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The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 20 (2015) pp 95-98

A Simple Method for Determining the Osmolarity of Aqueous SolutionsUsing Visibly-Colored Hydrogels: An Undergraduate Chemistry Experiment

Jeongha Kim, Kihoon Nam§, Simhyun Jung**, Jules J. Magda*,†, and In Suk Han

The College of Education, University of Utah, Salt Lake City, Utah 84112 USA; §Center for Controlled Chemical Delivery, Department of Pharmaceutics and Pharmaceutical Chemistry, University of Utah, Salt Lake City, Utah 84112 USA; **Department of Bioengineering, University of Utah, Salt Lake City, Utah 84112 USA; Department of Chemical Engineering, University of Utah, Salt Lake City, Utah 84112 USA,
Received February 4, 2015. Accepted April 20, 2015.

Published: 1 May 2015

Abstract. A laboratory experiment that investigates the use of visibly-colored hydrogels for determining the osmolarity of aqueous solutions is presented. Osmolarity, a thermodynamic property related to the number of solute molecules per unit volume, is a useful measure of water quality in desalination processes, as well as an indicator of the electrolyte-water balance in human physiology. Osmolarity is typically measured intermittently by removing a solution aliquot and performing freezing point depression measurements. However, when a polymer hydrogel material such as a contact lens is placed in an aqueous solution, it absorbs water without dissolving and reaches an equilibrium degree of swelling that depends on the osmolarity value of the surrounding solution. Hence measurements of the hydrogel degree of swelling can be used to determine the unknown osmolarity value of an aqueous solution. We present a method for synthesizing a highly visible cylindrical hydrogel, and show that it is possible to determine its degree of swelling solely by measuring its diameter. Therefore, visual measurements of the diameter of the hydrogel in solution using a simple magnifying glass could be used to monitor the osmolarity value of an aqueous solution. We also show that swelling rate of the hydrogel can be determined by fitting time-dependent values of its diameter to a simple equation. The proposed measuring system based on colored hydrogels is very simple to use, presents clear results, and allows one to determine the hydrogel swelling rate.

Key Words: Laboratories and Demonstrations; polymer chemistry, organic chemistry; hydrogels; thermodynamics

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