How to determine Trihydrate alumina in Bauxite by low temperature digestion.

1. PAGE 1 OF 2
Determination of THA (Trihydrate Alumina) in Bauxite
Reference: Lab in house developed method
Brief Description and its Principle :
The sample is digested in a Bomb digester using NaOH solution at 145º Centigrade
and Solution Made up to 500 ml and Dry filtered a suitable aliquot is taken and after
adding known amount of E.D.T.A Solution and PH adjusted to 5.3 to 5.5 and titrated
with ZnSo4 Solution using Xylenol orange as indicator .
Apparatus
1. Balance
2. Glass ware
3. Bomb Digester
Reagents
1. Xylenol Orange ( 0.5 % )
Dissolve 0.5 gm of xylenol orange in 100 cc water. Add 2 drops of dilute
hydrochloric acid
2. Digesting Solution-165 gpl ( as Na2CO3)
Dissolve 127.069 gm of NaOH pellets in 1000 cc DM water.
3. Methyl Red
Dissolve 0.5 gm of indicator in 100 cc Spirit.
4. EDTA (0.01 M)
Dissolve 3.7224 gm of EDTA in 1000cc of DM water.
5. Zinc Sulfate solution (0.01M)
Dissolve 2.8754 gms of zinc Sulfate in 50 cc of DM water a transfer into 1000ml
volumetric flask.
6. Buffer Solution ( pH 5.3)
7. Dissolve 21.5 gm of ammonium acetate in 300 cc distilled water and 2.0 l of
glacial acetic acid make the volume to 1000ml.

2. PAGE 2 OF 2
Procedure
Weigh 0.5000 gms of sample and place it in 50 ml bomb add 25 ml of Digesting
solution Cap the Bomb tightly and Mix the contents. Heat the bomb in Aluminum
block heaters to 145º Centigrade by adjusting Temperature Knobs and let the bomb
rotate for 30 minutes at 145º Centigrade. (Fix the bomb in Aluminum block heaters
when the instrument reaches set point 145º). Remove The bomb and cool and
transfer the contents to 500 ml Volumetric flask using Distilled water and Dry Filter
Using 1 Whatman Filter Paper and take an aliquot around 50 ml in 250 ml conical
Flask. Add 50 cc of 0.01 M EDTA, 1 to 2 drops of methyl red indicator .acidifiy with
1:1Hcl (adjust the Ph 5.3 to 5.5)
1st Titration
Add 15 cc buffer solution and boil for about 5 mins. Cool and about 3 – 4 drops of
xylenol orange indicator and titrate with std . Zinc Sulfate solution till colour change
from yellow to red.
2nd Titration
`Add about 2 gm of Ammonium fluoride in the above solution and boil for 5 – 10 mins.
Cool and add 15cc of buffer solution and about 2 to 3 drops of xylenol orange
indicator and titrate against std. Zinc Sulfate solution till the colour changes from
yellow to red.
Note the volume of zinc sulfate used (V)
Calculation
Trihydrate Alumina (THA) = V x M x 0.05098 x 100
W
Where,
V = ml of zinc sulfate solution required for second titration.
M = Molarity of the zinc sulfate solution W = Weight of the sample taken
W = Weight of the sample taken