A pH indicator is a chemical detector for hydrogen ions (H+) or hydronium ions (H3O+) in a sample solution. The pH (potential of hydrogen) value is used to express the amount of hydrogen in a solution. The negative logarithm of hydrogen ion concentration is related to the pH scale, which ranges from 0 to 14. Generally, the indicator causes the color of the solution to change depending on the pH.
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In chemistry, there are three types of indicators: natural indicators, artificial indicators, and olfactory indicators, they are used in different types of titrations like acid-base, redox, precipitation, isometric, etc. The two theories that explain the acid-base indicator are Ostwald's theory and the Quinonoid theory. Depending on the application they use litmus paper (red or blue), pH paper, pH indicator, phenolphthalein, methyl orange, and potentiometer and pH meter, etc.
What is pH paper?
A pH paper is a strip of paper that has been saturated with pH indicators and is used to determine the pH of a solution.Whether there is an acid or a base present, pH paper can change color depending on the pH of the sample solution. It is a paper treated with a natural water-soluble dye from lichens. pH paper is also called a universal indicator, at different concentrations of hydrogen ions in a solution show different colors.
In an acidic media, the pH indicator will display colors such as yellow, orange, red, and orangish-yellow. In the basic medium, it will turn purple or blue-violet, and in the neutral medium, it will turn green. However, there are various universal pH indicators in the market, most of which are modifications of the formula patented by Yamada in 1933.
What is pH paper made of?
The pH paper is made of paper; the basic raw materials used to compose pH paper are wood cellulose, lichens, and additional compounds.
The universal indicator is usually composed of water, phenolphthalein, methyl red, 1-propanol, thymol blue, bromothymol blue, sodium bisulfite and sodium hydroxide, etc. A pH paper is saturated with chemical compounds whose chromatophore is a different color at different pH concentrations.
How does pH paper work?
The pH paper is coated with a chemical indicator that changes color when hydrogen ions or hydroxide ions are present. A solution with a high concentration of hydrogen ions is acidic. A solution with a high concentration of hydroxide ions is basic or alkaline, and if the amount of hydrogen ions and hydroxide ions are equals that result in a neutral state.
pH indicators are weak acids or bases that change color at particular pH. Acids and bases are chemical compounds that exchange a proton. Acids donate a proton, whereas alkaline is substances is accept a proton. Acids are hydrogen donors and bases are hydrogen acceptors. Thus, the color changes when an acid or base accepts or donates a proton.
Types of pH paper:
Universal paper and litmus paper are the two most prevalent types of pH paper. Both types are widely used for acidity and basicity determination, which is depending on the type of measurement and degree of precision required.
Universal paper can indicate a range of pH values, while red and blue litmus paper kind of ready-made test filter paper that roughly indicate whether a sample is acidic or basic. When the pH is alkaline, the red paper turns blue, and when the pH is acidic, the blue paper turns red.
Nowadays, there are many different types of Whatman pH indicator paper available, each of which provides a different range of pH depending on the application and provides reliable and accurate results.
How to use a pH paper strip:
To determine the pH of any liquid, put a drop of the sample solution on the pH strip with a dropper. Or hold one end of the pH strip and dip the other end into the sample solution, then remove it after the required amount of time has elapsed.
When a color change occurs, it is measured against the color comparison chart (01-14). It is not necessary to immerse the entire strip in the test sample, as the dye could potentially contaminate the sample.
Applications of pH paper:
- The most common application of pH paper is to determine whether a solution is acidic, basic, or neutral.
- The pH paper is used for research, health care, educational institutions, pharmaceuticals, chemical laboratories, agricultural fields, environmental science, and water quality testing, etc. where it is necessary to determine the pH level of substances.
- It is also used in science classes (practicals/experiments) because it is rapid and easier to use than a pH meter.
FAQ (Frequently Asked Questions):
Is a pH paper strip reusable?
It is not recommended to reuse pH paper as the results may be inaccurate. It is not completely insoluble, and the paper tends to fall apart after a while, so it cannot be used indefinitely, although it will work a few more times.
What is pH paper coated with?
The pH paper is coated with a chemical indicator that changes color when hydrogen or hydroxide ions are present.
What is the pH value of a neutral solution give an example?
The pH value of any neutral solution is 7. Water and human blood are examples of neutral solutions.
What is the difference between litmus paper and universal pH paper?
The pH value of the sample solution can be determined using universal pH paper, whereas the acidity or alkalinity of the solution can be determined using litmus paper.
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