Borate buffers stabilise pH and reduce sanitiser demand. Here is what they do, and the honest case for and against using them in a hot tub.
Boric acid (the same compound used in eyewashes and as a mild antiseptic) is a powerful pH buffer. Dose it into a hot tub at 30 to 50 ppm and pH stops drifting between sessions, sanitiser demand drops by a noticeable margin, and the water feels softer. It is not standard kit in UK domestic tubs but is well worth knowing about.
They act as a secondary pH buffer alongside total alkalinity. Once dosed, pH becomes much harder to push out of band. Owners often find they go from weekly pH adjustments to monthly.
They have a mild algaestatic effect, useful for tubs run at lower temperatures or for swim spas.
The water feels softer to the touch. The reasons are debated but the effect is real and reproducible.
Pool-grade boric acid (not borax, which adds sodium and increases pH) is the right product. UK retailers sell it as a borate booster.
For a 1,200-litre tub aiming at 50 ppm borate, you need around 70g of boric acid. Dissolve in a bucket of warm tub water and add with the jets running.
Borates are stable. Once dosed, they last the life of the refill. Top up only when you refill or after a heavy partial drain.
Cost. Boric acid is more expensive per kilo than basic alkalinity products. The first dose for a typical hot tub is 5 to 10 pounds.
Not strictly necessary. A tub with good alkalinity holds pH well enough for most owners without borates.
Not tested by every test kit. Borate test strips exist but are uncommon. Most owners dose by calculation and trust the buffering effect rather than measuring directly.
On balance, worth trying once. If pH stability and softer feel matter to you, you will keep using them. If not, no harm in stopping at the next refill.
At 50 ppm, yes. The level is well below any concern for skin contact and below the level used in over-the-counter eyewash products.
No. Borate buffers pH without competing with chlorine or bromine. The reduced sanitiser demand comes from the more stable pH, not from any direct interaction.