Why not one drop

With a 32 fl oz batch and 0.05 mL drops, one drop per 10 ppm would need 379 g/L of potassium bicarbonate, more than dissolves (about 300 g/L), or 318 g/L of sodium bicarbonate, over three times its solubility. Magnesium sulfate would sit near 70% of saturation. At five drops the closest bottle, potassium bicarbonate, is at about a quarter of its limit.

Sodium instead

NaHCO₃ can replace the potassium salt at 63.5 g/L. That works, but it is about two-thirds of its solubility at 20 °C (about 93 g/L), and closer still in a cold kitchen. The potassium salt sits near a quarter of its limit, which is why it is the default here. Per 10 ppm of KH, the swap adds 4.6 mg/L Na⁺ instead of 7.8 mg/L K⁺.

Gypsum

CaSO₄·2H₂O is not usable as a drop stock. It would need about 65 g/L against a solubility near 2.4 g/L at 20 °C. Calcium goes in as the chloride, and sulfate comes from the magnesium bottle.

Keep them apart

Never combine concentrates. Calcium with bicarbonate at these strengths precipitates CaCO₃ and gives off CO₂. The calcium chloride and magnesium sulfate stocks mixed 1:1 would hold about 26 g/L of calcium sulfate, far more than stays dissolved (about 2 g/L of CaSO₄ in pure water, a few g/L in a salty solution), so gypsum crystallises out.

What drifts

A bicarbonate stock slowly loses CO₂ as some bicarbonate turns to carbonate. That raises its pH but conserves alkalinity, so a dose still adds the KH you expect. Water escaping through a loose cap concentrates any stock, so keep caps tight. The larger error is in the solid salts: CaCl₂·2H₂O is deliquescent and takes up water from the air, so an old jar weighs out less calcium than its formula says, while MgSO₄·7H₂O effloresces in dry air and weighs out more magnesium. Either can be a few percent off, however good the balance.

Verify

GH by EDTA titration with Eriochrome Black T at pH 10, which counts calcium and magnesium together. KH by titration with standard acid to about pH 4.5, with a bromocresol green–methyl red indicator. Titrate a test batch dosed from one bottle at a time: that checks the stock and your drop count together.

Solubilities are approximate values at about 20 °C, in grams of the salt as named (hydrate included) per litre of solution, used only to flag headroom. Check a reference before running a stock past about half of one. Ion amounts are computed from formula masses: 10 ppm as CaCO₃ is 0.0999 mmol/L of Ca²⁺ or Mg²⁺, or 0.200 mmol/L of HCO₃⁻ (0.200 meq/L either way).

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