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Sodium (Na) and water

Sodium and water: reaction mechanisms, environmental impact and health effects

For billions of years sodium is washed out from rocks and soils, ending up in oceans, where it may remain for about 50.106 years. Seawater contains approximately 11,000 ppm sodium. Rivers contain only about 9 ppm.
Drinking water usually contains about 50 mg/L sodium. This value is clearly higher for mineral water. In soluble form sodium always occurs as Na+ ions.

In what way and in what form does sodium react with water?

Elementary sodium reacts strongly with water, according to the following reaction mechanism:

2Na(s) + 2H2O → 2NaOH(aq) + H2(g)

A colourless solution is formed, consisting of strongly alkalic sodium hydroxide (caustic soda) and hydrogen gas. This is an exothermic reaction. Sodium metal is heated and may ignite and burn with a characteristic orange flame. Hydrogen gas released during the burning process reacts strongly with oxygen in the air.
A number of sodium compounds do not react as strongly with water, but are strongly water soluble.

Solubility of sodium and sodium compounds

A number of examples of water solubility of sodium are available. De most familiar sodium compounds is sodium chloride (NaCl), otherwise known as kitchen salt. At 20oC solubility is 359 g/L, in other words adequately water soluble. Solubility is nearly temperature independent. Sodium carbonate (Na2CO3) is also adequately water soluble. Solubility is 220 g/L at 20oC.

Why is sodium present in water?

Sodium compounds naturally end up in water. As was mentioned earlier, sodium stems from rocks and soils. Not only seas, but also rivers and lakes contain significant amounts of sodium. Concentrations however are much lower, depending on geological conditions and wastewater contamination.
Sodium compounds serve many different industrial purposes, and may also end up in water from industries. They are applied in metallurgy, and as a cooling agent in nuclear reactors. Sodium nitrate is often applied as a synthetic fertilizer.
About 60% of sodium is utilized in chemical industries, where it is converted to chlorine gas, sodium hydroxide or sodium carbonate, and about 20% of sodium is utilized in food industries as a preservative or a flavouring agent. The remainder is applied for example as brine in winter.
Sodium hydroxide may be applied to prevent clogging of sewer pipes, and sodium carbonate is applied in water purification to neutralize acids. Sodium hydrogen carbonate is a constituent of backing powder, and is applied in textile and leather industries and in soap and cleanser production. In sanitary cleanser the element is present as sodium hypo chlorite.
Using sodium metal, beryllium, thorium, titanium and zirconium can be extracted. In lamppost lights a small portion of sodium is often present in the neon lights, causing them to use fewer electricity.
The radioactive isotope 24Na is applied in medical research.

What are the environmental effects of sodium in water?

Sodium is attributed water hazard class 2, in other words it is a risk when present in water. Sodium chloride however is not a risk and is attributed water hazard class 1.
Sodium is a dietary mineral for animals. Plants however hardly contain any sodium. The LC50 value for gold fish is 157 mg/L. Sodium hypo chlorite from sanitary cleansers may contribute to chlorinated hydrocarbon formation, and may therefore heavily charge wastewater.
Only one sodium isotope occurs naturally, namely the stable 23Na. There are 13 instable sodium isotopes, which are mildly radioactive.

What are the health effects of sodium in water?

Sodium is present in the human body in amounts of about 100 g. It is a dietary mineral, partially responsible for nerve functions. Blood serum contains 3.3 g/L sodium. It regulates extra cellular fluids, acid-base balance and membrane potential, partially together with potassium.
One may overdose on sodium from kitchen salt. This causes increased blood pressure, arteriosclerosis, oedema, hyperosmolarity, confusion and increased risk of infection from excessive Na+ intake. Sodium shortages may lead to dehydration, convulsion, muscle paralysis, decreased growth and general numbness.
Generally, humans require about 300 mg sodium chloride per day to warrant a balanced sodium level. People that have diarrhoea or other health effects that increase salt requirements need a higher dietary amount of sodium than usual. Adult intake of kitchen salt is on average 9 g per day, which translates to approximately 4 g of sodium. People with heart and kidney disease are recommended a sodium poor diet.
Kitchen salt solution was applied as vomiting provoker in the old days. Caustic soda can deeply affect tissues.

Which water purification technologies can be applied to remove sodium from water?

To remove sodium chloride from water, one may apply reverse osmosis, electro dialysis, distillation techniques or ion exchange. Reverse osmosis is most economical considering energy and money requirements.
Sodium is applied in water purification. It may function as a counter ion of calcium and magnesium in water softeners. Caustic soda and sodium per carbonate are applied to neutralize acids. Sodium bisulphite (NaHSO3) is applied as a reductor for strongly oxidizing chemicals, sodium sulphide (Na2S) for precipitation of complexed metals.

Literature and the other elements and their interaction with water

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