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EFFECT OF WRONG USE OF INDICATOR
The success of titration exercises depends on the use of the correct indicator. Wrong use of indicators will definitely give wrong results. For instance, let’s consider a case of the titration of a solution of a wrong acid say HCl with that of a weak base says Na2CO3, methyl orange is the suitable indicator, but if phenolphthalein indicator is used instead, the endpoint will appear when only half of the weak base has been used up. This can then be represented with the following equation.
HCl + Na2CO3phenolphthalein NaHCO3 + NaCl
Indicator
This happened because the phenolphthalein is sensitive to a weak acid such as Na2CO3.
| TITRATION | EXAMPLE | PH RANGE | SUITABLE INDICATOR |
| 1.Strongacid vs. strong base(3-11) | H2SO4(aq) and KOH(aq) | 3.5 – 9.5 | Any indicator is suitable. |
| 2.Weak acid vs. strong base(7-11) | H2C2O4and NaOH | 7.0-9.5 | phenolphthalein |
| 3.strong acid vs. Weak base(3-7) | HCl(aq)andNH3(aq) or K2CO3(aq) or Na2CO3(aq) or Ca(OH)2(aq) | 3.5-7.0 | Methyl orange or screened Methyl orange |
| 4 Weak acid vs. Weak base | CH3COOH(aq) and NH3(aq) | No sharp change | No suitable indicator. Or phenol red. |
IMPORTANCE OF VOLUMETRIC ANALYSIS
- Standardize unknown solution
- Calculate molar mass, water of crystallization and solubility.
- Determine the purity of substances.
- Determine the masses of substances dissolved.
- Faster and more convenient
GENERAL PROCEDURES AND PRECUTIONS DURING ACID –BASE TITRATION
STEPS INVOLVE IN FILLING THE BURETTE WITH DILUTE ACIDES
STEP1. Clamp the burette in a vertical position to avoid error due to parallax while taking reading. Rinse it with the given acid solution, alloing part of the acid to pass through the tap and the jet. Then cloe the tap.
STEP2.With the funnel at the top of the burrette, fill the burettewith acid to a desired level ensure that the jet is filled with acid to avoid air bubbles inside it.tightly close the buette tap to avoid leakages. Remove the funnel toavoid droplets of acid from it.Hold a piece of white paper behind the burette so that the acid level is clearly shown and note the reading of the burette.
STEPS INVOLVE IN MEASURING 25.00Cm3 (20.00Cm3) of standard base solution.
STEP1.Collect four conical flasks and wash with distiled water only. Make sure that none of the flasks contain any acid or alkali.
STEP2.Rinse the pipette with the given base solution.
STEP3.suck the base solution into the pipette until the level is about 1cm3 or2cm3above the forefinger. Gently release the pressure until the bottom of the concave meniscus is at the graduation mark,with the eye level aligned.
STEP4.While filing the pipette ensure that the jet lies well inside the base solution to aviod air bubbles in the pipette.
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