Chemlab: Chemistry 3/5


Natural Salt Solutions 1: Ion Exchange

Overview

Getting Started

Techniques

Procedure

FAQ

Full Lab Manual

Introduction & Goals

Chemistry & Background

Key Questions

Prelab Problems

Safety

Procedure

In Your Write-up

Experiments Index

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Before Lab
Before coming to lab, write your prelab, including the objective, reference, procedure, and sample calculations or analysis flowchart. This is the first week that your write-up requires calculations using measured data, so for the first time your prelab should include sample calculations or an analysis flowchart or outline. This should illustrate how you will calculate or estimate the capacity of your column, in milliequivalents, from the result of two different experiments. There are five prelab problems for you to complete before coming to lab.

In Lab
Make careful observations of your experiments in your lab notebook. The observations made at each step in the procedure should be interpreted in terms of the chemistry and properties of ion exchange resins. Use chemical equations like those in the introduction to describe your observations.

Uncertainty Analysis
Uncertainty analysis is used to evaluate the precision of measured and calculated results. The first step in an uncertainty analysis happens in the lab, each time you make a measurement. Each time you record a number, you should estimate its uncertainty, either explicitly as a value or by recording the correct number of significant figures. For this week's experiment, estimate the uncertainty in the height of the blue band of copper ions and the total column height, when you measure them. Use this to calculate the relative uncertainty of the height of the blue band and of the height of the total column. A complete uncertainty analysis would use error propagation to calculate the uncertainty in your calculated column capacity, but we will save that for another experiment. This week you will determine the absolute uncertainty values and, from this, calculate the relative uncertainty of some of your data. Definitions of absolute and relative uncertainty are given in the section at the start of this manual called Computing Uncertainty in Laboratory Data and Results. Your data sheet will include the calcuclation of the absolute and relative uncertainties of the heights of the blue band and the entire column.

In addition to considering the uncertainty in the column and blue band height, record the absolute uncertainty of your mass measurements and calculate the relative uncertainty for one of your KHP samples.

After Lab
The Results section should contain two calculations of the column capacity, from two of your experiments. This capacity tells how many millimoles of positive charges, or milliequivalents, a column can bind at the active sites. In one case, the capacity may be estimated from the distance a known amount of Cu2+ ions (blue band) travels down the column and comparing to the total column height. In the second case, a capacity estimate can be obtained by observing how much Na+ must be bound to the column to reach saturation. These two methods estimate the column capacity using different assumptions; however, the two results should be reasonably consistent. The prelab problems may be helpful in carrying out these calculations. The two column capacity estimates should be compared and reasons for any difference between them should be included. The uncertainties for your measurements should be compared and discussed. Explain the difference between relative and absolute uncertainty and why the relative uncertainties differ for the three measurements. Complete the data sheet in your manual and turn it in for grading.

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