19 January 2017

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EFFECTIVENESS OF RED CABBAGE AS ACID – BASE INDICATOR BY MICHELLE D. MIRANDA

Litmus paper is the most recognized member of chemical indicator. According to Nelson (2009), acid-base indicators provide a great platform for a variety of home chemistry experiments that anyone can do. Many colored molecules react with acids and bases in ways that change their colors. Chemists use this color change to indicate how acidic or basic a solution is. Litmus paper is one such indicator, which is made of wood cellulose, the main component of a tree's cell walls. Of all the different products used in chemical testing, litmus paper is the most well-known. As with all forms of pH testing paper, litmus will change colors depending on what it comes in contact with. It is a chemically sensitive paper that when exposed to an acid, it either changes into a pink or red color. If a strip of litmus paper touches a base, then the paper changes to a shade of blue. The simple pH scale ranges from 0-14 with 0 being the most acidic, 7 being neutral, and 14 being the most basic or alkaline.

Moreover, Litmus paper is commonly used in educational science classes particularly in

laboratory experiments or activities to check whether a certain substance is acidic, basic or neutral. It is a way to characterize the relative acidic or basic nature of a substance based on its hydrogen ion concentration. Also, once students understand pH, they can be introduced to the idea that many familiar and innocent-looking substances may in fact be hazardous or toxic substances. They can learn to beware of the possible effects of these substances on health and the environment.

In addition, litmus paper allows an observer the opportunity to assess a sample's pH. As

stated by Delisle et. al., (2001), pH is an important biological indicator because most life forms have a very small range of pH in which they can survive. For example, the acid-base ratio in the human body is a delicate balance. Even a slight change in the bloods pH in either direction can result in death. A change in blood or urine acidity can be critical in determining and diagnosing the presence of disease (Watkins, 2006). Plants are also susceptible to minute pH changes in the soil. That is why soil that is too acidic for a plant is neutralized with calcium carbonate fertilizer, a base.

Litmus is a dye made from species of lichens (fungi), most commonly from Rocella

Tinctoria, lichen that found in South America. Paper dyed with this substance is called litmus paper, and it is used to test the pH of a substance. In addition, many Lichens contain highly colored pigments and have been used for dying fabric throughout the ages. Mauve, purple, beetroot red, cyan, fawn, lilac, yellow, cream and various shades of brown are readily achievable, given the right mordant. However, lichens are getting scarcer and many are now protected from being harvested for dyestuff. In the study of Delisle et. al., (2001), they found out that since litmus paper is made from extracted lichens, some varieties of it are becoming extinct. But the use of litmus

19 January 2017

Publications

paper will most certainly continue to be used extensively in education especially in science class. However, there are a large number of other indicators available. A very common class of indicators is the group of pigments called anthocyanin (from the Greek roots for flower and blue). These molecules reflect red light in acid solutions, and blue light in basic solutions. Many of the colored fruits, leaves, and flowers we encounter owe their color to anthocyanin, and to the levels of acid in the plant. Examples are the red of apple skins and cherries, the red or purple of red cabbage leaves and eggplant skins, the blue of blueberries, the red of red wines, the purple of purple corn, the blues and reds of pansies and violets, and many more.

Anthocyanin has the ability to widely change its color depending on the concentration of

positively charge H+ ions in the water, which make it pretty sensitive natural indicator of pH. Shakhashiri (2009) stated that red cabbage is useful as a pH indicator because the leaves contain a pigment molecule from the family of anthocyanin which are responsible for many of the red/purple/blue colors observed in fruits and vegetables. In neutral solutions, the color observed is purple. As the solution becomes more acidic a red color results. Increasing the pH in basic solutions changes the color from blue to green and at high pH the solution is observed as greenish-yellow.

Students can used this red cabbage as acid – base indicator in the absence of litmus paper

or if it is not available in the school. With the proper guidance of teacher, students can perform a laboratory experiments regarding to the acidity and basicity of the said substance and serve as an alternative way than a common used of litmus paper.

References: Delisle et. al.,"Litmus Paper. How Products Are Made.” 2001. Retrieved September 30, 2014 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-2897000062.html 2001 Nelson, Acid-Base Reactions, NCSU – Dept. of Chemistry – Lecture Demonstrations. 2009 Shakhashiri, In Chemical Demonstrations: A Handbook for Teachers of Chemistry; The University of Wisconsin Press: 1989; Vol. 3, p 50-57; 65-69. 2009 Watkins, “What Is the Function of Litmus Paper?: Retrieved from: http://www.ehow.com/facts_5072700_function-litmus-paper.html. 2006