LAB TECHNIQUE - GRAVIMETRIC ANALYSIS


Introduction

Gravimetric analysis is a technique through which the amount of an analyte (the ion being analyzed) can be determined through the measurement of mass. Gravimetric analyses depend on comparing the masses of two compounds containing the analyte. The principle behind gravimetric analysis is that the mass of an ion in a pure compound can be determined and then used to find the mass percent of the same ion in a known quantity of an impure compound. In order for the analysis to be accurate, certain conditions must be met:

  1. The ion being analyzed must be completely precipitated.
  2. The precipitate must be a pure compound.
  3. The precipitate must be easily filtered.


Quantitative Transfer Techniques

Quantitative transfer techniques are essential for accurate results in this investigation. Here are some techniques or tips to effectively filter a precipitate.   




Digestion

Digestion is a process where the precipitate and the solution it is produced in is heated.  The heating process creates larger particles of the precipitate. These larger particle are easier to trap in the filter paper. 

*We will usually skip this step in lab.  You should only perform if explicitly told to do so. 


Filtering

When quantitatively transferring the precipitate to a filter paper there are techniques to maximize the amount of precipitate that ends up on the filter paper:
 
  • Wet the filter paper with distilled water and turn on the faucet/aspirator.  Make sure the filter paper is laying flat on the funnel, so no precipitate can circumnavigate/avoid the filter.
  • Use a stirring rod to guide the flow of liquid into the filter. You want to flow of the liquid to hit the center of the filter paper so it is immediately pulled through the filter paper.
  • Let precipitate settle to the bottom of the beaker before starting  to filter the mixture. Decant the supernatant (the liquid above the settled precipitate) into the funnel. Decanting means to pour in such a way as to leave the solid material on the bottom of the beaker. Do not let the precipitate material collect on the filter paper until almost all of the supernatant has been filtered. By pouring most of the supernatant off this will speed up the filtration.
  •  Do not over fill the funnel when pouring into the filter paper.  If using a Buchner funnel, where the filter paper lays flat on the funnel, you do not want the supernatant to reach the edges of the filter paper.  Liquid which overflows can no longer be filtered. Use a rubber policeman (rubber attached to a stirring rod - see below) to make sure all the precipitate has been transferred from the beaker to the filter. It is important that the precipitate is quantitatively transferred to the filter.
  • Rinse the beaker with distilled water (technically the solvent, which is usually distilled water) to get all the precipitate that stuck to the bottom to the filter paper. Wash the precipitate with small portions of distilled water once the beaker has been empty.  This is done to wash any impurities that are trapped in particles of the precipitate.  Be careful not to exceed the capacity of the filter flask.
  • If you notice some of the precipitate passes through the filter and ends up in the flask, stop the filtration immediately pour the liquid in the flask back into the beaker.  Use a second piece of filter paper (be sure record its dry weight before you wet it).


Preparation for Weighing The Precipitate

After separation, the material must be prepared for weighing. This is accomplished by heating up the precipitate to drive off excess solvent and volatile electrolytes. Low temperature drying may be used for some lyophobic (solvent phobic) precipitates. High temperature drying, called ignition, is used for the lyophilic (solvent liking) precipitates. The thing to remember is that drying or ignition may change the chemical composition of the precipitate.


Background Information - LabTechniques Used in This Experiment


How to Set Up Vacuum Filtration




Gravity Filtration Set Up 


Use this video to FOCUS ON folding the filter paper. The set up is best described by the picture above. We will usually not use the buret clamp, rather a ring for support.




OPTIONAL:
Decanting and Filtration
Observe the set up with a ring stand and how to decant (pour out) the liquid. 
This is another version of the first video "Quantitative Transfer"