pH & Alkalinity Reducer - Sodium Bisulphate

Lower hot tub or pool pH safely with sodium bisulphate (dry acid). Brings high pH down into the 7.2-7.6 sanitiser-effective band without the fumes of liquid acid.

• Reduces pH by approximately 0.2 units per 16 g per 1000 L.
• Lowers total alkalinity at the same time, so dose carefully.
• Restores chlorine and bromine sanitising power, which collapses above pH 8.
• Prevents calcium scale forming on heaters, jets and waterline tiles.
• Eliminates skin and eye irritation caused by alkaline water.

Sizes and prices

• 1kg: £5.89
• 2.5kg: £7.95
• 5kg: £11.38
• 7kg: £14.12
• 10kg: £18.23
• 14kg: £23.72
• 20kg: £31.94
• 25kg: £38.80

When you need it

• A fresh test reads above pH 7.6.
• Sanitiser is being used quickly but free chlorine still drops fast.
• White scale is forming at the waterline or on jets.
• Bathers report itchy skin or stinging eyes after a soak.
• You have just topped up with hard tap water.

How to use

1. Test pH and total alkalinity with a fresh strip or liquid kit.
2. Calculate the dose using the table below; add no more than half the calculated amount in one go.
3. Switch the circulation pump on and remove the floating dispenser.
4. Pre-dissolve the dose in a clean bucket of cool tub water (about 5 L) and stir until clear.
5. Pour the dissolved solution slowly across the surface with the pump running.
6. Wait 30-60 minutes, then retest pH and alkalinity before adding any more.

Dosing guide

Add half the calculated dose, retest after 60 minutes, then dose the balance only if needed. Total alkalinity drops at roughly the same rate; adjust TA second if both are high.

• For 1,000 L (small spa), add 16 g to reach -0.2 pH.
• For 1,500 L (typical hot tub), add 24 g to reach -0.2 pH.
• For 2,000 L (large hot tub), add 32 g to reach -0.2 pH.
• For 5,000 L (swim spa), add 80 g to reach -0.2 pH.
• For 10,000 L (small pool), add 160 g to reach -0.2 pH.
• For 20,000 L (mid pool), add 320 g to reach -0.2 pH.

How it works

Sodium bisulphate (NaHSO4), also known as dry acid or sodium hydrogen sulphate. Supplied as fine, fast-dissolving granules with at least 93% active content.

Sodium bisulphate releases hydrogen ions on dissolution, which neutralise carbonate and bicarbonate alkalinity. The reduction shifts the carbonate equilibrium and lowers measured pH. Because both pH and TA fall together, you must rebalance one then the other.

At pH 8.0 less than 25% of free chlorine exists as the active hypochlorous acid; at pH 7.2 it is more than 65%. Holding pH 7.2-7.6 makes every gram of sanitiser work two-to-three times harder, slows scale formation and protects bather comfort.

Frequently asked questions

What pH should a hot tub be?

7.2-7.6 for hot tubs using chlorine or bromine. Stay below 7.8 to keep sanitiser effective.

Does pH minus also lower alkalinity?

Yes. Sodium bisulphate lowers both pH and total alkalinity at roughly the same rate.

How long until I can bathe after dosing?

Wait 60 minutes with the pump running, then retest. If pH is in band you can bathe.

Why dry acid instead of liquid muriatic acid?

Granules are safer to store and transport, do not give off fumes, and are easier to dose precisely in domestic settings.

How much will 5 kg cover?

Roughly 80-100 typical doses for a 1500 L hot tub or 12 months of monthly maintenance for a 30 m3 pool.

Can it be used in salt-water pools?

Yes. Salt chlorinators run pH high, so you will use pH minus more often than with a tablet system.

Why does chlorine work better at lower pH?

Chlorine exists as hypochlorous acid (HOCl) below pH 7.5 and as hypochlorite ion (OCl-) above. HOCl is roughly 80 times more effective at killing bacteria.

Does sodium bisulphate raise sulphates?

Yes, each 100 g per 1000 L adds about 80 ppm sulphate. Refresh part of the water if sulphates exceed 300 ppm.

Why should I dose half then retest?

Heated water reacts faster than expected. Half-dosing prevents pH crashing below 7.0 and avoids etching.

What if I overdose?

Add an equal weight of pH plus (sodium carbonate) to neutralise; retest after 60 minutes.

Why does my pH keep climbing back?

Either total alkalinity is above 150 ppm or carbonates are dosed in via tap water. Lower TA first to fix the drift.

Why is foam appearing after I lowered pH?

Acidified water can release dissolved organics into foam. Add Foam Away at 25 ml per 1000 L and clean the filter.

pH minus vs pH plus?

pH minus (sodium bisulphate) lowers pH and TA. pH plus (sodium carbonate) raises pH with little TA effect.

pH minus vs muriatic acid?

Same chemistry, different form. Muriatic acid is faster but more hazardous to handle. Sodium bisulphate is the domestic-safe equivalent.