Understanding purified water
What is purified water and why do we need it?
Because water passes through minerals in its ongoing natural cycle, it contains ions (the atoms of dissolved
minerals). Laboratories need purified water – water that has had these ions removed – as an inert media for the
basis of any sample analysis, because to measure and test samples for gases, organics, minerals and bacteria there
must be no impurities already present in the water.
Purified water is water that has been mechanically filtered
or processed to remove impurities.
There are different grades of water purity. For techniques such as tissue and cell culturing or PCR a high grade of
water purity is required. For some uses in the lab – such as glass washing, or media and reagent make up – a lower
grade of water purity can be used.
To understand the different grades of purity we first need to take a closer look at the water supply itself
Hard and soft water
When it rains, water runs through the land and is subsequently evaporated into the air,
where it condenses and becomes rain again, completing the water cycle.
If you live or work in an area where the land has a high limestone content the rainwater
dissolves the limestone into calcium and magnesium ions, which flow into the water source
causing ‘hard’ water. This can stick to pipes and kettles, producing limescale which is hard
to remove. It also causes problems in water systems, which is why the minerals have to be
removed.
Where the land is low in limestone content the water tends to be high in organics (longchain carbon molecules which flow into the source water from naturally decaying vegetation
and fertiliser) and therefore contains low concentrations of ions, making the water ‘soft’.
High limestone content = hard water
Low limestone content = soft water
Conductivity and resistivity
Just as a teaspoon of sugar dissolved in a cup of tea makes it sweeter, minerals dissolved in water make it more
conductive. The dissolved minerals split into cations and anions. These ions carry charges that make the water
conductive, so the more ions in the water, the higher the conductivity. In contrast, water that has low mineral content
is less conductive and is said to have more resistivity
High mineral content = high conductivity
Low mineral content = high resistivity
Total Organic Carbon (TOC)
Water purification also requires the removal of organics. As mentioned above, organics are long-chain carbon
molecules which flow into the source water from naturally decaying vegetation and fertiliser, so if you live or work
in an area where the surrounding land is farmland or marsh land there may be high levels of organics in the source
water. Organics can also be manmade and include pesticides, herbicides and detergents. The lower the TOC content,
the purer the water. Total Organic Carbon is measured in ppb or ppm.
Below are pdf file for more model of Purite Water Purification System