experiment 8: determination of calcium by complexometric titrations

Question 1

a titration which involves the complexation of a chelating agent with a metal

Answer 1

complexometric titration

Question 2

chelating agent

Answer 2

a compound which provides more than one pair of electrons for complexation

Question 3

example of chelating agent

Answer 3

ethylene diamine tetraacetic acid EDTA

Question 4

6 covalent bonds to a metal -
4 covalent bonds to a metal -

Answer 4

6 covalent bonds to a metal - tetrabasic form (EDTA^-4)
4 covalent bonds to a metal - dibasic form (H2EDTA^-2)

Question 5

define water hardness

Answer 5

amt of calcium ad magnesium ions in the water sample

Question 6

water hardness is usually reported as

Answer 6

ppm CaCO3

Question 7

true or false:
analysis does distinguish between Ca2+ and Mg2+

Answer 7

false - does not

Question 8

why does the high concentrations of Ca +2 and Mg +2 are undesirable

Answer 8

due to the formation of insoluble carbonate deposits and formation of soap scum

Question 9

CaCO3 ppm
0-60:
61-120:
121-180:
above 180:

Answer 9

0-60: soft
61-120: moderately hard
121-180: hard
above 180: very hard

Question 10

how can water hardness be determined

Answer 10

titration with a standard EDTA solution using EBT indicator

Question 11

at what pH is titration conducted and why

Answer 11

at pH 10

so that EDTA will maintain its dibasic form and also to prevent precipitation of Mg(H)

Question 12

what happens when all of the Ca+2 and Mg+2 have already reacted with EDTA (dibasic form)

Answer 12

the excess EDTA will interact with the Mg-EBT indicator complex producing a color change from wine red to blue.

Question 13

titration with EDTA can also used to determine …

Answer 13

the percentage of Ca in an unknown solid

Question 14

reagents

Answer 14

MgCl2.6H2O
distilled water
6.0 M HCl
6.0 M NaOH
pre-dried standard calcium carbonate
ethylene diaminetetraacetic acid disodium (EDTA)
eriochrome black T (EBT) indicator
pH 10 Buffer methyl red indicator

Question 15

disposal of waste

Answer 15

down the sink in running water

Question 16

weigh how many grams of EDTA disodium dihydrate

Answer 16

0.93 to 0.95g

Question 17

preparation of EDTA solution

Answer 17

weigh EDTA
transfer it into 250ml volumetric flask
clean beaker, weigh 0.052g MgCl2 6H2O and dissolved into 50mL distilled water
transfer to volumetric flask
invert the flask thrice
transfer content to clean colored glass

Question 18

weigh how many grams of standard calcium carbonate

Answer 18

weigh 0.012 to 0.015g

Question 19

EDTA ethylene diaminetetraacetic acid
chemical formula -
structure -

Answer 19

chemical formula - C10H16N2O8
structure - nil

Question 20

how many mL of 6.0 M HCI
then add how many ml of water

Answer 20

2 mL using a 5mL serological pipette
80ml

Question 21

how many mL of 6.0 M NaOH

Answer 21

Until first appearance of a yellow color

Question 22

why do we need to boil the solution for 2 minutes in the standardization of EDTA solution

Answer 22

to remove the carbon dioxide

Question 23

add how many drops of methyl red indicator then what is the next step

Answer 23

3 drops
add 6.0 M NaOH dropwise until the first appearance of a yellow color

Question 24

add how many drops of EBT indicator and ml of pH 10 buffer solution then what is the next step

Answer 24

2mL of pH 10 buffer
3 drops
EBT indicator

then titrate usingthe standard EDTA solution till color changes from wine red to blue