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Monday, October 11, 2010

My task:Gravimetric determination of calcium

Gravimetric Determination of Calcium as CaC2O4.H2O
(Experiment 2)

Calcium ion can be analyzed by precipitation with oxalate in basic solution to form CaC2O4.H2O.  The precipitate is soluble in acidic solution because the oxalate anion is a weak base.  Large, easily filtered, relatively pure crystals of product will be obtained if the precipitation is carried out slowly.  This can be done by dissolving Ca2+ and C2O42- in acidic solution and gradually raising the pH by thermal decomposition of urea (see Reaction 27-2 in the textbook).


UNKNOWN

You will be given about 150-mL of a solution containing ~ 0.1-0.2 M Ca2+.  (The solution was prepared by dissolving solid CaCO3 or CaCl2 in dilute HCl.)  The objective is to determine the exact molarity of the Ca2+ solution.


PROCEDURE

1.   Dry three medium-porosity, sintered-glass funnels for 1 h at 105° C; cool them in a desiccator for 15 min (more if still warm) and weigh them on the analytical balance.  Repeat the procedure with 15-min heating periods until successive weighings agree to within ± 0.3 mg.
      Note: This type of repetitive heat-cool-weigh procedure is known as "drying to constant weight."  Obviously, you must keep track of your own items in the drying ovens.  A good method is to put your three funnels in a single beaker with your name on it.  Handle the hot beakers and funnels with tongs or a towel.  Never handle filter funnels, weighing bottles, etc. with your bare fingers because oils from your skin may effect the weight.  Instead, use a dry towel, tongs, or latex gloves. 

2.   Use a few 2-3 mL portions of the unknown solution to rinse a 25-mL transfer pipet, and discard the washings.  Use a rubber bulb, not your mouth, to provide suction! Transfer exactly 25 mL of unknown to each of three, dry 250 or 400-mL beakers.  Dilute each with ~75 mL of 0.1 M HCl.  Add 5 drops of methyl red indicator solution (Table 11-4 in the textbook) to each beaker.  This indicator is red below pH 4.8 and yellow above pH 6.0.

3.   Add ~25 mL of ammonium oxalate solution to each beaker while stirring with a glass rod.  Remove the rod and rinse it into the beaker with a little DI water.  Add ~15 g of solid urea to each sample, cover with a watch glass, and boil gently for ~15-30 min until the indicator turns yellow.


4.   Filter each hot solution through a weighed funnel, using the "house" vacuum as demonstrated by your instructor.  Add ~3 mL of ice-cold water to the beaker, and use a rubber policeman to help transfer the remaining solid to the funnel.  Repeat this procedure with small portions of  ice-cold water until all of the precipitate has been transferred.  Finally, use two 10-mL portions of ice-cold water to rinse each beaker, and pour the washings over the precipitate.

5.   Dry the precipitate, first with suction for 1-2 min, then in an oven at 105° C for 1 h. Bring each filter to constant weight.  The product is somewhat hygroscopic, so only one filter at a time should be removed from the desiccator, and weighings should be done rapidly.  The water of crystallization in the "dry" CaC2O4.H2O is not lost.

6.   Calculate the average molarity of Ca2+ in the unknown solution.


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