Question

principle of estimation of calcium from milk​

Answer 1

Answer:

This method, called a complexometric titration, is used to find the calcium content of milk, the 'hardness' of water and the amount of calciumcarbonate in various solid materials. The method uses a very large molecule called EDTA which forms a complex with calcium ions. ... This solution is then titrated with EDTA.

Answer 2

Answer:

Safety

Lab coats, safety glasses and enclosed footwear must

be worn at all times in the laboratory.

Note that the concentrated (8mol L-1) sodium

hydroxide solution used is highly corrosive and

should be handled with extra care: ideally wear

rubber gloves when preparing and handling it.

Introduction

This method, called a complexometric titration, is used

to find the calcium content of milk, the ‘hardness’ of

water and the amount of calcium carbonate in various

solid materials.

The method uses a very large molecule called EDTA

which forms a complex with calcium ions. EDTA stands

for ethylenediaminetetraacetic acid. A blue dye called

Patton and Reeder’s indicator (PR) is used as the

indicator. This blue dye also forms a complex with the

calcium ions changing colour from blue to pink/red

in the process, but the dye–metal ion complex is less

stable than the EDTA–metal ion complex. As a result,

when the calcium ion–PR complex is titrated with EDTA

the Ca2+ ions react to form a stronger complex with

the EDTA.

For the titration, the indicator is added to the sample

solution containing the calcium ions and forms the

pink/red calcium ion-indicator complex (Ca-PR). This

solution is then titrated with EDTA. The endpoint occurs

when the solution turns blue, indicating that the Ca-PR

complex has been completely replaced by the calcium

ion-EDTA complex and the PR indicator reverts to its

blue colour.

The reaction is:

Ca-PR + EDTA4− → PR + [Ca-EDTA]2−

Note: Ca-PR is pink/red and PR is blue.

Equipment Needed

10 and 20 mL pipettes

250 mL conical flasks

100, 250 and 500 mL volumetric flasks

pH indicator paper

10 mL and 100 mL measuring cylinders

burette and stand

Solutions Needed

EDTA: ethylenediaminetetraacetic acid 0.025 mol L−1

solution. If possible, dry 5 g of the disodium salt of EDTA

for several hours or overnight at 80°C, allow to cool.

Weigh 4.65 g of the dried EDTA salt and dissolve it in

500 mL of distilled water in a volumetric flask.

Patton-Reeder indicator triturate: a small amount

may be available from Outreach at the University of

Canterbury, see contact details on back page.

Sodium hydroxide solution: (8 molL-1). (See safety

notes) Weigh 32 g of solid sodium hydroxide into a 250

mL conical flask and carefully dissolve in 100 mL of

distilled water. The solution will get very warm as the

NaOH dissolves; the temperature may be controlled by

sitting the bottom of the flask in a small basin of cold

tap water.

Dilute hydrochloric acid solution: (1-2 mol L−1)

Dilute sodium hydroxide solution: (1-2 mol L−1

Explanation:

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