Pure water is neutral, and so is paraffin. Indicators are substances that change colour when they are added to acidic or alkaline solutions. Litmus, phenolphthalein, and methyl orange are all indicators that are commonly used in the laboratory. Litmus indicator solution turns red in acidic solutions, blue in alkaline solutions, and purple in neutral solutions.
Litmus paper comes as red litmus paper and blue litmus paper. The table shows the colour changes it can make:. The human eye perceives a pink hue in the solution. The stronger the alkaline solution is, the more the phenolphthalein indicator changes and the darker the pink hue will be. The pH scale runs from 0 to 14, with a pH of 7 being neutral. A substance below pH 7 is considered acidic; above pH 7 is considered basic.
Phenolphthalein is naturally colorless but turns pink in alkaline solutions. The compound remains colorless throughout the range of acidic pH levels but begins to turn pink at a pH level of 8. In , the German chemist Adolf von Baeyer discovered phenolphthalein by fusing phenol and phthalic anhydride in the presence of sulfuric acid or zinc chloride, the manufacture process still used today. In the chemistry laboratory, phenolphthalein is mostly used in acid-base titrations. A solution of known concentration is carefully added into one of unknown concentration.
The phenolphthalein indicator is added into the unknown concentration. When the solution turns from colorless to pink or vice versa , the titration or neutralization point has been reached, and the unknown concentration may be calculated. In the past, phenolphthalein has been used as a laxative. It was a constituent of Ex-Lax for the over-the-counter relief of constipation.
However, it was banned from use in the United States in after research showed it as a possible carcinogen cancer-causing agent. See the graphic for more indicators, colors, and pH ranges. Magic Pitcher Demonstration:. Phenolphthalein is an indicator of acids colorless and bases pink.
Sodium hydroxide is a base, and it was in the pitcher at the beginning, so when added to the phenolphthalein in beakers 2 and 4, it turned pink top half of the graphic. The equilibrium shifts right, HIn decreases, and In - increases.
As the pH increase between 8. The third beaker has only the NaOH but no phenolphthalein, so it remained colorless. The first beaker contain acetic acid and is skipped over at first. Bottom half of the graphic: When the pitcher is then poured back into beakers 2, 3, 4 it is a pink solution. In the first beaker, a strange thing happens in that the pink solution coming out of the pitcher now changes to colorless.
This happens because the first beaker contains some vinegar or acetic acid which neutralizes the NaOH, and changes the solution from basic to acidic. Under acidic conditions, the phenolphthalein indicator is colorless. Use equilibrium principles to explain the color change for phenolphthalein at the end of the demonstration. As OH - ions are added, they are consumed by the excess of acid already in the beaker as expressed in the above equation.
The hydroxide ions keep decreasing and the hydrogen ions increase, pH decreases. See lower equation: The indicator equilibrium shifts left, In - ions decrease. Below pH 8. Click for larger image. Color changes in molecules can be caused by changes in electron confinement. More confinement makes the light absorbed more blue, and less makes it more red.
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