[cmquer]

To perform gravimetric analysis, a constant weight porcelain crucible should be prepared first. The procedures are as follows,

1. A porcelain crucible and lid are cleaned with deionized water and then placed in air oven at 110 ﾟC for 15 mins to dry.

2. The porcelain crucible and lid are placed into a desiccator and cool it to room temperature.

3. Weigh the porcelain crucible and lid using an analytical balance before heating.

4.  Heat the porcelain crucible and lid to dull redness for 15 mins with a Bunsen burner or in a muffle furnace at 800 ﾟC.

5. Cool the porcelain crucible and lid in a desiccator to room temperature.

6. Weigh the porcelain crucible and lid.

7. Repeat the steps 4 to 6 until a constant weight is obtained.

What I confused is the differences between steps 1 and 4. I know step 1 is to evaporate the deionized water on the porcelain crucible and lid. But how about step 4? What is its purpose?

[RIPPIN]

Porcelain is a material that will absorb water.  The purpose of step 4 is to bring the crucible (s) to constant weight. 

Usually step 4 is performed in a muffle furnace.  It is heated, removed, then stored in a desiccator. Then when cooled it is weighed.  This is usually repeated a minimum of 3 times to get the crucible to constant weight.  Three trials will tell you if there is variation. 

In analytical chemistry this usually is defined as a weight that does not deviate by more than 0.004 g...(that figure can be variable depending on the experiment).

I am attaching a link that will explain the WHYS? for differet types of gravametric analyses:

Link: http://www.iugaza.edu/users/Abdellatif/Chem%202310/Contents/Gravimetric_Analysis.htm

Another good resource is Skoog and West:  Analytical Chemistry, 8th Edition...

Hope that helps...