Steps in an Acid-Base Titration

By Paul Dohrman

In chemistry, acid-base titration is used to determine the unknown concentration of an acid or base solution. Any given acid and base neutralize each other molecule-by-molecule in a known simple ratio, like 1-to-1 or 2-to-1. Therefore, you can add a known amount of acid (base) to the unknown amount of base (acid). The known amount of one that neutralizes the other tells you its unknown amount. You know that neutralization has been reached in a titration usually by using a color indicator.

Terminology

The acid or base solution of unknown concentration is called the "titer." The added solution is called the "titrant." The titrant is said to "titrate" the titer. When the titrant has completely neutralized the titer, the "equivalence point" has been reached and the water is neutral, with a pH of 7.0. A "buret" is a long, thin graduated cylinder with a stopcock at the bottom to drip the titrant into the titer in a thin stream at a controlled rate.

Setting Up the Equipment

The buret is filled with the titrant because a narrow cylinder spreads out the demarcations of volume. This increases accuracy in measuring the amount of titrant added to the titer. The stopcock allows you to add the titrant by drops as you near the equivalence point. The titer is held in an Erlenmeyer flask. Its triangular shape allows you to swirl it to fully react the acid and base. This is important so that you don't prematurely assume equilibrium has been reached.

Adding the Indicator

Titration indicators added to the titer change color when the titer passes a pH of around 7.0 (neutral). Litmus is a good indicator for an acid-base titration, because it changes color at a pH of around 6.5. This is close enough because the concentration of hydronium (H3O+) and hydroxide ions (OH-) is very small around pH 7.0. (These are the ions that determine the pH level.) Adding just a few drops of acid or base to neutral water throws these very small concentrations off by an order of magnitude or more. The wide variation of the pH near equivalence means with a few drops, the water will pass right through a pH of 6.5 within just one or two drops of its passing through a pH of 7.0. Just a few drops of indicator are added, to minimize how much interferes with the neutralization process.

Titration and Calculation of Unknown Concentration

The titrant is then added to the titer, getting slower and slower as the indicator changes color more and more. The flask is swirled constantly. When the equilibrium point is near, add the titrant one drop at a time, swirl the flask after every drop. When the indicator has fully changed color throughout the flask, even after swirling, you stop adding titrant. You then measure how much volume of titrant solution you did add, using the gradations on the buret. Then you use its known concentration to calculate the number of molecules of titrant added (measured in moles). From knowing the molecular ratio in which the acid and base combine (remember, the concentration wasn't known but the molecule was), then the number of molecules of the titer can be determined as well, and therefore its concentration in the solution from which the titration sample was taken.

About the Author

Paul Dohrman's academic background is in physics and economics. He has professional experience as an educator, mortgage consultant, and casualty actuary. His interests include development economics, technology-based charities, and angel investing.