Determination of acid ionization constants for weak acids by osmometry and the instrumental analysis self-evaluation feedback approach to student preparation of solutions.

Abstract

One focus of this work was to develop of an alternative method to conductivity for determining the acid ionization constants. Computer-controlled osmometry is one of the emerging analytical tools in industrial research and clinical laboratories. It is slowly finding its way into chemistry laboratories. The instrument's microprocessor control ensures shortened data collection time, repeatability, accuracy, and automatic calibration. The equilibrium constants of acetic acid, chloroacetic acid, bromoacetic acid, cyanoacetic acid, and iodoacetic acid have been measured using osmometry and their values compared with the existing literature values obtained, usually, from conductometric measurements. Ionization constant determined by osmometry for the moderately strong weak acids were in reasonably good agreement with literature values. The results showed that two factors, the ionic strength and the osmotic coefficient, exert opposite effects in solutions of such weak acids.

Another focus of the work was analytical chemistry students solution preparation skills. The prevailing teacher-structured experiments leave little room for students' ingenuity in quantitative volumetric analysis. The purpose of this part of the study was to improve students' skills in making solutions using instrument feedback in a constructivist-learning model. After making some solutions by weighing and dissolving solutes or by serial dilution, students used the spectrophotometer and the osmometer to compare their solutions with standard solutions. Students perceived the instrument feedback as a nonthreatening approach to monitoring the development of their skill levels and liked to clarify their understanding through interacting with an instructor-observer. An assessment of the instrument feedback and the constructivist model indicated that students would assume responsibility for their own learning if given the opportunity.

This study involved 167 students enrolled in Quantitative Chemical Analysis from fall 1998 through spring 2001. Surveys eliciting students' reactions to the instrument feedback approach showed an overwhelmingly positive response. The results of this research demonstrated that self-evaluation with instrumental feedback was a useful tool in helping students apply the knowledge they have acquired in lectures to the practice of chemistry. A demographic survey to determine whether part-time or full-time jobs had a negative impact on their experiment grades showed a small but significant correlation between hours worked and grade earned. However, the study showed that grades students earned on this experiment were predictive of overall semester lab grades.