Pounds of Active Chemical in a Tank

A gauge on a chemical tank reads volume, but inventory, dosing and safety paperwork all want mass of active ingredient. Three multiplications get you there: volume, then specific gravity to account for the solution being heavier or lighter than water, then the percent active. In trade units, gallons × 8.345 lb/gal × SG × %/100. A 500 gallon tank of 50% caustic soda at SG 1.53 holds 500 × 8.345 × 1.53 × 0.50 = 3,192 lb of NaOH: nearly a ton and a half, in a tank that only reads "500 gallons."

Specific gravity is where the money and the mistakes are. Concentrated chemicals are dense: 50% caustic is 1.53, 93% sulphuric acid is 1.835, 12.5% sodium hypochlorite is about 1.16, and 40% ferric chloride is 1.42. Ignoring SG on the caustic tank understates your inventory by a third. Two field notes: 50% caustic freezes at 12 °C (54 °F) and will plug an outdoor line solid, which is why northern plants buy 25% and pay to ship the water; and SG drifts with temperature, so a hydrometer reading taken on a hot summer afternoon will underread concentration unless you correct it back to the reference temperature on the supplier's table.