Contributed by: Shaan D.

Date of creation: 9/30/11
Phenolphthalein can be used to indicate if a liquid is an acid or a base. There are other things it can be used in like dyes and invisible ink. It can be a powder or a liquid, but can only be used as a liquid(adding water to the powder makes it able to be used as an indicator) Phenolphthalein changes color from colorless to bright pink, which starts in a pH of 8 (colorless in acids and pink in bases). The process that the phenolphthalein undergoes during this color transition is called ionization. This is when an ion is added to or removed from a molecule it can change the molecule into an ion, depending on if the ions are positive or negatively charged. Phenolphthalein has positively charged hydrogen ions. The reason phenolphthalein changes color is because although the phenolphthalein molecules are colorless, when they are mixed with an alkaline solution the molecules convert into ions and phenolphthalein ions are bright pink. The higher on the pH scale the substance is, the darker the pink color will be because more ions will be removed. Therefore, when you expose phenolphthalein to acidic solutions, the ions are not removed and the phenolphthalein remains colorless.

Figure 1: Shows what phenolphthalein looks like in basic or acidic solutions.

Litmus paper is another acid-base indicator and it is more common than phenolphthalein. Using litmus paper is fairly easy. All you do is place the strip of paper in whatever substance you want to test and soon it will start changing color. Litmus paper turns red under a pH of 4.5 and blue and red over a pH of 8.2. So something that has a neutral pH, such as pure water, turns the litmus paper into a color in between red and blue.
Figure 2: Litmus paper turns blue in basic solutions.

To completely understand an acid-base indicator like phenolphthalein, you need to know what acids and bases are. They can be found in a lot of everyday things. For example, oranges are acidic and baking soda is basic. The strengths of acids can vary greatly. Some very strong acids are corrosive and some are so weak that you can digest them. It is similar with bases; strong bases are used in manufacturing or for breaking down substances such as animal tissue. Soap, a weaker base, breaks down a very tiny layer of skin and washes of the bacteria. To determine if something is an acid or a base or to see how strong or weak an acid or a base is, a pH scale is used. The lower the pH of a substance, the more acidic it is. It is easier to see what pH level litmus paper is showing than phenolphthalein because with litmus paper, you usually get something that tells you what each color indicates on the pH scale.

Ion: an electrically charged atom or group of atoms formed by the loss or gain of one or more electrons
Phenolphthalein: a white, crystalline compound used as an indicator in acid-base titration and as a laxative
Molecule: the smallest physical unit of an element or compound, consisting of one or more like atoms in an element and two or more different atoms in a compound.
Litmus paper: a strip of paper impregnated with litmus, used as a chemical indicator.


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