Gravimetric Determination of Ca

Marble is just CaCO 3

GOAL OF EXPERIMENT:

To determine the % calcium in antacid tablets (CaCO 3 )

METHOD:

Gravimetric Analysis Isolate calcium as calcium oxalate precipitate and compare mass of calcium in precipitate (known formula CaC 2 O 4 •H 2 O) with original mass of sample.

SCHEDULE: Today

• Demonstration of cleaning crucibles • Students check-out materials from stockroom and clean crucibles and place in oven • Demonstration of reaction • Obtain weight of clean, dry, cool crucibles • Prepare Samples Grind antacid tablet Dissolve samples in acid Add ammonium oxalate precipitating agent, indicator, and urea • Digest sample (start by 4 PM) • Dry product

Next Week:

• Filter sample • Weigh resulting precipitate • Clean Crucibles • Calculate % Ca in samples (individuals, partners, and class)

Working with “partners”

Preparation of Crucibles Drying Procedure Handling of Desiccators Transfer of Hot Crucibles to Beaker Return after Crucibles in Oven

The Chemistry of this experiment

( CaCO CaCO 3 (s) NH 4 ) 2 3 ( s C 2 )  2 ⇄ Ca 2+(aq) O 4 ( H s )  + CO 3 2-(aq) ; CO 3 2-(aq)  ( aq ) 2 H  ( aq )  Ca + 2H +(aq) 2 2  ( NH aq ⇄ 4  ) H 2 CO 3(aq) (  aq CO )  → 2 H ( g ) CO 2(g) 2 C  2 O H 4 2 + H 2 O (l) ( dissolution//acid/base O ( l ) aq ) dissolution//acid/base

base Urea

( NH 2 ) 2 CO ( s )  H 2 O ( l )    2 NH 3 ( aq )  CO 2 ( g )

oxalic acid

hydrolysis

Very slow reaction; imperceptible at room temperature and neutral pH What do we do to increase rate?

Heat and go to acidic conditions (low pH) What so some bacteria do?

enzyme: increases rate by 10 14 !!!!

oxalate

2 NH 3 ( aq )  H 2 C 2 O 4 ( aq )  2 NH 4  ( aq )  C 2 O 4 2  ( aq ) acid/base Slow production of NH 3 to make good crystals!!!

makes reaction slow so ppt is slow in order

Ca

2  (

aq

) 

C

2

O

4 2  (

aq

)

CaC

2

O

4 

H

2

O

(

s

) precipitation

• • • • • • • • • • • • •

PROCEDURE: Obtain mass of clean, dry crucibles Grind tablet (lab assistant checks out mortar & pestal) Weigh out samples (0.35-0.38 g) to nearest 0.1 mg Dissolve in 100 mL H Add pH indicator (20 drops) Add (NH 4 ) 2 C 2 O 4 2 O, 6 mL 6M HCl, adjust pH if necessary precipitating agent (20 mL sat. sol. + 1 mL 6 M HCl) Add 15 g urea (use top-loading balance to mass) Heat at near boiling until color changes to yellow and precipitate forms Two samples ~simultaneously, store beakers covered with watch glass and store crucibles in beaker covered with watch glass Return CRUCIBLE HOLDER to stockroom ********************************************************************** Week 2 – to be discussed next lab period Filter hot solutions into crucibles (careful using paper towel insulator) Wash and dry precipitate (water and acetone) Weigh precipitate Clean Crucibles and put all equipment away in proper location Do calculations and turn in report

WEEK 2

•

Warm solutions – one at a time

•

Filter hot solutions into crucibles (careful using paper towel insulator)

•

Wash and dry (1hr) precipitate - water (10 mL, 2x) and acetone (10 mL). NO OPEN FLAMES AROUND THE ACETONE!!!!

•

Dry 1hr – precipitate in crucible (in ovens by 3PM)

•

Weigh precipitate/crucible after complete cooling

•

Clean crucibles (remove ppt with spatula), 5-10 mL of HCl, 2 x water

•

Put all equipment away in proper location, do calculations, and turn in report by 5 PM the next day