Standardization of NaOH
acid phthalate(KHC8H4O4, or KHP) is one of the most common primary
standards for sodium hydroxide titrant. The compound
is a salt of phthalic acid (H2C8H404 , or H2P) : only one proton per molecule of the potassium
salt is acidic. The two molecules are represented:
Potassium acid phthalate reacts with sodium
hydroxide according to the equation:
The divalent phthalate ion is produced and
is responsible for the basicity of the solution at the equivalent
point. Phenolphthalein is employed as an indicator (acid form
colorless, basic form red-pink ; transition interval 8.0 - 9.6).
same reaction is used for the determination of impure potassium
acid phthalate unknowns ; this is sometimes called the determination
of the total acidity of a substance.
Preparation of 0.1N Sodium
Boil about 1
liter of distilled water for 5 minutes to remove carbon
dioxide. Allow to cool covered with a watch
glass ; transfer while warm (40oC)
to a 1 liter bottle (bottles are stored
in hall cabinet).
7 mL of a clear saturated (1:1) solution
the bottle, using a transfer pipet and a rubber bulb.
Mix thoroughly and keep covered. (1)
A saturated (1:1) solution of
sodium hydroxide is usually available in the laboratory
. Such a solution may be prepared by mixing 50
g of sodium hydroxide pellets with 50 mL of water in
a polyethylene bottle.*
Let stand for
a week to allow the isoluble sodium carbonate to settle.
*Be sure to cool the polyethylene bottle when
dissolving the sodium hydroxide. Use a water bath cooled
Standardization of 0.1N
Dry 4 to 5 g
of primary-standard potassium acidic phthalate
in a weighing bottle at 110oC for 2 hours. Allow to cool 30
minutes in a desiccator before weighing (Why should
the desiccator lid be left slightly "cracked"
during the cool-down period?)
out 800 to 900 mg of potassium acid phthalate
into each of three 250mL flasks. This is about
4 to 4.5 meq and should require 40 to 50 mL of
distilled water, warming if necessary. Cool before
Review the steps
on the use of the buret. Read buret to hundredths
places, e.g. 10.43.
Add 3 or more
drops of phenolphthalein indicator the first flask and
titrate dropwise in the vicinity of the end point. If
necessary, adjust the volume of phenolphthalein indicator
added to the other two flasks, and titrate them also.
Try to obtain the same intensity of pink color
at the end point for all three titrations. Be sure to
perform preliminary calculations in the laboratory.
Repeat any experiment in which an unsatisfactory
result is obtained.
normally of the sodium hydroxide by using an equivalent
weight of 204.22 for potassium acid phthalate. Calculate
relative average deviation. The precision should