The pH of a solution is a measurement of its acidity or alkalinity, which is determined by the colorimetric method, which uses indicator solutions or papers, and the electrometric method, which uses electrodes and a pH meter, are the two ways for measuring pH.
What is pH?
pH is a logarithmic measure of an aqueous solution's hydrogen ion concentration. pH = -log(H+), where (H+) is the hydrogen ion concentration in moles per liter and a log is the base 10 logarithm.
"pH is defined as the negative logarithm of hydrogen ion concentration"
A pH of less than 07.00 indicates that an aqueous solution is acidic, whereas a pH of higher than 07.00 indicates that it is basic. A pH of 07.00 is considered neutral, with water being an example. The pH scale typically ranges from 0 to 14, while very strong acids can have a negative pH and very strong bases can have a pH greater than 14. Søren Peter Lauritz Sørensen, a Danish biologist, was the first to describe the word "pH" in 1909.
Importance of pH:
The chemical conditions of a solution are reflected by pH, which is an important quantity. The pH of a solution can control nutritional availability, microbial activity, biological functions, and chemical behavior. As a result, a wide range of applications relies on controlling or monitoring the pH of water, soil, and food or beverage products. pH plays a very important role in everyday life, it is important in our digestive system, in our blood system.
pH measurement is used in different applications such as agriculture, pharmaceutical, wastewater treatment, food and beverage, chemical industrial operations, research and development, cosmetic industry, environmental monitoring, and to verify the quality of soil, water, and pesticides. etc.
There are many activities in the science classroom (practical/experiment) that require pH testing. Titration in chemistry, environmental science water quality testing, and biological process laboratories are among them. To determine the pH they use liquid pH indicators, pH test strips, and pH meters, etc.
How pH is measured:
Two methods are used to determine pH: the colorimetric method and the electrometric method.
The electrometric method can provide accurate results, however, it requires the use of special apparatus. The colorimetric method is easy to perform, does not require expensive equipment, and is accurate enough for most applications. However, color, free chlorine, turbidity, high saline content, and other oxidants and reductants can all cause interference.
Determination of pH by the electrometric method:
The pH of the sample is measured electrometrically using a glass electrode or a combination electrode. To determine the pH by electrometric method a pH meter apparatus is used that consists of a potentiometer, a glass electrode, a reference electrode, and a temperature-compensating mechanism. A series of reference or standard buffer solutions of known pH such as pH 07.00, 04.00, and 09.20 are used to calibrate the measuring instrument.
Principle of electrometric method:
The principle behind the electrometric pH measurement is to determine the activity of a hydrogen ion using a standard hydrogen electrode and a reference electrode via potentiometric measurement.
Determination of pH by the colorimetric method:
Color is one of the easiest ways to determine the pH of a solution. If the pH of an aqueous solution falls within a certain range, many chemical compounds dissolved in it will change color.
Litmus or pH paper is a general laboratory application of this principle, which uses a color-changing chemical compound saturated on a paper strip to change color when immersed in a sample solution. The approximate pH value for the solution can be measured by comparing the final color of the paper to the reference chart.
Principle of colorimetric method:
The colorimetric technique is based on the ability of an acid-base indicator dye to change color depending on the pH of the sample. The color change can be detected spectrophotometrically as an absorbance change.
3 methods of pH determination:
The three most common methods for determining the pH of a substance are as follows.
pH indicators:
Liquid acid-base indicators are weak organic acids or bases which appear as different colors in their acid and base state. An indicator has a defined pH range in that it changes from an acid to a base form.
A universal indicator is a pH indicator composed of different compounds such as phenolphthalein, water, sodium hydroxide, bromothymol blue, thymol blue, 1-propanol, methyl red, and sodium bisulfate, etc. which produces a new color that is used to match the colors on the pH chart.
The most widely used pH testing tools are pH indicators such as phenolphthalein, which produces colorless to pink, bromothymol blue, which produces yellow to blue, and litmus which produces yellow to blue.
pH test papers:
Litmus paper is probably the most well-known and widely used pH paper in science classes, laboratories, etc. It comes in three types: red, blue, and neutral, and is used to determine if a sample solution is acidic or basic. In basic pH, red litmus turns blue, blue litmus turns red, and neutral litmus (often purple) turns blue in basic pH and red in acidic pH. It is made by mixing water-soluble dyes that are absorbed into the filter paper strips. It is impregnated with lichens, which gives it the ability to change color when exposed to acids or bases.
If you need to accurately measure the pH of a sample, you need a pH test paper or strip that is more accurate than litmus 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. It is mainly composed of paper, in which wood cellulose, lichen, and auxiliary compounds are the basic raw materials. It covers the entire pH range, is fast and simple to use, and is much less expensive than a pH meter.
Both papers are used by putting a drop of the sample solution on the paper. Alternatively, hold one end of the strip and dip the other into the sample solution, then remove it when the required time has passed.
pH meter:
A pH meter is a device used to measure the pH of a sample solution by measuring the electrical potential difference between the pH electrode and a reference electrode. The membrane of a pH meter allows H+ ions to pass through, allowing current to flow and providing voltage. The pH meter compares the voltage of the unknown solution to that of the buffers to determine the pH of the sample solution.
It is the most precise of the 3 test options, compared to other pH measuring techniques; pH meters often require calibration and maintenance. The electrodes are used as probes, dipped into the test solutions, and kept there long enough for the hydrogen ions in the test solution to equilibrate with the ions on the surface of the bulb on the glass electrode, allowing the pH of the solution to be measured.
Glass electrode: The sensor electrode is a unique glass bulb in contact with an internal reference electrode that contains a fixed concentration of hydrochloric acid (HCl) and a buffered chloride solution.
Reference electrode: A constant mV is generated by the reference electrode, which contains electrolyte or filling solution, and is transferred to the silver/silver chloride (Ag/AgCl) wire. The signal is then passed to the electrode's cable via the wire.
Frequently Asked Question (FAQ):
What are 3 ways to measure pH?
The three methods for measuring the pH of a substance are pH indicators, pH test papers, and a pH meter.
Why is the electrometric method of pH determination more accurate than the colorimetric method?
In general, the electrometric method is more accurate since it is closer to the definition compared colorimetric method. The electrometric method gives a single value which is depending on the hydrogen ions concentration of the sample, while the colorimetric method uses an acid-base indicator to provide a range of values for the particular sample, and is, therefore, less accurate compared to the electrometric method.
Difference between the colorimetric and electrometric method of pH determination
The major difference between the colorimetric and electrometric methods of pH determination is that to determine the pH of the sample the colorimetric method uses indicator solutions or papers that give a rough idea about acidity, while the electrometric method uses a pH meter which provides precise pH value.
- Mendham, J.; Denney, R. C.; Barnes, J. D.; Thomas, M. J. K. (2000), Vogel's Quantitative Chemical Analysis (6th ed.).
- https://www.mt.com/mt_ext_files/Editorial/Generic/1/Guides_to_Electrochemical_Analysis_0x000248ff00025c9a00093c4a_files/guideph.pdf
- Bates, Roger G. Determination of pH: theory and practice. Wiley, 1973.
- https://www.sciencedirect.com/topics/medicine-and-dentistry/ph-measurement
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