pH is the measure of the concentration of H+ ions in a solution while pOH is the measure of OH- ions in solution. Acids contain the H+ ion and bases contain the OH- ion in the traditional Arrhenius sense. The pH of acids is usually below 7 while the pH of bases is usually above 7. pH and pOH are measured on a scale that is between 1-14 with 7 being neutral meaning that concentration of H+ ion roughly equates to the OH- ion neutralizing each other. pH and pOH are related to H+ and OH- through the math treatment -log(Molarity of substance). This means that the number on the pH and pOH scale is representative of the power of how much of that ion is currently in solution. The larger the pH number the exponentially smaller the amount of hydrogen ions as it is symbolic of a negative exponent.
pH indictators are a tools used to discover the pH of a acid or base. Universal indicator is used to find the pH of an unknown acid or base and works with almost every reaction. This can be done as it is a mix of many indicators. Other indicators are also useful if the pH of a substance is known and they can be used to give off a distinctive color to verify the pH.
If one neutralization reaction occurs at a pH of 5 and another occurs at a pH of 9 different indicators should be used. This allows distinction between the two reactions and with the indicators known to give off color at the certain pH this allows the reaction to be verified that it occured at the proper pH.
Indicators are usually composed of weak acids and bases that interact with the H+ and OH- ions in a system to give off a distinctive color change when their conjugate acid or base forms. Indicators are usually composed of giant complex molecules that are for the most part organic. Indicators are limited however, this is observed as they give a rough estimate of pH, occur at certain pH’s (excluding universal indicator), and can only detect either acid or base. Such as the Alazarine yellow can only detect half the the acids used in the lab while the universal was able to detect all.