Determination of pH

Our Objective

Our objective is to determine the pH of the given sample using;

  • pH paper 
  • Universal indicator

The Theory

There are millions of chemical substances in the world, some are acids, others are bases and some are neutral.  

Acids and Bases

Acids are  substances that produce free hydrogen ions (H+ ions) when dissolved in water.   Bases are substances that produce hydroxyl ions (OH- ions) when dissolved in water. Acidic solutions are rich in hydrogen ions and basic solutions are poor in hydrogen ions.

Some acids dissociate only partly, releasing very small amounts of H+ ions, and are called weak acids. Others dissociate completely, releasing large amounts of H+ ions, and are called strong acids. In the same way, bases that dissociate partly are called weak bases and those that dissociate completely are called strong bases.

pH of a Solution

The acidic or basic property of substances is measured in terms of pH. It is a measurement of the hydrogen ion concentration. pH is defined as the negative logarithm (base 10) of hydrogen ion concentration.

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If the hydrogen ion concentration is very high, the pH value is very low. This is determined using a scale ranging from 0-14 called the pH scale. It was introduced by a Danish chemist Soren Peder Lauritz Sorensen.

Substances with pH lower than 7 are acidic, those with pH equal to 7 are neutral and those with pH greater than 7 are basic in nature.

The pH of pure water is 7. i.e.,

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pOH of a Solution

pOH is used to measure the concentration of hydroxyl ions (OH- ions) or the alkalinity of a solution. pOH is defined as the negative logarithm (base 10) of hydroxyl ion concentration.

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pOH is derived from pH and are related by the equation,

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Measurement of pH

pH of a solution is measured using pH indicators. pH indicators are substances that changes colour when in contact with acidic, basic or neutral solutions.

Let us discuss some pH measurement methods.

pH paper

pH paper is a strip of special paper that is prepared by dipping the strip in different chemical compounds and then drying it. It can be used to find the approximate pH of any solution. These paper changes colour when immersed in acidic or basic solutions. To determine the pH, the pH paper is dipped in a given sample solution and the colour developed in the paper is compared with the colour chart and the approximate pH of the solution can be identified. It is commerically available as test papers.

Universal Indicator

Universal indicator is a pH indicator composed of different chemical compounds.  When the indicator is poured in the sample solution, the mixture obtained exhibits a smooth colour change over a pH value ranging from 1-14 that indicates the acidic or basic property of the solution. It is commercially available as test solutions. 

pH Meter

pH meter is an electronic instrument consisting of a special bulb that is sensitive to hydrogen ions that are present in the test solution. The signal produced by the bulb is amplified and sent to an electronic meter connected to the bulb, which measures and display the pH reading. It gives more precise values than the pH papers.

For very precise measurement, the pH meter should be calibrated before each measurement. The calibration should be performed with at least two buffer solutions with known pH. For general purposes, buffer solutions with pH 4 and pH 10 are used. For more precise measurements, three buffer solution calibrations are preferred.

After each single measurement, the bulb is rinsed with distilled water or deionised water to remove any traces of solution being measured. Then the bulb is blotted with a blotting paper to remove remaining water that could dilute the sample and alter the reading. When not in use, the bulb must be kept wet at all times to avoid dehydration of the pH sensing membrane. 

Red Cabbage Juice

This is a blue-violet coloured liquid. When it comes in contact with acidic substances, it becomes red. When it comes in contact with basic substances, it becomes green or even yellow.

Applications of pH

  • It helps to determine the acidic and basic strength of acids and bases.
  • The pH of sea water plays an important role in understanding the oceans carbon cycle.

Learning Outcomes

  • Students understand the terms like pH, pOH, acidic, basic & neutral solutions and univeral indicator.
  • Students acquire the skill to measure the pH of the solution using pH paper or universal indicator solution.
  • Students acquired the skill to classify the samples as acidic, basic or neutral based on the pH value.
  • Students acquire the skill to measure the concentration of H+ and OH- ions present in a solution based on the pH value of the solution.