ByDr.Khalida Bano

Water and pH

WaterWater constitutes a principal end product

of oxidative metabolism of foods and most abundant substance of the body,making about 65% to 70% of body mass.

It solvate a wide range of organic molecules.

Water has a slightly ability to dissociate into hydroxide ions and protons.

Structure of water:Water is a dipole molecule with electrical

charge distributed asymmetrically about its molecule.

Hydrogen bond exists between water molecules and by providing great internal cohesive forces make water a liquid at room temperature and give it solvent properties.

Hydrogen bond is responsible for viscosity and surface tension properties of water.

Shape of water is v-shaped at an angle of 104.5’.

Polar in nature due to high electro negativity of oxygen relative to that of hydrogen.

Water is a compound that consists of two hydrogen atoms and one oxygen atom attached together by two sigma bonds and with two lone pairs of electrons around the oxygen. This attachment of the hydrogen nucleus to the central oxygen atom by electrons is called a covalent chemical bond. These components of a water molecule generally form a tetrahedral arrangement around the oxygen atom. 109.5° is the expected bond angle between each component; however, this is not the case due to the repulsive forces of the lone electron pairs..

. As a result, the electrons push the hydrogen atoms closer together, resulting in a bond angle between the hydrogen atoms of 104.5°. The geometry of water molecules is typically referred to as "bent" or "angular". Since this is a bent structure and not a linear structure the distribution of charges is not symmetrical. Therefore giving a polar trait to water molecules

Intermolecular StructureThe partial charges on the oxygen and the

hydrogen allow for water to participate in hydrogen bonding. The partial negative charge on the oxygen is attracted to the partial positive charge on the hydrogen of another water molecule.

The oxygen atom is partial negative because the oxygen nucleus draws away the electrons from the two hydrogen atoms.

The net charge of the hydrogen atoms becomes partial positive.

A water molecule exhibits an attractive force to other water molecules and has ability of ionization due to this hydrogen bonding characteristic.

Hydrogen bonding is one of the strongest electrostatic interactions. Hydrogen bonding in water is responsible for many of water's unique properties, such as high melting and boiling points, high heat of vaporization, and high surface tension.

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Chemical and Physical Properties of Water

Hydrogen Bonding High melting point, boiling point, and heat of

vaporization are unique qualities of water caused by the attractions between adjacent water molecules. Each hydrogen of a water molecule shares an electron pair of the central oxygen atom. Because oxygen is more electronegative, its nucleus attracts electrons more strongly than the hydrogen nucleus. The result gives each hydrogen a partial positive charge, and the oxygen a partial negative charge. This allows for hydrogen bonding the electrostatic attraction between the hydrogen of one water molecule to that of another

High Heat Capacity Specific heat is a measure of heat

required to raise the temperature of 1 gram of water 1°C. Water has a high heat capacity, meaning that it changes temperature slowly after gaining or losing energy.

The heat capacity of water is a property directly resulting from hydrogen bonding.

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When heat is absorbed, hydrogen bonds are broken and water molecules can move freely. When the temperature of water decreases, the hydrogen bonds are formed and release a considerable amount of energy.

High Melting Point Water has an unusually high melting point due

to the hydrogen bonding between water molecules.

High Boiling Point water molecules also have a higher boiling

point reason for this is the hydrogen bonding

between neighboring water molecules. Because hydrogen bonding is a relatively strong intermolecular force, high heat energy is

required to break up the force.

High Heat of Vaporization Heat of vaporization is the quantity of

heat required to transform 1 gram of liquid water into its gaseous form. Similarly to the high specific heat of water, the high heat of vaporization is also due the hydrogen bondings.

Cohesion of Water MoleculesHydrogen bonds are very fragile in the

liquid form of water. The property of cohesion is seen when hydrogen bonds, collectively, hold water molecules together.

Surface tension is related to cohesion, Water has higher surface tension .

Water is known to be amphoteric, which means it can act as a base or an acid.

Water as solventBetter solvent.many crystalline salts and

other ionic compounds readily dissolve in water.water dissolves sodium chloride because of the strong electrostatic attraction between water NaCl ions.

Water also dissolves some non ionic compounds such as sugars,simple alcohols,aldehydes and ketones.

pHpH is a measure of the acidity or basicity of

a solution. Pure water is said to be neutral, with a pH close to 7.0 at 25 °C . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline.

low pH indicates a high concentration of hydronium ions, while a high pH indicates a low concentration

Mathematical definition pH is defined as a negative decimal logarithm

of the hydrogen ion activity in a solution. pH+=-log(H+)

Significance of pHpH of pure water is 7 at 25’c.i.e neutral.pH falls below 7 means solution is acidic.pH rises above 7 means solution is alkaline.It is important that pH involves taking the

negative log of hydrogen ion(H+)concentration.therefore each degree decrease or increase in Ph value denotes 10 times more or less number of H+ respectively.

PH detectionAcidity or alkanity can be detected by

LITMUS PAPER.If it turns red,it means solution is acidic and if it turn blue,it means the solution is alkaline.

Ph scale

pH measurementA pH meter is an electronic instrument

used to measure the pH (acidity or alkalinity) of a liquid (though special probes are sometimes used to measure the pH of semi-solid substances). A typical pH meter consists of a special measuring probe (a glass electrode) connected to an electronic meter that measures and displays the pH reading.

Ph meter

pH indicatorA pH indicator is a halochromic chemical

compound that is added in small amounts to a solution so that the pH (acidity or basicity) of the solution can be determined visually. Hence a pH indicator is a chemical detector for hydronium ions (H3O+) or hydrogen ions (H+).e.g congo red,phenolphthalein,methyline blue.

The indicator causes the color of the solution to change depending on the pH. At 25° Celsius, considered the standard temperature, the pH value of a neutral solution is 7.0. Solutions with a pH value below 7.0 are considered acidic, whereas solutions with pH value above 7.0 are basic.

ApplicationpH indicators are frequently employed in

titrations in analytic chemistry and biology experiments to determine the extent of a chemical reaction.

Henderson–Hasselbalch’s equationHenderson–Hasselbalch equation

describes the derivation of pH as a measure of acidity (using pKa, the acid dissociation constant) in biological and chemical systems. The equation is also useful for estimating the pH of a buffer solution and finding the equilibrium pH in acid-base reactions.

SignificanceIf the pH of a buffer solution is known then

the ratio of salt/acid can be calculated.For research purposes the buffer solution of

known pH can be prepared by adjusting the ratio of salt/acid.