Alkalinity is not hardness and the two are easy to confuse because both get quoted as CaCO₃. Hardness is what scales — calcium and magnesium. Alkalinity is the water's capacity to neutralise acid, its buffer, and it is carried by three species: bicarbonate, carbonate and hydroxide. A water can be high in one and low in the other, and the two ledgers are kept separately.

$\mathrm{TA} = 0.8202\,\mathrm{HCO_3} + 1.6679\,\mathrm{CO_3} + 2.9425\,\mathrm{OH}$ — read aloud T-A equals point eight two oh two H-C-O-three, plus one point six six eight C-O-three, plus two point nine four three O-H. $\mathrm{TA}$ is the total alkalinity in mg/L as CaCO₃. $\mathrm{HCO_3}$ is bicarbonate, $\mathrm{CO_3}$ carbonate and $\mathrm{OH}$ hydroxide, each in mg/L as its own ion — which is exactly why each needs its own factor. Solve forward for the total, or backwards for one species when the titration and the other two are known.

Those three factors are the same $50.04/\mathrm{EW}$ from the last lesson, already evaluated: bicarbonate 61.02 g/eq, carbonate 30.00, hydroxide 17.01. Hydroxide is the lightest by far, so each of its milligrams is worth nearly three and a half times what a milligram of bicarbonate is worth. In practice, below about pH 8.3 the alkalinity is essentially all bicarbonate; carbonate and hydroxide only appear after lime softening, caustic dosing, or in a boiler.

Then the pump. $\dot m = \Delta\mathrm{Alk}\;Q\;\dfrac{\mathrm{EW}}{50.04}\;\dfrac{100}{p}$ — m-dot equals delta-Alk, Q, E-W over fifty point oh four, times a hundred over p. $\dot m$ is the feed rate of acid PRODUCT — what comes out of the drum, in kg/h. $\Delta\mathrm{Alk}$ is the alkalinity you intend to remove, in mg/L as CaCO₃ — the drop, not what remains. $Q$ is the water flow in m³/h. $\mathrm{EW}$ is the acid's equivalent weight: sulphuric is 98.08/2 = 49.04, hydrochloric 36.46/1 = 36.46. And $p$ is the product's strength, percent acid by weight.

Read the shape and it is two honest corrections in a row. $\mathrm{EW}/50.04$ trades equivalents for equivalents — one equivalent of acid per equivalent of alkalinity, which is the only exchange rate chemistry offers. $100/p$ pays for the water in the drum, because commercial acid is deliberately dilute and you are pumping all of it.

Last, the operating nugget. Never trim alkalinity to zero. The buffer is what keeps pH from swinging on a whim, and a tower or a loop with no alkalinity left will chase pH all over the chart on the smallest upset — and corrode while it does. Leave 50 mg/L or so, and trim towards a target rather than towards nothing.