Re: what's the method and calculation to determine Mg(OH)2 in milk of magnesia?

It is difficult to determine Mg(OH)2 using either gravimetric procedure. 
You might think that a gravimetric method would be possible since you 
could just evaporate a specific weight of MOM and calculate the weight 
percentage based on the weight of the dried residue and the molecular 
weight of Mg(OH)2. You could do it this way but the results would not be 
accurate because you end up with a dried residue of a hydrated form of Mg
(OH)2. There is also always some residue unevaporated water. If you heat 
the residue to a high enough temperature to drive off all the residual 
water, you also start to decompose the Mg(OH)2 to MgO + H2O 
gravimetrically. The usual way to determine Mg(OH)2 in MOM is by acid-
base titration. However direct neutralization of the base is not 
performed. This is due to the cloudiness of the Mg(OH)2 suspension and 
the fact that the solid particles cling to the glass walls of the 
container.

Briefly, this is how it is done: use a back titration procedure that is 
performed by adding 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. This may be a little complicated for your 
grade level but should be familiar to you. The reactions and procedure 
are as 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. Acid-base colorometric indicators must be 
used for each of these reactions. Since the final titration is the 
titration of a strong acid with a strong base, the choice of indicator is 
not critical.

1) Accurately weigh the a vial with cap which contains magnesium 
hydroxide of MOM 1-1.5 g MOM should be good.

2 Rinse the contents of the vial into a 250 ml Erlenmeyer flask, then 
with distilled water rinse vial and cap a few times to ensure that all of 
the MOM will be analyzed.

3) Dry the rinsed vial in the oven (about 1 hour), do not dry the cap in 
the oven: It will melt.  After the vial cools to room temperature, weigh 
the vial and cap. Determine the weight in grams of MOM by difference.

3) Add a measured amount of standardized 0.1M HCl with a buret into an 
Erlenmeyer flask which contains the sample of MOM 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 0.1M 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 as 
follows:

HCl(L)*0.1(M/L) - NaOH(L)*0.1(M/L) = excess moles of HCl

Moles Mg(OH)2 remaining = excess moles of HCl * (1 mole Mg(OH)2 / 2 moles 
HCl)

Wt Mg(OH)2 in grams = Moles Mg(OH)2 * 58.32 grams/mole Mg(OH)2

Wt % Mg(OH)2 in MOM = 100 * Wt Mg(OH)2 in grams / Wt MOM in grams