By The Zagosa Team. Published .
Reviewed or updated .
Boric acid, the same compound found in eyewashes and used as a mild antiseptic, is a strong pH buffer. Dose it into a hot tub at 30 to 50 ppm and pH stops wandering between sessions, sanitiser demand drops noticeably and the water feels softer. It is not standard kit in UK domestic tubs, but it is well worth knowing about.
What borates actually do
They act as a second pH buffer alongside total alkalinity. Once dosed, pH becomes much harder to knock out of band. Owners often shift from weekly pH tweaks to monthly ones.
They have a mild algaestatic effect, handy for tubs run cooler or for swim spas.
The water feels softer to the touch. The reasons are argued over, but the effect is real and repeatable.
How to dose borates
Pool-grade boric acid is the right product, not borax, which adds sodium and pushes pH up. 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 it in a bucket of warm tub water and add with the jets running.
Borates are stable, so once dosed they last the whole refill. Top up only when you refill or after a heavy partial drain.
The argument against
Cost. Boric acid costs more per kilo than basic alkalinity products, and the first dose for a typical tub runs 5 to 10 pounds.
Not essential. A tub with good alkalinity holds pH well enough for most owners without borates.
Not on every test kit. Borate test strips exist but are uncommon, so most owners dose by calculation and trust the buffering rather than measuring.
On balance, worth a try. If steadier pH and a softer feel matter to you, you will keep using them. If not, no harm in stopping at the next refill.
FAQ
Is it safe to bathe in borates?
At 50 ppm, yes. That is well below any concern for skin contact and below the level in over-the-counter eyewash products.
Do borates interfere with sanitiser?
No. Borate buffers pH without competing with chlorine or bromine. The lower sanitiser demand comes from steadier pH, not from any direct reaction.