with the optical head separated from the transmitter so the electronics sit outside the hazardous or hot zone. A material caution that is regularly missed: hot concentrated caustic above roughly 50 % and 100 °C will stress-corrosion crack standard 316L. For that duty use the optical route or a qualified alloy; do not assume a stainless fork will survive because it survived the 32 % line.

3. Hydrochloric acid (HCl)

   Hydrochloric acid attacks almost all standard stainless steels, including 316L, and chloride stress-corrosion cracking can occur well below the concentrations people expect. Recommended: PS7110 (sapphire prism, PTFE wetted body) or PS7400 with a fully PTFE-coated fork. Confirm the coating covers every wetted surface including the process seal — a pinhole in a coating fails faster than bare metal would, because attack concentrates at the defect. For vapour-phase exposure above the liquid, check the housing material as well, not only the wetted parts.

4. Mixed acids (H2SO4 + HNO3 / HF + HNO3)

   Mixed acids are where single-parameter methods most often fail, and the reason is worth understanding: two components that both raise density or both raise refractive index cannot be separated by one scalar reading. If both components vary independently and both must be known, no density or refractive instrument will deliver it — you need a method with chemical specificity, which in practice means spectral measurement, or an additional independent input such as a titration or a material balance. Being clear about this before purchase avoids a long and unwinnable commissioning argument. If one component is held constant by the process, the problem reduces to a binary system and the simpler methods become viable again.

5. Hydrofluoric acid (HF)

   HF attacks glass, quartz and sapphire, which removes the optical route that solves most other aggressive-acid problems. It also attacks most metals. Recommended: PS7400 with a fully PTFE-coated fork, and confirm the coating specification and the seal material explicitly. Treat any published compatibility figure with caution and verify against the actual concentration and temperature. HF is also a severe personnel hazard, so the installation review should cover leak containment and maintenance access, not only measurement performance — the right answer here sometimes is a sampling loop that keeps the instrument away from the main line.

6. Chlor-alkali sodium hypochlorite (multi-component)

   The classic multi-component case: available chlorine, free caustic and salt all vary, and they influence density in overlapping ways, so a density reading alone cannot report available chlorine. Recommended: PS7100 spectral measurement, which responds to the chemistry rather than the bulk property, or a combination of density with an independent input. Hypochlorite also decomposes, releasing gas, so bubble tolerance is a real selection criterion here and not a formality. Confirm the working temperature, since decomposition accelerates with it and the measurement problem changes as it does.