Freezing Point Studies of Ethylene Glycol Solutions Freshman Chemistry Christopher L. White, Paul E. Reinbold, Daryl G. Cox

In the Past….. Freshman Chemistry students at SNU have been performing an analysis of an unknown mixture of ethylene glycol solutions using freezing point curves. The equipment used has changed twice since the introduction of this lab to the General Chemistry sequence three years ago. This work describes the use of Vernier Lab Pro devices along with laptop computers and the thermocouple probe (TCA-BTA) to acquire freezing point curves in a digital form.

Project Developments Initially, a digital thermistor probe was used to acquire the freezing point data in an isopropyl bath. This method caused some complications The Bath was not remaining in a slurry The thermister provided data inaccurate with the data obtained from the other methods used in the previous years.

Project Developments Instrumentation Change Bath Composition Change The thermocouple (TCA-BTA) was used. It provided data comparable with past data. Bath Composition Change The bath composition was changed to 90% ethanol and 10% isopropyl. This change corrected the problem of the bath freezing. By making the changes, the freezing point depressions were obtained, and the data acquired was accurate with the past data.

Cooling curves on display The Procedure To begin the analysis of the unknown mixtures of ethylene glycol, the freezing point curve of deionized water is first obtained and then the curve for the unknown. With the Vernier Lab Pro devices and software, both cooling curves are displayed in real time on the computer. Analysis of the two cooling curves then allows for the determination of the freezing point depression (∆T). Cooling curves on display

Acquiring the Data A mixture of ice and deionized water was used as the standard for the thermocouple. This provided a fairly consistent curve with which to analyze the unknown mixtures against.

Finding the %EG of the Unknown Since it is known that the ∆T is proportional to the molality (m=∆T/K), by knowing the molar mass of ethylene glycol to be 62.08 g/mol, simple calculations can be used to calculate the mass per kg of water. Using the equation below, students can calculate the %EG in their unknown. %(w/w) EG= mass EG x 100/ mass of solution

Data Analysis ΔT=8.13 m=8.13/1.86= 4.37m The Vernier software allows for easy analysis of the curves once they are obtained. Linear fits make it simple to calculate the ΔT. ΔT=8.13 m=8.13/1.86= 4.37m mass EG= (4.37m)*(62.08g/mol)= 271.29g/kg of water Total mass of solution= 1000 g H2O + 271.29g EG %EG={271.29/1271.29}*100= ~21.34% EG

Known Mass % of Solution Summer 2003 Data Known Mass % of Solution Reported Delta T value Molality Mass of EG g/kg water Calculated Mass % 5 1.35 0.73 40.05 4.13% 10 3.31 1.78 110.46 6.95% 15 4.35 2.34 145.16 12.68% 20 8.13 4.37 271.31 21.34% 25 10.19 5.48 340.05 25.38%

Known Mass % of Solution Past Student Data Known Mass % of Solution Average %EG Calculated Standard Deviation Range Number of samples tested 10 8.18 % 2.78 1.96 to 11.78 31 15 14.33% 1.56 10.46 to 16.68 21 20 21.13% 2.67 13.31 to 25.02 25 26.58% 0.76 25.39 to 27.38 8 30 32.15% 2.57 28.42 to 37.53 14

Conclusion The thermocouple worked well for the analysis of the EG/Water solutions. The new composition for the bath was sufficient. 30% solutions are not easily analyzed. It is suspected that the thermocouple does not provide linear results as the temperature change increases. Once the ∆T reached between 17 and 19oC, the results were not accurate.

Lab Revision Because of the developments of this project, the lab procedure for General Chemistry is being revised. The various changes mentioned are being incorporated. At the end of this semester General Chemistry students will be able to use the laptops and Vernier devices to perform this experiment.

References SNU General Chemistry Lab Procedure, “Freezing Point Depressions in Ethylene glycol-water mixtures.” Experiments in Physical Chemistry, Shoemaker. McGraw Hill. New York, 1981.

Acknowledgements Southern Nazarene University, Department of Chemistry For the opportunity to perform this research project Dr. Paul E. Reinbold For the help and advice given throughout the project. Science Equipment Fund For helping fund the purchase of the new laptops and Vernier devices.