Borax or Sod. borate or
Sod. Tetraborate
Ch i l fformula
Chemical
l N
Na2B4O7.10H
10H2O
Mwt.= 381.4 gm
g
• Physical properties: Borax is sod.tetraborate
decahydrated, should be kept in well closed
container, soluble in 20 parts of water.
• Uses; antiseptic and astringent in mouth paints.
• Procedure: 10mL of unknown
10mL of distilled water
2drops of methyl red indicator
Titrate with N/2 HCl solution,
solution the color of sol
sol.
changes from yellow
pink–orange(1st end
point).
point)
Record the volume of N/2 HCl consumed.
Boil
o aand
d coo
cool tthee so
solution,
ut o ,
Add 4gm mannitol and 2drops of phenolphthalein
i d The
ind.
Th color
l off sol.l iis pink
i k
Now titrate with N/1 NaOH until the color of sol.
changes from pink yellow
pink(2nd end
point).
point)
Record the volume of N/1 NaOH consumed.
1st
titration
N/2 HCl solution
10mLL off unknown
10
k
10mL of D.W
2drops of methyl
red indicator
2nd titration
N/1 NaOH solution
Boil , cool and add
4gm mannitol and
2drops of ph.ph ind.
• Chemical principle:
™ Borax
B
is
i a salt
l off weakk acid
id with
i h strong b
base.
Na2B4O7.10H2O + 2HCl
2NaCl+ 4H3BO3+5H2O
™ Boric acid(H3BO3) will be liberated, it is very weak acid.
™ Boil the solution to remove CO2 .
™ Add mannitol because boric acid is weak acid to
convert it into strong complex acid which is strong
enough
h tto b
be tit
titrated
t d di
directly
tl with
ith N
NaOH.
OH
™ Mannitol should be used in high conc. to prevent the
hydrolysis of the complex
complex.
4H3BO3 + 4NaOH
Mannitol
4NaBO2 + 8H2O
sod. metaborate
™Mannitol should be used in high conc. to
prevent the hydrolysis of this complex.
• Chemical factor: home work
¾For borax with 0.5N HCl (0.09535 gm)
¾For borax with 1N NaOH (0.09535 gm)
¾For sod
sod. carbonate with 0.5N
0 5N HCl (0
(0.0265gm)
0265gm)
¾For boric acid with 1N NaOH (0.06184gm)
• Calculations:
we must first correct the volume of HCl & NaOH
1. If we have p
pure borax, the vol. of N/2 HCl must
be equal to the vol. of 1N NaOH.
V’of N/2
/ HCl = V’of 1N NaOH ((the differences up
p to 0.3 mL))
take the average of the two volumes & calculate the %
w/v of Borax.
2. Impure borax may contain Na2CO3 or boric acid
as impurities.
p
a) Mixture of Borax & Na2CO3
Na2CO3 is basic; it can react with HCl
HCl, so both
borax and carbonate will react with HCl
N 2CO3 + 2HCl
Na
2N Cl + H2O + CO2
2NaCl
Whilee NaOH
aO react
eact only
o y with
t boric
bo c acid
ac d liberated
be ated
from borax. Then after vol. correction;
V1 of N/2 HCl > V2 of 1N NaOH
V1- V2= vol.
vol of N/2 HCl react with Na2CO3
Calculate the % w/v for Borax and Na2CO3.
b) Mixture of Borax & boric acid
Na2B4O7.10H
10H2O + 2HCl
2NaCl+ 4H3BO3+5H2O
liberated boric acid
H3BO3 + NaOH
N OH
Mannitol
N BO2 + 2H2O
NaBO
Liberated & free boric acid
S b
So
both
th th
the lib
liberated
t d and
d ffree b
boric
i acid
id reactt with
ith NaOH,then
N OH th
Correct
Co
ect the
t e volumes
o u es and
a d co
compare:
pa e
V2 of 1N NaOH
> V1 of N/2 HCl
V2 - V1 = vol. of 1N NaOH react with free boric acid
C l l
Calculate
the
h % w/v
/ ffor Borax and
d ffree b
boric
i
acid.
Example on the assay of borax
A 10 mL- sample of borax solution containing 2.78 % w/v boric acid was
subjected to analysis using 0.54 N hydrochloric acid solution and 1.11 N sodium
hydroxide solution. If you know that the volume of sodium hydroxide solution
used in the second titration was 13.2 mL, what is the percent content of borax in
this sample?
(Note: refer to the periodic table to obtain the atomic weights so that you can
calculate the necessary chemical factors depending on the related balanced
chemical equations)
Answer
.
10
0.278 g of boric acid present as a contaminant in the 10 mL-
sample
Calculations of the chemical factors show that:
0.06184
1
0.09535
1
.
1
2
.
0.278
0.06184
4.5 mL the volume of 1 N sodium hydroxide solution reacted
with the free contaminating boric acid only
1.11
13.2
1
14.65 mL the total volume of 1 N sodium hydroxide solution reacted with
both the free contaminating and liberated boric acid.
14.65
4.5
10.15 mL the volume of N/2 HCl consumed in titrating borax (during
the first titration)
.
.
.
.
10.15
.
0.967 g the weight of borax in the sample
100
0.09535
96.9 % w/v the percent content of borax in the sample