2. What is a pH indicator and its PH range….
Preparation of the Beetroot indicator and
identification of its pH range.
Applications of acid-base titrations.
Discussion.
Conclusion.
References.
3. A pH indicator (An acid-base indicator ) is either a weak acid
or weak base that exhibits a color change as the
concentration of hydrogen (H+) or hydroxide (OH-) ions
changes in an aqueous solution.
Examples:
Thymol Blue, Phenolphthalein, Methyl Orange, Bromocresol Green,
Bromophenol Blue and etc;
An Indicator’s colour change occurs over a range of
hydrogen ion concentrations. This range is termed
the colour change interval. It is expressed as the Indicator’s
pH range.
Reference: Anne Marie Helmenstine, Ph.D., (2011). Acid-Base Indicators. Available:
http://chemistry.about.com.
4. Beetroot Red, is a red glycosidic food dye obtained from its
aglycone, obtained by hydrolyzing the glucose molecule, Betanin.
The colour of Betanin depends on pH. It is bright bluish-red,
becoming blue-violet as the pH increases. Once the pH reaches
alkaline levels Betanin degrades by hydrolysis, resulting in a
yellow-brown colour.
Moira Sheehan, Carol Chapman. (2003). Make your own indicator.
Available: http://www.pearsonschoolsandfecolleges.co.uk.
5. Preparation of the Indicator:
1. 300.0 g of chopped Beetroot was added to 50.00 cm3 of
boiling water in a beaker and was boiled continuously for 45
minutes.
2. After allowing the beaker to cool for 15 minutes, boiled
Beetroot were squashed and the liquid was filtered.
3. The residues were squeezed once again and the liquid
was evaporated to get a highly concentrated portion of
the indicator.
6. Detection of the pH range:
A series of solutions in the range pH 1.0 -13.0 was prepared by
using 1.0 M NaOH and 1.0 M HCl acid.
When the indicator was added the to the prepared series of
solutions the colour was changed from red to yellow in between
pH 10.0 -12.0.
In order to get the exact pH range, another series of solutions was
prepared by diluting the pH 10-12 solution range into 10.10,
10.20,……..,12.00 and 0.05 ml of the indicator was added to each
solution.
A colour change was observed in
the pH range 10.80-11.20, which is
the working pH range of Beetroot.
7. i) Strong Acid – Strong Base Titration
Colour Change:
Red Yellow
ii) Strong Acid – Weak Base Titration
A colour change
was not observed.
8. iii) Weak acid – Strong Base Titrations
Colour Change:
Red Yellow
With 0.1M NaOH
.
A colour change
was not observed.
With 0.001M NaOH
9. A colour change was observed only in strong acid – strong base
titration and weak acid-strong base titration (with 0.10 M NaOH), as
the working pH range of the indicator falls to the vertical portion of
those reactions.
But when the base is diluted, a colour
change was not observed, as the pH range
of the Beetroot indicator doesn’t fall
within the rapid pH change of the reaction.
Although a colour change was observed in
above two titrations, the most applicable
titration is weak acid-strong base titration, because the vertical
portion of the reaction shifts upwards.
10. Working pH range of BEETROOT is pH 10.80 - 11.20
Acidic Basic
RED YELLOW
(Above result was obtained on 28th of March 2012 at 11.50 a.m. at
B2 102 laboratory under the room temperature 270 C.)
BEETROOT is a good indicator.
But it is not a very suitable indicator for laboratory
experiments as it should be prepared at the time the
experiment is done. The prepared indicator also cannot be
kept for a long time as it is a biological sample.
11. Anne Marie Helmenstine, Ph.D., (2011). Acid-Base Indicators.
Available: http://chemistry.about.com. Last accessed 5th April 2012.
Australian Broadcasting Corporation(ABC). (2007). Teaching Science -
Cool Chemistry(PH Indicator). Available: http://www.abc.net.au. Last
accessed 4th April 2012.
Lange's Handbook of Chemistry, 8th Edition, Handbook Publishers Inc;
1952.
Moira Sheehan, Carol Chapman. (2003). Make your own indicator.
Available: http://www.pearsonschoolsandfecolleges.co.uk. Last
accessed 2nd April 2012.
Volumetric Analysis, Kolthoff & Stenge, Interscience Publishers, Inc;
New York, 1942 and 1947.
