Need HELP: Volumtric determination of Calcium via EDTA

[Jumpshooter]

Hi,
I need desparate help in verifying this "calcium titration" method for yellow
corn meal samples.

Gen. procedure = 1 g corn meal; add 25 mL of 0.1% HCl; add 25 mL water; let
sit at Troom for 30 mins; then add 15 mL of 4.3% NaOH; add 3 dr calcon
indicator; then titrate the mixture with 0.01M EDTA.

But when we do this; we observe no color change!? Supposedly the color
change is to be from tawny reddish to faint green. But we see no change
even though we are 100% positive that calcium is spiked into sample.

Why is our Problem?
could it be the yellow color is masking the color change?

b) does any body here know the reason WHY or How this indicator functions?
In what direction is the pH of the solution changing when the indicator is
supposed to be indicative of calcium titration? Any help would be much
appreciated. Thank you.

[Beth Bray]

check out http://www.sjsu.edu/faculty/chem55/55mgca.htm
http://www.natscience.com/Uwe/Forum.aspx/analytical/984/Calmagite-EDTA-titration-for-Ca-Mg

Check to see if your pH = 12 for Ca titration

Post by Jumpshooter
Hi,
I need desparate help in verifying this "calcium titration" method for yellow
corn meal samples.
Gen. procedure = 1 g corn meal; add 25 mL of 0.1% HCl; add 25 mL water; let
sit at Troom for 30 mins; then add 15 mL of 4.3% NaOH; add 3 dr calcon
indicator; then titrate the mixture with 0.01M EDTA.
But when we do this; we observe no color change!? Supposedly the color
change is to be from tawny reddish to faint green. But we see no change
even though we are 100% positive that calcium is spiked into sample.
Why is our Problem?
could it be the yellow color is masking the color change?
b) does any body here know the reason WHY or How this indicator functions?
In what direction is the pH of the solution changing when the indicator is
supposed to be indicative of calcium titration? Any help would be much
appreciated. Thank you.

[David Stone]

Post by Beth Bray

Post by Jumpshooter
Hi,
I need desparate help in verifying this "calcium titration" method for yellow
corn meal samples.
Gen. procedure = 1 g corn meal; add 25 mL of 0.1% HCl; add 25 mL water; let
sit at Troom for 30 mins; then add 15 mL of 4.3% NaOH; add 3 dr calcon
indicator; then titrate the mixture with 0.01M EDTA.
But when we do this; we observe no color change!? Supposedly the color
change is to be from tawny reddish to faint green. But we see no change
even though we are 100% positive that calcium is spiked into sample.
Why is our Problem?
could it be the yellow color is masking the color change?
b) does any body here know the reason WHY or How this indicator functions?
In what direction is the pH of the solution changing when the indicator is
supposed to be indicative of calcium titration? Any help would be much
appreciated. Thank you.

check out http://www.sjsu.edu/faculty/chem55/55mgca.htm
http://www.natscience.com/Uwe/Forum.aspx/analytical/984/Ca
lmagite-EDTA-titration-for-Ca-Mg
Check to see if your pH = 12 for Ca titration

I would second the recommendation to use Calmagite as the indicator,
rather than Calcon. You should check the pH of your solution BEFORE
adding the indicator. We also add an ammonia/ammonium hydroxide buffer
to make sure that the pH stays in the correct range during the titration.
(This is for water samples and calcium supplement tablets).

Vogel suggests using a pH of 12.3 for Calcon using a diethylamine buffer
(5 mL per 100 mL of solution) with colour change from pink to blue.
(But your sample is coloured, which will affect what you see.)
The pH is important to prevent interference from Mg, which should be
precipitated out as the hydroxide at this pH.

One other general point about EDTA titrations - if the EDTA is too dilute
relative to the calcium concentration, it is very hard to get a good
end-point. You might wish to take a sample, prep it, then add more
concentrated EDTA (say, 0.1 M) just to establish a rough ball-park, and
ensure that you are getting to an end-point.

[Samite Alchemist]

If possible, confirm your results by AA or ICP. Filter 10mL of the
dilute HCl leachate before addition of NaOH for that analysis.

[Bob M]

Remember corn contains phytic acid which chelates calcium. This may
give low results depending on the indicator used.

Also plant materials contain phenolics which yellow in alkaline
solution.

Suggest you try HCl /H2O2 digestion to destroy interfering materials
before EDTA titration.

I normally use calcon carboxylic acid (aka paton and readers reagent)
indicator. But I am not sure whether it will work in the presence of
phytic acid.

Also the lack of an endpoint may be due to endpoint not being reached
try stronger EDTA.

Bob M
www.molab.co.nz

[Jumpshooter via NatScience.com]

loads of great help!

Post by Bob M
Remember corn contains phytic acid which chelates calcium. This may
give low results depending on the indicator used.
Also plant materials contain phenolics which yellow in alkaline
solution.
Suggest you try HCl /H2O2 digestion to destroy interfering materials
before EDTA titration.
I normally use calcon carboxylic acid (aka paton and readers reagent)
indicator. But I am not sure whether it will work in the presence of
phytic acid.
Also the lack of an endpoint may be due to endpoint not being reached
try stronger EDTA.
Bob M
www.molab.co.nz