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DOUBLE INDICATOR

TITRATION

Choosing an Indicator in Acid-Base Titrations
Titration curves are used to deternne which indicator(s) is/are suitable fora particular titration

Titration of NezCOs with HCI
enopointI mauesol purpa {,9*9
...
2-4
...
of trant

te

a

pil

tne

rd

ndie
...
0)

***

endpolnt 2methy oranga (3
...


p


...
2-2
...
4-1
...
0-5
...
6

-8
...
8-


...


ATId olo
orang
yclogreen

Base color

O n y r o l hiue 3nd p h n o l red

Sreenea indicato

8

6 b7
...
0

yelow

orngered

ACId C

green

green

dimethryf yellaw and methyfene blua

blue-green

bluevlolot

and metyl orane

*d-violet

romocreot green

violet-:
hlue

promoaeiol greon and chloropiionot red

metryi red and mett»ylene blue

ixed Indicator

? arin yeliovw

cor
moi blye

Dheroi red

ltma

Actd

Choosingan indicatorin Acid-Base

f

su, t

range

at the end-point

colors at the

Indicators are chOsen
...
if the reaction involves
strong base and a weak acid

strong acid and a weak base

Neutral if the reaction involves
a strong acid and a
strong base
Acidic
...


step 2 ind the equivalence point
I t is the steepest part of the curve

The equivalence point can be used to
determine the equivalent weight (molar

mass) of the acid

unsutable: azerin yekow (tco early)

Thration of NaOH with HC

good indicators: bromothymol bue,

Bomus and pheno red

unsutadle: orange IV (too late)

vol
...
methyl orange indicator (ptH-4)

Vino

volume required to reach the imo endpolnt

vofume requlred to reach phe endpoint

vpn *

Vph =Vmo
Vuoztr
Vph

where
Vmo

Co2+H
+

H

+

HOos
H20
Vph< Vmo
Ne2Co3 2Vph
NaHCO3Vmo-Vph

NaHcOg

mo endpoint

phe endpoint

has been converted to

HCO

Na,CO

cO

NaHCO with a Strong Acid

wRh he NaHCO, Is #he some as Uhat
a c t n g with the Na,C0,

(B NacO

ghenaiphiaieh thange1

AO h e i (oe

Is reached when

Step 1 phenolphthalein indicator (pH-9)

[used
certain indicators change color at different stages of a
tiwo end points in
the proportions of the components of certain mixtures byy observing
A
...
50em
Volume of HCI added I cmJ

NaCO

C
...
Titration of a Mixture containing Na,CO3 and Naol with a Strong Acid

oH)
A

-

2Vmo

Vpn

Vph> Vmo

taOH

Nn2C03

Is neutralized and
End polnt using phenolphthalelin is reached when NaOH
halfneutralized (1
...
converted to NaHCO)

neutralization reaction with

bcg endpoint

is

and

bromcresol green indicator

directly

points

(bcg)
NaHCO3 Mixture with a Strong Acid using Bromcresol green

the total volume for the 2 end

Na,c0, (conversion of C0,* to CO,)

strong acid using

StrongAcid using Bromcresol green (bcg) indicator

acid

reaction of NaHCO with an acid
End point using methyl orange is reached when complete
OCCurs
for the phenolphthalein end point
Volume acid methyl orange end polnt < volume

Indicator

TItration of Na,C03

If only Na,CO, is present Vph* Vn* Voxg Or Vph Vmo1/2 Vbog

using phe and methyl orange

Votume of the strong acid added to reach the

completes the

a t the start of the titration
...
Titration of Na,CO, with a

E
...
addition
of CO33 to cO2) and with NaHco3
the neutralization reaction with Na,CD3 (conversion

a

(HCO, to CO2)
In

NaHco, present in the

NaCO, to HOOS
with

F
...
200-g sample of a mixture of NaOH and Na,CO, with
with 0
...
With phenolphthalein as the indicator, thesolution turns colorless ater the adition

at the start of the titration
...
addition of the strong acid using phe indicator converts only the

NaCO, to HCO and the OH of NaOH is neutralized

30
...
Methyl orange is then added, and 5
...
500N

w
...
200g

Vto 5
...
00 ml

Using above Figure 1:
ls neutralized first
...
All this requires 30
...
00 ml of the acid, of the total 35
...
00 mL must have reacted with the Na;CO, and therefore, 25
...
00mL

x

asoo0 N

NaOH
/ml
1000 meq/ma

x

100

NaOH z

41
...
s000 N x * L O } p 7 o
2000 meq/moi

- x 10 22
...
5

Exercise:
A 1
...
NaHO, and inert impurities is ttreted cold with 0
...
and 22
...


NH
V
V2a0D

C

V C2

0-S9


...
SM) x

40A ghpet


...
5 0

NatCo vh

=CnonL



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