Hydrochloric Acid Standardized Solutions

Around the time when Emperor Charlemagne was crowned Emperor in Dark Age Europe (800 AD), hydrochloric acid was discovered by a Persian scientist, Jabir ibn Hayyan during the the Middle East’s Golden Age. Today HCl is produced on a massive scale by combining hydrogen with chlorine gas. Our mammalian stomach produces it as well for digestion. It is about 0.18 Normal/Molar. HCl is used for pickling steel, food processing, water treatment, pH regulation, PVC plastics, and many other purposes.

In the modern laboratory the hydrochloric acid normality solution is one of the most commonly used acids. It has a high pKa dissociation constant  – i.e. it is a strong acid, along with Sulfuric or Nitric Acids. The molarity is the same as the normality for HCl. The concentrate is typically 12 normal. The most popular normality strengths for accurate/dependable titrations are between 0.1N and 1N.

Several different indicators are used for such titrations. For most analyses the preferred indicator is 1% phenolphthalein in isopropanol. However, the plotted titration curve shows such a sharp rise in slope upon approaching the endpoint that the differences between indicators are very minimal. As for weaker acid/base titrations, an indicator such as methyl orange provides a poor color transition.

Also affecting the endpoint of weak acid/base titrations is the effect of carbonates dissolved in the solutions, whether titrant or sample. This is referred to as fading endpoint. This is discussed further in another article in Exaxol’s “101’s and How To’s”.   The typical base with which to titrate HCl normality solutions is a sodium hydroxide standardized solution. Potassium hydroxide can also be used. NaOH can further be tested to a primary standard , typically potassium hydrogen phthalate, or KHP.