NAME: Aqilah Mahabir LAB DAY: Wednesday

NAME: Kwasi Worrell WEEK: 8

DEMONSTRATOR: Sherlund James DATE: March15th 2011

EXPERIMENT 3: The Determination of Calcium in Milk by EDTA Titration

Introduction/Theory:

Calcium is an important mineral essential to the human body for the formation of healthy and proper formation of teeth and bones. It can be found in fruits, vegetables and mostly in dairy products. In this experiment the amount of Calcium present in powdered milk will be determined using a Complexometric back titration with EDTA.EDTA is a large molecule which creates a complex with a metal ion (in this case Ca2+), bonding through six coordination sites (see fig 2) Ca2+ + EDTA→ Ca[EDTA]4-.

The indicator used was Solochrome Black and you see the characteristic royal because the dye complexes to the metal ion (Ca2+), But as more EDTA is added all of the calcium ions are complexes with the EDTA and becomes unavailable for bonding to the dye. When enough EDTA has been added to coat all of the metal ions present, there is nothing left for the dye to complex with, and so the royal blue color is lost and replaced with a purple color. This is, of course, the endpoint of The titration.

Figure 1: EDTA(ethylenediaminetetraacetic acid) molecule

Image taken from: http://www.chm.bris.ac.uk/motm/edta/EDTA.gif

[Accessed 3/22/2011]

Figure 2: A structure of how EDTA bonds with the calcium ion

Image taken from: http://openlearn.open.ac.uk/file.php/2986/S_ 1_002i.jpg[Accessed 3/22/2011]

PROCEDURE: Carried out as stated in lab manual (pg 10 - 11)

Results:

Table 1: Showing the mass of powdered milk used.

Sample # 1 2 3Mass of milk powder used /g 1.019 1.0222 1.0244

Table 2: Showing the masses of EDTA and CaCO3 used.

Table 3: Titre volumes of CaCO3 standard solution vs. 25mL 0.04M EDTA and Milk solutions

Sample # 1 2 3Final Burette Reading /cm3 36.30 35.00 35.00Initial Burette Reading /cm3 0.00 0.00 0.00Volume of CaCO3 /cm3 36.30 35.00 35.00

Table 4: Titre volumes of CaCO3 standard solution vs. 25mL 0.04M EDTA.

Sample Blank # 1 2 3Final Burette Reading /cm3 42.10 41.90 42.50Initial Burette Reading /cm3 0.30 0.00 0.90Volume of CaCO3 /cm3 41.8 41.90 41.6

EDTA CaCO3

Mass of salt used /g 3.7218 0.6040

Treatment of Results:

Determination of the concentration of EDTA

Mass of EDTA used = 3.7218 gMr EDTA= 372.24 gmol-1

# of moles EDTA = 0.04 mols

372.24 g of EDTA = 1mol

So 3.782g = 3.782× 1372.24

= 0.01002 moles of EDTA

In 250 ml there is 0.01002 moles of EDTA

So in 1000 ml there is = 0.01002× 1000

250 = 0.04008M

Determination of the concentration of CaCO3

Mass of CaCO3 used = 0.6040 gMr CaCO3: 100.09 gmol-1

100.09 g of CaCO3 = 1mol

So 0.6040 g =0.6040 ×1

100.09 = 0.006041 mols

In 250 ml there are 0.00604 mols present

So in 1000 ml there is 1000× 0.006041

250 = 0.02416 M CaCO3 present

Determination of the number of moles of Ca that reacted with 25mL of EDTA

Sample 1 used 41.8 mL of CaCO3 (from table 4)1000 ml of CaCO3 contains 0.02416 M

Therefore 41.8 ml will contain 41.8 ×0.02416

1000 = 0.00101 moles

So 0.00101 moles of Ca reacted with 25mL of EDTA

Sample 1 used 36.3 mL of CaCO3

1000 ml of CaCO3 contains 0.02416 M

Therefore 36.3 ml will contain 36.3× 0.02416

1000 = 8.77× 10 -4 moles

So 8.77× 10 -4 moles of Ca reacted with 25mL of EDTASo the number of moles of calcium in sample 1

= Total Ca to react with 25 mL of EDTA – moles of Ca that react with residual moles of Ca= 1.01x10-3 moles - 8.77× 10 -4 moles = 1.33 × 10-4 moles

Determination of percentage of Ca in milk sample 1

Mr Ca = 40.078 gmol-1

Mass = moles x Mr = 1.33 x10-4 moles x 40.078 gmol-1 = 5.33 x10-3 g of Ca in 1.0019 g of milkSo the amount of Ca in 100g of powder milk = 5.33 x10-3 x 100 = 533 mg

Average calcium concentration

= (533 mg+667 mg+638)

3= 612.667mg

Standard deviation = 70.5

Therefore calcium concentration in milk samples = 612.667 ±70.5 mg

Conclusion/Discussion:

The calcium concentration in the milk samples was found to be 612.667 ±70.5 mg. the standard deviation shows a large error within the experiment. The error could have been caused by the following;

1) The presence of other metal ions such as copper, iron, manganese and zinc.’2) Inaccurate volumes and masses of samples used3) Assumption that pure milk was used. This could not have been guaranteed since the brand

of milk used wasn’t specified.

Solochrome Black was used as the indicator. In the beginning the solution was a royal blue color well before endpoint (excess Ca2+ ions present to complex with indicator), this then turned a purple color at endpoint (where all of the Ca2+ ions complexes by EDTA,and the indicator completely uncomplexed). In the case of the samples color changes were also noticed as they each started off as cloudy (opaque) bluish sample solutions well before the end point, followed by a trace of bluish-purple color just before endpoint an finally a purple color at endpoint.

Answers to Questions:

1) EDTA is one of the most common chelating agents used in chemistry.

                                                  EDTA

EDTA has this chelating effect because it forms octahedral complexes with calcium as shown

below. This complex is more favored than the monodentate ligands of water and other bidentate

ligands. So these ligands are replaced by EDTA. They also react in a 1:1 ratio Ca2+ + EDTA4-→

[CaEDTA]2-.

Diagram 1 showing CaEDTA] 2-

1. An Amperometic titration method can be used to determine the calcium concentration in the

presence of barium. This method of titration measures the concentration of analytes in

solution by means of an electric current. It takes advantage of the fact that the calcium is

smaller than the barium ion so it will move faster if a voltage is applied. Voltage is applied

across the indicator electrode (Pt) and reference electrode (Hg/calomel) is held constant, the

current passing through the cell is measured as a function of the titrated volume added. So

an electrical cell has to be made and titre volumes are added by a microburet. Then a graph

current vs. volume of reagent added is plotted. To straight lines with different gradients one

will be a negative gradient and the other positive will be obtained after which it will be

extrapolated to where they intercept. This point is the volume of reagent added.

References:

1. CHEM 2460 Principles of Chemical Analysis Laboratory Manual.pg 10-11

2. James N Miller & Jane C Miller, “Statistics and Chemometrics for Analytical” Chemistry, 5th Ed(2005) Publ. Pearson Education Limited pg 114

3. http://www.csudh.edu/oliver/che230/labmanual/calcium.htm [Accessed 3/22/2011]4. http://www.ajcn.org/cgi/reprint/83/2/310.pdf [Accessed3/22/2011]5. http://www.cerlabs.com/experiments/10875404367.pdf[Accessed3/22/2011]6. http://www.chemteach.ac.nz/investigations/documents/calcium.pdf {\ [ accessed 3/22/2011]