Acid/Base Titrations
Titrations involving NaOH, KHP, Na2CO3 and weak acids (HA)
H+1 + OH-1  H2O
Vocabulary1) Aliquot
2) Back Titrate
3) Buret
4) End-Point
5) Equivalence point
6) Erlenmeyer Flask
7) Indicator
8) KHP
9) Meniscus
10) Parallax
11) Phenolphthalein
12) Pipet
13) Primary Standard
14) Standardization
15) TC
16) TD
17) Titer
18) Titration
19) Volumetric Flask
Procedurea) Prepare 1000 mL of approximate 0.25 N HCl solution by adding
21.5 mL of 12 MHCl to 1000 mL of De-ionized water.
Store in a clean glass bottle.
b) Prepare 1000 mL of approximate 0.25 N NaOH solution by
adding approximately 10 grams of solid NaOH pellets to
1000 mL of boiled and cooled De-ionized water. Store in
a clean polyethylene bottle.
c) Remember the magic number three.
i. Always rinse rinse titration burets three times
with 2 to 5 mL of the solution that they will
contain.
ii. Always rinse titration flask, Erlenmeyer Flask
three with DI water.
iii. Always add three drops of indicator.
iv. Always do three good trials.
d) Determine the Acid/Base ration by titrating approximately
20.00 mL of the HCl with the NaOH.
i. Be sure to read and record the initial and final buret
readings and report your answer to the nearest .01 mL.
ii. Don’t forget to add the indicator. Use either methyl
orange or phenolphthalein.
iii. Be sure to rinse the sides of the E-Flask down with
DI water just before the endpoint.
e) Determination of a titer (You use the data from the first trial
to speed up the process.) Remember, you should
always use different amount in each trial.
Titer
mL Base needed
Trial x
= mL NaOH
mL
HCl
X
Trial #1
ml HCl
Trial x
Trial #1
Titer
mL acid needed
Trial x
=(mL acid used
mg Na2CO3
Trial #1
)(mg Na2CO3
Trial x)
Trial #1
Titer
mL base needed
Trial x
=(mL base used Trial #1)(mg KHP
mg KHP Trial #1
Trial x)
f) Standardize the HCl by titrating a sample of pure sodium
carbonate ( a primary standard). Measure accurately
between 0.4 to 0.5 grams of Na2CO3. Dissolve in 50 mL of
DI water and add 3 drops of bromocresol green indicator
solution. Fill both the acid and base buret. Titrate to a
yellow color, then heat to drive off the CO2 formed the
carbonate. If the indicator goes back to its original blue
color finish titrating with acid to the faintest of yellow. If
too much acid has been added back titrate with the NaOH.
Do three trials.
g) Or standardize the NaOH with KHP. Measure accurately 1.0 to
1.2 grams of KHP. Dissolve in 50 mL of DI water and add
3 drops of phenolphthalein indicator. Titrate to the faintest
of pink endpoint. If necessary you can back titrate with the
acid. Do three trials.
Errors-The biggest error problem is the carbon dioxide dissolve in the
water used to make the NaOH solution.
Calculations-
mL X N =
meq = mg/GEM
a) Acid Base Ratio
Ratio = mlHCL/mLNaOH
b) Standardization of HCl
Normal-titration
NHCl =
(mgNa CO /GEMNa CO )(1/ mLHCl)
2
3
2
3
Back-titration
meqacid - meqbase = (mgNa CO /GEMNa CO )
2
3
2
3
(mLacid) (Nacid) - (mLbase) (Nbase) = (mgNa2CO3/GEMNa2CO3)
(mLacid) (Nacid) = (mLbase) (Nbase)
(Nbase) = (Nacid)(mLacid/mLbase)  Titer
(Nacid)(mLacid) - (mLbase) [(Nacid)(mLacid/mLbase)] = (mgNa2CO3/GEMNa2CO3)
(Nacid) [(mLacid) - (mLbase) (mLacid/mLbase)] = (mgNa2CO3/GEMNa2CO3)
(Nacid) = (mgNa2CO3/GEMNa2CO3) (1/[(mLacid) - (mLbase) (mLacid/mLbase)] )
c) Standardization of NaOH
Normal titration
NNaOH =
(mgKHP/GEMKHP)(1/ mLNaOH)
Back-titration
meqbase - meqacid = (mgKHP/GEMKHP)
(mLbase) (Nbase) - (mLacid) (Nacid) = (mgKHP/GEMKHP)
(mLacid) (Nacid) = (mLbase) (Nbase)
(Nacid) = (Nbase)(mLbase/mLacid)  Titer
(Nbase) (mLbase) - (mLacid) [(Nbase)(mLbase/mLacid)] = (mgKHP/GEMKHP)
(Nbase) [ (mLbase) - (mLacid) (mLbase/mLacid)] = (mgKHP/GEMKHP)
(Nbase) = (mgKHP/GEMKHP) (1/[(mLbase) - (mLacid) (mLbase/mLacid)] )