Experiment 8 % Calcium in Egg Shells By EDTA Complexometric Titration
Announcements • Extra Credit Presentations • Final Project Plans Due Next Week • 1 per group
What are Egg Shells Made Of? • Protein • CaCO3, the material that makes the egg shells hard • We want to determine the amount of calcium. • The calcium in egg shells is in the form of CaCO3. Therefore, we can also determine how much CaCO3 is in them. • CaCO3 is a carbonate. • Are carbonates water soluble? Do egg shells dissolve easily in water?
Goals • To dissolve a sample of egg shells in water • To quantitatively transfer the dissolved sample • To titrate aliquots of the sample to determine the amount of Ca2+ present
Hazards • EDTA • Egg Shells • HCl • Buffer • EBT
Refresher • What is a titration? • What must you know in order to perform a titration? • When do you stop titrating? • How do you determine the endpoint?
EthyleneDiamineTetraacetic Acid HOOC-CH2 CH2-COOH N-CH2-CH2-N HOOC-CH2 CH2-COOH Chelating Agent = “loves” to bind metals Tetradentate= has 4 binding sites
The Ionic form -OOC-CH2 CH2-COO- N-CH2-CH2-N -OOC-CH2 CH2-COO- Forms a very strong complex with Ca2+ Ca2+ + EDTA4- => [Ca-EDTA]2-
The Experiment • Weigh out egg shells • Add HCl to make soluble • Why? • Quantitatively transfer • Why? • How? • Vacuum Filter • Set up just like demo in lab • Removes protein. Why do this? • Dilute to volume in volumetric flask (100mL) • Use pipet to transfer aliquot (10mL) • Titrate aliquot with EDTA
The Indicator • Eriochrome Black T (EBT) • Also forms complex with metal ions • EBT prefers metals to H+ (actually a little more complex than this, but for this course…..) • EBT<EDTA as a complexing agent • At pH 10 (use buffer) • EBT w/ Ca2+ is _____ • EBT w/ H+ is _________ • As pH decreases, _____________________, this is why we add buffer at last minute
How it Works • So, EBT competes with EDTA for Ca ions in solution (use as little EBT as possible) • In the beginning, you have just Ca ions, no EDTA • When the EDTA has bound some Ca from the EBT • When the EDTA has “grabbed” up all of the Ca ions, EBT can no longer be bound to the Ca
Data and Results • Known • MEDTA (mol/L) • Volume EDTA delivered (mL => L) • Mass of sample (g) • Only used 1/10 of sample for each titration • 1mol EDTA: 1mol Ca2+ • Atomic mass Ca (40.078g/mol)
Applications of This Method • Used to determine concentration of metal ions in solution • Can be used to determine how much calcium (or magnesium or other minerals) is in a food product (milk, orange juice, etc.) • Can be used to determine concentration of good/bad metals in the environment (ie water source)