Objective: To determine the percentage by weight of magnesium hydroxide in
commercially available milk of magnesia.
Introduction:
According to the U.S. Pharmacopeia, medicinal
preparations of milk of magnesia should contain at least 7% magnesium hydroxide
by weight. An accurate analysis must
measure the total of the suspended and dissolved magnesium hydroxide. To take representative samples for analysis,
the contents of the bottle must be shaken well first, transferred to a large
flask and stirred occasionally while samples are being prepared.
Because the sample is a white opaque suspension,
direct titration poses several analytical problems. Since the suspended magnesium hydroxide particles adhere to the
walls of the flask (out of contact with the titrant) inaccurate results are
likely to give a fading end point. The
opacity of the sample may also cause difficulties in the detection of the color
change of the indicator at the end point.
A better approach is to use a back titration
procedure that is to add a measured excess of hydrochloric acid to react with
all of the dissolved and suspended magnesium hydroxide to give a clear
solution. The excess (unreacted) acid
is then (back) titrated with standard sodium hydroxide. The reactions are the following:
OH-1(aq) + H+1
(aq) = 2H2O(aq) Standardize of NaOH & HCl
Mg(OH)2 (s) +
2H+1 (aq) = 4 H2O
(aq) + Mg2+ (aq) Initial titration
H+1(aq, excess) +
OH-1(aq) = 2H2O
(aq) Back titration
The first two reactions occur when the excess of standard HCl is added to the suspension of Mg(OH)2. The third reaction occurs when the excess acid is titrated with standard NaOH. Since the final titration is the titration of a strong acid with a strong base, the choice of indicator is not critical.
EXPERIMENTAL
Reagents
HCl (0.1M)
NaOH (0.1M)
KHP, oven dried for 1-2 hours at 110 0C.
Phenolphthalein
indicator
Methyl Orange
indicator
Timesaver Hint:
Perform Part C Step 1 first.
While the sample is in the oven start at Part A step 1 and proceed as
described.
2-50 ml burets, one is for acid titration, one is
for base titration.
5-250 ml Erlenmeyer flasks
Procedure
Part A: Standardization of NaOH
1) Accurately weigh 0.4xxx g KHP into each of three
Erlenmeyer flasks. Add 75 ml distilled
water dissolve KHP, add 3 drops of
phenolphthalein indicator to each flask
2) Titrate with NaOH solution until a sudden color
change occurs. Record the volume of
NaOH used and calculate the concentration of NaOH. Repeat this step two times.
Part B: Standardization of HCl
1)
Using
a buret accurately add 30.00 ml NaOH
into each of three Erlenmeyer flasks,
add 70 ml distilled water. Then add 3 drops methyl
orange indicator to one of the flasks.
2)
Titrate
with the HCl solution until a sudden
color change .Record the volume of HCl and calculate the concentration of
HCl. Repeat this experiment two times..
Part C: Determination of Magnesium hydroxide in Milk of Magnesia
1)
Accurately
weigh the vial with cap which contains magnesium hydroxide of milk,
report the weight to 0.1mg. Rinse the contents of
the vial into a 250 ml Erlenmeyer flask, then with distilled water rinse vial
and cap few times to ensure that all of the magnesia will be analyzed.
2)
Dry
the rinsed vial in the oven (about 1 hour), do not dry the cap in the oven: It
will
melt. Be
sure to identify your vial by placing your vial in a beaker which is labeled
with your name. After the vial cools to
room temperature, weigh the vial and cap.
3)
Add
a measured amount of standardized HCl with a buret into an Erlenmeyer flask
which contains the sample of magnesia milk until the
solution is clear, then add 1-2 ml more to ensure that excess HCl has been
added. Record the exact volume of HCl which has been added. For 1 gram (1 ml)
sample, this volume should be at least 30 ml (no more than 50 ml should be
required.)
4)
Add
3 drops of phenolphthalein indicator into Erlenmeyer flask which contains the
sample, titrate the solution with standard NaOH.
This titration should not require more than a few ml of NaOH. Record the volume
of NaOH.
5)
Repeat
this procedure for at least three samples.
6)
Calculate
the weight percent of magnesium hydroxide for each run.
Part A: Standardization
of NaOH using KHP
(20 points)
(ml) (ml) (ml) NaOH
(M)
(1)
(2)
(3)
Average Molarity of NaOH after titrations
95% Confidence Interval (show work)
(20 points)
Concentration
of NaOH
(ml) (ml) (ml) HCl
(M)
Average Concentration of HCl
95% Confidence Interval
Part C:
Determination of Magnesium hydroxide in Milk of Magnesia
(30 points)
1 2 3
Wt (samp +
cont), g
Wt (sample), g
V(HCl), mL
V(NaOH), mL
Average Wt% Mg(OH)2 (+/- std dev)
Notebook : Final Grade: