A Toilet rim block is a disinfectant block used in flush toilet

Flush toilet

A flush toilet is a toilet that disposes of human waste by using water to flush it through a drainpipe to another location. Flushing mechanisms are found more often on western toilets , but many squat toilets also are made for automated flushing...

s which slowly dissolves in water. They often come sold in a small holder that is attached over the rim of a toilet and hangs down into the bowl, so as the toilet gets flushed, the water passes through the holder coming into contact with the block, however, they also come loose for placement directly in-cistern (so are also usable with squat toilet

Squat toilet

A squat toilet is a toilet used by squatting, rather than sitting. There are several types of squat toilets, but they all consist essentially of a hole in the ground...

s), although these tend to be slightly different in composition, so as to dissolve slower, due to the constant contact with water. These may also contain a colouring, which shows up in the water (typically blue or green).

Composition and action

The composition of toilet blocks can vary, but they may contain, among other things borax

Borax

Borax, also known as sodium borate, sodium tetraborate, or disodium tetraborate, is an important boron compound, a mineral, and a salt of boric acid. It is usually a white powder consisting of soft colorless crystals that dissolve easily in water.Borax has a wide variety of uses...

, troclosene sodium

Sodium Dichloroisocyanurate

Sodium dichloroisocyanurate is a chemical compound used as a disinfectant, biocide, industrial deodorant and detergent. It is found in some newer water purification tablets/filters. It is more efficient than formerly used halazone water disinfectant...

 or sodium percarbonate

Sodium percarbonate

Sodium percarbonate is a chemical, an adduct of sodium carbonate and hydrogen peroxide , with formula Na2CO3 · 1.5H2O2. It is a colorless, crystalline, hygroscopic and water-soluble solid...

 (a form of oxygen bleach

Bleach

Bleach refers to a number of chemicals that remove color, whiten, or disinfect, often via oxidation. Common chemical bleaches include household chlorine bleach , lye, oxygen bleach , and bleaching powder...

), sodium carbonate

Sodium carbonate

Sodium carbonate , Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well-known for its everyday use as a water softener. It can be extracted from the...

 (washing soda) and a perfume.

Recently, there has been the introduction of 'Liquid rim' which has the block, as it were, in liquid form, with the liquid held in a small bottle above and connected to the holder slowly releasing the liquid into the bottom of the holder which is beneath the toilet rim, so coming into contact with the water when the toilet is flushed.

Besides disinfecting and giving fresh scent to the toilet, these blocks are also used to help prevent the build up of limescale

Limescale

Limescale is the hard, off-white, chalky deposit found in kettles, hot-water boilers and the inside of inadequately maintained hot-water central heating systems...

in the bowl.