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The litmus test is a tool to measure the degree of acidity or alkalinity of solutions, that is, their concentration of hydrogen ions. To weigh this concentration of hydrogen ions, the measure called pH is used, which means “potential of hydrogen” or “potential of hydrogen ions”. Litmus paper is single-use and only works with substances in aqueous solutions. Therefore, it is impossible to measure the pH of an oil using litmus paper.
Litmus paper should not be confused with pH paper. Unlike this, the pH paper provides a more precise measurement in numerical values, instead of being limited to changing color according to the acidity or basicity of the measured solution.
Use of litmus paper and pH paper
These measuring papers are used in a wide variety of disciplines. This is because the maintenance and regulation of the pH of a solution is not only important for scientists and researchers. For example, it is impressive how much this tool is used in the manufacture of food. Products such as jams, alcoholic beverages, and even milk products are frequently tested by manufacturers to determine if their acidity or alkalinity levels are adequate or safe for the consumer.
Another field where litmus or pH paper is tested to measure acidity or basicity is in the proper maintenance of swimming pools. In swimming pool water there is a wide variety of chemical substances: users’ sweat, sunscreens and skin creams, chlorine for cleaning, insects and other even more unwanted substances. For this reason, they must be constantly maintained at a safe pH for any user, which requires a large availability of test papers.
How is litmus paper?
It is a paper treated with natural dyes extracted from lichen formations (symbiotic biological units formed by fungi and algae) found in nature. The dyes of these react changing color according to the acidity and alkalinity to which they are exposed. Litmus paper is available in red or blue. Red paper is used to measure substances suspected of being alkaline. Blue is used to measure substances suspected of being acidic. If the wrong paper is used for the type of substance to be measured, it will not change color.
And what happens with the pH paper?
pH paper is yellow in color and changes when exposed to solution. Since the pH level is usually measured on scales from 1 to 14, this paper often includes a scale with corresponding colors and numbers to compare when it changes. The scale is divided into fourteen color spaces that match a specific level of acidity or alkalinity. Space number one is used to indicate the most acidic point on the scale and fourteen the most alkaline. The paper will change color with the solution and the redder it becomes, the more acidic the measured substance would be, and the bluer the more alkaline. Solutions that do not change the color of the pH paper at all are called neutral.
How do these tools work?
The protons of the dyes that are on the paper undergo a process of protonation or deprotonation when exposed to the solutions to be examined. This changes the color of these dyes. There are various dyes that are used as indicators, such as phenolphthalein, methyl orange, bromophenol blue, methylene blue, and silver nitrate, among others.
How are they used?
These simple steps are the most common when using these measurement tools:
- Make sure the solution is aqueous.
- Immerse the pH paper in the solution.
- Hold the dive for ten to fifteen seconds. The solution should completely soak into the paper.
- Remove the paper strip from the substance after this time.
- Wait for the tape to change color and compare it to the scale (this is often included with pH test kits).
- If the sample is a gas, you must modify step number one and, previously, impregnate the paper with distilled water. By bringing it closer to the source of the gas, it will dissolve in the water that impregnates the paper tape.
- If you do not see any color change on the paper, it is a solution or a neutral value gas.
Why use pH paper or litmus paper?
We know that today there are more precise or faster ways to know the alkalinity or acidity of a solution, but sometimes the use of a tool remains through the years thanks to a simple reason: costs and efficiency. A pH test paper may not be as fast a method as using an electronic pH meter, for example, but in many of the settings that are used, a high level of accuracy is not necessary. Sometimes you don’t even need numerical precision. A common pH kit can be enough to do up to 100 pH tests and usually costs less than US$7. In contrast, despite the numerical precision, an electronic pH meter can cost US$56 or more, depending on whether or not it includes reagent solutions. Besides, It is a device that is only used in very specific circumstances in which it is cumbersome to soak the paper and wait for the reaction time. In addition, it requires calibration before starting to use it. Traditional pH tests have been around for decades and are so common that many high schools teach students how to use them. In addition, the fact that its manufacture is so established in the industry has lowered the costs to a great extent.
References
Pearson Education. (2005). Chemistry. Available at: https://books.google.co.ve/books?id=3V1Kr-FXwcsC&dq
Vasey, C. (2010). The importance of acid-base balance: A practical and comprehensive overview. Available at: https://books.google.co.ve/books?id=KHNa_LFP6iUC&dq
