Fundamentals of water systems

Kishor Datar

In any compound the number of protons is equal to the number of electrons; it would appear that all compounds are electrically neutral. But, that is not always the case.

One end of the water molecule is slightly negative and the other end is slightly positive. Water possesses some unique properties because of its molecular structure. The polarity of water allows it to line up with itself (positive end to negative end, a chemical property called hydrogen bonding) as well as with other polar molecules. For this reason, ice is lighter than water, which causes it to float over liquid water. Water is the only molecule that behaves this way.

Hydrogen bonding plays an important role in many water purification processes. All polar compounds will generally dissolve other polar compounds. This applies to water as well it being a polar compound. However, such polar compounds will not dissolve non-polar compounds like oil.

Let us take a look at this simple experiment: Get a thin stream of water flowing from a faucet. Build up a static charge on a plastic comb or plastic pipe by rubbing it briskly on a silk cloth. Hold it near the stream of water. The charged comb / pipe will actually deflect the stream of water. The above experiment clearly indicates that although it is a stable molecule, water has polarity and hence gets deflected due to static charge created on a charged pipe.

How does it affect the properties of water?

Water has high melting point, high boiling point, high surface tension, expansion on freezing and most important, its ability of dissolving many polar compounds. These abilities make water a ‘universal solvent’. That is why we find water of great use in all industrial applications. Water is used as, solvent in process industry besides universal cleaning agent in many industries.

Properties of water

Physical Properties: Water is unique in that it is the only natural substance that is found in all three physical states liquid, solid (ice), and gas (steam) at the temperatures normally found on earth. Earth’s water is constantly interacting, changing and in movement. Water freezes at 0° C (32° F) and boils at 100° C (212° F). In fact, water’s freezing and boiling points are the baselines with which temperature is measured: 0° on the celsius scale is water’s freezing point, and 100° C is water’s boiling point. Water is unusual in the solid form. Ice is less dense than the liquid form, which is why ice floats. Water has a high specific heat index. This means that it can absorb a lot of heat before it begins to get hot. This is why water is valuable to industries and in your car’s radiator as a coolant. The high specific heat index of water also helps to regulate the rate at which air changes temperature. This is why the temperature change between seasons is gradual rather than sudden, especially near the oceans.

Water has a very high surface tension. In other words, water is sticky and elastic, and tends to clump together in drops rather than spread out in a thin film. Surface tension is responsible for capillary action, which allows water (and its dissolved substances) to move through the roots of plants and through the tiny blood vessels in our bodies.

Water temperature: Water temperature is not only important to swimmers and fishermen, but also to industries and even fish and algae. A lot of water is used for cooling purposes in power plants that generate electricity. They need cool water to start with and they generally release warmer water back to the environment. The temperature of the released water can affect downstream habitats. Temperature also can affect the ability of water to hold oxygen as well as the ability of organisms to resist certain pollutants.

pH: pH is a measure of how acidic/ basic water is. The range goes from 0—14, with seven being neutral. pH of less than seven indicates acidity, whereas a pH of greater than seven indicates a base. pH is really a measure of the relative amount of free hydrogen and hydroxyl ions in the water. Water that has more free hydrogen ions is acidic, whereas water that has more free hydroxyl ions is basic. Pollution can change water’s pH, which in turn can harm animals and plants living in the water. For instance, water coming out of an abandoned coal mine can have a pH of 2, which is very acidic and would drastically affect any fish crazy enough to try to live in it. By using the logarithmic scale, this mine-drainage water would be 100,000 times more acidic than neutral water so stay out of abandoned mines.

Specific Conductance: Specific conductance is a measure of the ability of water to conduct electrical current. It is highly dependent on the amount of dissolved polar solids (such as salt) in the water. Pure water, such as distilled water, will have a very low specific conductance, and sea water will have a high specific conductance. Probably, in school you’ve done the experiment where you hook up a battery to a light bulb and run two wires from the battery into a beaker of water. When the wires are put into a beaker of distilled water, the light will not glow. But, the bulb does light up when the beaker contains salt water (saline). In the saline water, the salt has dissolved, releasing free electrons, and the water will conduct an electrical current.

Turbidity: Turbidity is a measure of the cloudiness of water. It is measured by passing a beam of light through water and seeing how much light is reflected off the particles in the water. Water cloudiness is caused by material, such as dirt and residue from leaves that is suspended (floating) in the water. Crystal-clear water, such as Lake Tahoe and Lake Manasarovar) has a very low turbidity.

Dissolved oxygen: Although water molecules contain an oxygen atom, this oxygen is not what is usable to aquatic organisms living in our natural waters. A small amount of oxygen, up to about 10 molecules of oxygen per million of water, is actually dissolved in water. This dissolved oxygen is breathed by fish and zooplankton and is needed by them to survive. Rapidly moving water, such as in a mountain stream or a large river dissolves a lot of oxygen, while stagnant water contains very little oxygen. The process where bacteria in water consume organic matter, such as that which comes from a sewage-treatment plant consumes oxygen. Thus, excess organic material in our lakes and rivers can cause an oxygen-deficient situation to occur. Aquatic life can have a hard time in stagnant water that has a lot of rotting, organic material in it, especially in summer, when dissolved-oxygen levels are at a seasonal low.

Hardness: The amount of dissolved calcium and magnesium in water determines its ‘hardness.’ Water hardness varies throughout the sources. If you live in an area where the water is ‘soft,’ then you may never have even heard of water hardness. But, if you live in Florida, New Mexico, Arizona, or Solapur, where the water is relatively hard, you may notice that it is difficult to get a lather up when washing your hands or clothes. Industries in your area might have to spend money to soften their water, as hard water can damage equipment. Hard water can even shorten the life of fabrics and clothes. Does this mean that students who live in areas with hard water keep up with the latest fashions since their clothes wear out faster?

Suspended sediment: Suspended sediment is the amount of soil moving along in a stream. It is highly dependent on the speed of the water flow, as fast-flowing water can pick up and suspend more soil than calm water. During storms, soil is washed from the stream banks into the stream. The amount that washes into a stream depends on the type of land in the river’s drainage basin and the vegetation surrounding the river. If land is disturbed along a stream and protective measures are not taken, then excess sediment can harm the water quality of a stream. You’ve probably seen those short, plastic fences that builders put up on the edges of the property they are developing. These silt fences are supposed to trap sediment during a rainstorm and keep it from washing into a stream, as excess sediment can harm the creeks, rivers, lakes, and reservoirs. Sediment coming into a reservoir is always a concern; once it enters it cannot get out—most of it will settle to the bottom. Reservoirs can ‘silt in’ if too much sediment enters them. The volume of the reservoir is reduced, resulting in less area for boating, fishing, and recreation, as well as reducing the power-generation capability of the power plant in the dam.

Aqueous Solution Geochemistry: Acid = substance containing hydrogen which gives free hydrogen (H+) when dissolved in water Base = substance containing the OH group that yields free (OH-) when dissolved in water. An acid solution is one containing an excess of free H+ , and a base is one containing excess of free OH-. A reaction between an acid and a base is usually called neutralisation.

HCl (acid) + NaOH (base) ==> H 2 O + NaCl which are dissociated into ions:

H + + Cl – + Na + + OH – ==> H 2 O + Na + + Cl – i.e. Na + and Cl – are unaffected.

pH = inverse log of the concentration (activity) of free H+ , or pH = -log [H+]

Water dissociates into H+ and OH- ; the dissociation constant is: Kwater = [H+] [OH-] =10 -14 So there has to be 10-7 moles each of H+ and OH- in a kilogram of neutral solution at standard temperature of 25°C. One mole is 6.023 x 1023 atoms (or molecules) and H2O has a molecular weight of 18 grams per mole.

One kilogram of water has about 1000/18 = 55.6 moles of water or about 3.35 x1025 atoms of oxygen and about twice that number (6.7 x 1025 atoms) of H+ (the amount of free H+ or free OH- is relatively small compared to the amount of undissociated H2O).

pH ranges at 25°C from 0 to 14; pH < 7 = acidic solution; pH > 7 = basic solution. If HCl or another acid is added then pH decreases; if NaOH or another base is added then pH increases.

pH increases as carbonic acid (a weak acid) dissociates: When carbon dioxide combines with water, such as what happens in the atmosphere when fossil fuels are burned, carbonic acid is formed: H2O + CO2 ==> H2CO3 . Free H+ are made available during successive dissociations:

H2CO 3 ==> H+ + HCO3- carbonic acid to bicarbonate, occurs at pH ~6.4

HCO3 ==> H+ + CO3– bicarbonate to carbonate, occurs at pH ~10.3

Remember, free H+ is available only when acidic, or when pH < ~7. The dissociation of bicarbonate to carbonate occurs when there is too much OH- in the system and H+ is “released” to balance out the base.

Dissolved Cations and Anions in water

Cations = electron donors, positively charged: Na+, K+ , Mg++ , Ca++ , Fe++ or Fe+++ , Mn++ , Al+++

Anions = electron acceptors, neg. charged: Cl- , F- , I- , Br- , SO4—, CO3—, HCO3- , NO3—, NO2-

Metals=act like cations mostly: Cu, Zn, Pb, Co, Ni, Cr, As, Se, Mo, etc.

Water Analyses – Need to have cation-anion balance

Millequivalent (MEQ) = mole equivalent charge or anion or cation, measure of total charge due to the ion in question dissolved in the solution. Start with concentration, divide by mole wt., multiply by charge: XX mg/L / MW x CHG = MEQ

Example: NaCl in solution, Na=50 mg/L (50 ppm): 50/23 x 1 = 2.17 MEQ

Cl = 77 mg/L (77 ppm): 77/35.5 x -1 = -2.17 MEQ

So, if the total cation and anion MEQ’s are not balanced, some error exists in the analysis. The ability of water to dissolve polar materials, also finds its utility in Pharmaceutical industry as,

1. Solvent (To prepare Solutions) – To be considered as raw material
2. Cleaning Agent

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