Chlorine-Based Chemicals

Chlorine-Based Chemicals

Pool Sanitising Chemicals at Incredible Prices

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Chlorine is the most widely used chemical for pool sanitisation. It is inexpensive, easy to use, and many people like its clean, fresh smell. Chlorine breaks down into Hypochlorous acid and Hypochlorite ions which oxidise bacteria until they are neutralised and can no longer reproduce. For domestic use, chlorine is supplied in liquid form, or as granules or tablets, and is available in 'Stabilised' or 'Unstabilised' formulas.

To discuss your requirements, and for prices, please call us on 0208 941 6618 or email us

Stabilised & Unstabilised Chlorine Formulas

Stabilised Formula - If cyanuric acid is part of the formula then the chorine is said to be ‘stabilised’ - this means that it lasts longer because it is attacked less by UV light. Using some stabiliser is therefore valuable, but problems arise if it becomes too concentrated and a condition called 'Chlorine Lock' occurs. In this situation, the chlorine remains in a combined form so is not available to kill micro-organisms. To stop this, excess stabiliser must be diluted out, since it does not get used up like chlorine.

Unstabilised Formula ('Shock Chlorine')- Chlorine without cyanuric acid is ‘unstabilised’ and is referred to as 'Shock Chlorine'. When shock chlorine is added to a pool, it works rapidly killing bacteria and algae and is used up quickly.

The best solution when sanitising a pool with chlorine is to use a combination of ‘stabilised’ and ‘unstabilised’ chlorine. A low concentration of ‘stabilised’ chlorine of 1-2 ppm should be maintained and periodically, say once a week, the pool should be shock dosed with ‘unstabilised’ chlorine up to 5ppm. This will kill any microorganisms that are resistant to low levels of chlorine. Ideally this should be done at night so by the next day the chlorine level has fallen.

Types of Chlorine Products

Unstabilised Liquid Chlorine (Sodium Hypochlorite) - This is unstabilised, and is very effective at sanitising a pool. However, it has a short shelf life (less than 3 months), is dangerous to use, bleaches clothing, and comes in heavy 20 litre bottles.

Unstabilised Shock Granules (Calcium Hypochlorite) - This is unstabilised, acts quickly, and has a long shelf life. As some of the calcium does not fully dissolve it can cause some cloudiness unless it is mixed with water and added through the skimmer. The new quick-dissolving shock overcomes this, but with a slight increase in cost.

Stabilised Chlorine Granules (Sodium Dichloroisocyanurate)  - Long lasting and with a long shelf life, these granules should be added regularly to maintain a concentration of 1-2 ppm. More will be needed during hot weather, in warm water, when the pool is in high use, or when the pool cover is off for extended periods of time. Please note that with liner pools, chemicals should be dissolved first before adding to the water, as there is a risk of undissolved chlorine bleaching the liner. Never dissolve two chemicals together.

Stabilised Chlorine Tablets (Sodium Trichloroisocyanurate) - Long lasting and with a long shelf life, these tablets must not be put directly in the pool but held in a feeder where they slowly dissolve. Floating feeders are simple and inexpensive. Some are ready filled and disposable when empty, and others are refillable. Inline feeders in the pump house cost around £100 and provide a semi-automatic chlorination system. The amount of chlorine they release is easily adjusted to suit the needs of a particular pool and will deliver chlorine whenever the pump is running. Once set up it requires little attention other than the weekly shock dose of unstabilised chlorine. Some people put chlorine tablets in their skimmer basket. Although this works it will shorten the life of the basket as it makes the plastic brittle and could be dangerous if children put their hands in. Please note, you should never mix pool chemicals together in their undissolved, concentrated form as they can react together. Stabilised and unstabilised chlorine, for example, form an explosive mixture if mixed in, say, a bucket or skimmer basket.

Important Notes for Using Chemicals in Your Pool

Although it is common practice to put chlorine tablets in the skimmer, we strongly advise against it (apart from very occasionally when on holiday perhaps). The strong bleach solution will quickly perish and crack the basket and eventually weaken the skimmer body itself, and can also discolour the pool lining. The recommended way to disperse chlorine or bromine tablets is to put them in a proper feeder designed for the purpose - either a cheap floating refillable feeder or an erosion feeder permanently plumbed-in to the pool water return line with an O-ring sealed screw top lid. Please contact us for more information and prices.

Chemical Concentration

The concentration of chlorine required in domestic pools is lower than that required in public pools. You should always use the correct quantities for your pool size. Test the chlorine concentration of your pool using DPD No1 in a test kit - available on request. To work out the water capacity of your pool (in gallons), you can use the following formulas:

Imperial (gallons):

For square or rectangular pools - length (ft) x width (ft) x average depth (ft) x 6.25 = volume (gallons)
For circular pools - 3.1428 x radius2 (ft) x average depth (ft) x 6.25 = volume (gallons)

Metric (m3):

For square or rectangular pools - length (m) x width (m) x average depth (m) = volume (m3)
For circular pools - 3.1428 x radius2 (m) x average depth (m) = volume (m3)

Pool pH

All chlorine-treated pools should have a pH between 7.2 and 7.6. A low pH is acidic and corrosive, and will damage pool parts and sting bather's eyes. A high pH is alkaline and stops any chlorine present working as well as being scale forming.

Test the pH of your pool using Phenol Red in a test kit - available on request. If your pool pH is not within the required limits (either too high or too low), you can add a pH minus or a pH plus to bring levels within the required limits.

pH minus - A pH above 7.6 ('high') is lowered by adding pH minus (sodium bisulphate or pool acid)
pH plus - A pH below 7.2 ('low') is raised by adding pH plus (sodium carbonate or pool alkali)
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