How Is Boric Acid Assayed?

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Purpose and Preparation

  • The purpose of performing an assay is to determine the percentage of purity of a given amount of the boric acid. To perform the assay, the equipment you will need includes a burette (approximately 50ml), a 10ml volumetric pipette, 100ml volumetric flask, 250ml conical flask, a weighing bottle and a scale. Chemicals required include boric acid, KHP, 0.1M sodium hydroxide, glycerin and phenolphthalein.

Performing Assay

  • To begin the experiment, weigh out a specified amount of boric acid and add equal parts of water and glycerol while dissolving into solution. Next, add a few drops of phenophthalein, which acts as an indicator for the titration. Once added, begin titrating with sodium hydroxide until the solution takes on and maintains a pink color. For the results to be more accurate, repeat the titration three times, which provides more confidence in the data. To compare the boric acid to a standard, complete a titration on a solution containing just glycerol and water. This can be used to calculate the percent purity of the boric acid solution.

Theory of the Assay

  • Boric acid is a weak acid that must be combined with a polyhydroxy compound, in this case glycerin, for a titration assay to be performed. Once combined, it can be titrated against a strong base like sodium hydroxide, which causes the indicator to change colors. This is because the glycerin esterfies the boric acid to produce a complex that behaves like an acid, which in turn allows the titration to be carried out.

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