Experiment 5 - Double Indicator Titration

MALAYAN COLLEGES LAGUNA

EXPERIMENT NO. 5

DOUBLE INDICATOR TITRATION

Experiment 5: Double Indicator Titration Page | 1

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I. OBJECTIVES

Upon completion of the experiment, the student should be able to:

determine qualitatively the components of a carbonate mixture;

determine quantitatively the components of a carbonate mixture;

calculate the percent of the components of a carbonate mixture in the original sample; and

apply the techniques involved in the analysis of unknown solutions for double indicator

titrations.

II. A. LABORATORY EQUIPMENT / INSTRUMENTS

Apparatus Quantity

50 mL beaker 1 250 mL Erlenmeyer flask 2 125 mL Erlenmeyer flask 2 100 mL volumetric flask 1 50 mL volumetric flask 1 Glass funnel 2 Buret holder 2 50 mL buret 2 10 mL pipet 1 10 mL graduated cylinder 2 Aspirator 1 Hot plate 1 Iron stand 1

B. CHEMICALS AND REAGENTS

Chemicals/Materials Quantity

Standard NaOH and HCl (from Exp 3) 1 Phenolphthalein 1 Methyl orange 1 10% BaCl2 solution 1 Unknown carbonate mixtures 2 Distilled water in wash bottle 1


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III. DISCUSSION OF FUNDAMENTALS

Introduction

In the previous experiments, it is cited there that there are many types of titrations, and they

differentiate according to what type of reaction it is. This experiment talks about an acid-base

titration reaction, except that it revolves under a wider concept than the former experiments done,

which is called as double-indicator titrations. Double indicator titration is defined as another

titration method that is applied on the observation of polyprotic bases and acids. Practically,

polyprotic acids and bases contain more than one equivalence point, and for which different

indicators should be used in the process to determine the different endpoints of different pH. In this

experiment, the carbonate species is to be used. Carbonate is in the form of CO32-, meaning it can

protonate twice, leaving NaHCO3, which when dissolved in water, Na+ becomes a counter ion, and

what’s left is the bicarbonate, an amphiprotic molecule, which can act as a base (deprotonate the H)

or as an acid (become carbonic acid). In its bicarbonate stage, however, from the carbonic acid, it

can readily act with the hydronium ions present in the solution, and form carbon dioxide and water.

The reaction of carbonate to H3O+ forming bicarbonate falls on a pH 9.0-10.0, from which methyl

orange is the one which can detect this pH change. With these stepwise reactions, one must not

need to perform a double indicator titration to analyze the content of the solution. Another example

for doing double indicator titration is on mixed samples containing Na2CO3 (CO32- counterpart),

NaHCO3 and NaOH. By doing qualitative analysis, the possible contents of the sample are to be

determined, and by that, one can now compute for the percentage of the components present in

the solution.

Body

Double indicator titration is a titration method applied to analysis of polyprotic acids and

polybasic bases. These samples pose more than one equivalence point and thus would need more

than one indicator to determine its endpoint. Sodium carbonate is one example of a polybasic base

and can be titrated to give two end points corresponding to the stepwise additions of protons to

form bicarbonate and carbon dioxide, which entails equal volumes of the same acid for each step as

shown below:

CO32- + H3O

+ HCO3- + H2O

HCO3- + H3O

+ CO2 + H2O


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The first step is completed at pH 9-10, where the appropriate indicator is phenolphthalein and

the next step is completed at pH 4-5, for which methyl orange is suitable.

Double indicator titrations can also be done when the samples contain a combination of any of

the following bases, NaOH, NaHCO3, and / or Na2CO3. The first part of the analysis of a carbonate

mixture requires the determination of the individual components of the mixture. After which, the

amounts of each of the components can then be determined. The qualitative analysis of the

composition is done by titrating two identical samples with the same standard acid where one

sample makes use of phenolphthalein as the indicator and the other makes use of methyl orange.

The volumes of acid required to reach the endpoint for each titration is then compared to

determine the composition of the mixture (Table 1).

Table 1. Qualitative analysis of carbonate mixtures using volumes at end points

Component present Volume relationship of standard acid used to reach end points*

NaOH Vph = Vmo

NaHCO3 Vph = 0

Na2CO3 Vph = ½ Vmo

NaOH - Na2CO3 Vph > ½ Vmo

NaHCO3 - Na2CO3 Vph < ½ Vmo

*Vph – volume of acid used to reach end point using phenolphthalein

Vmo – volume of acid used to reach end point using methyl orange

The combination of NaOH and NaHCO3 results to an unstable mixture since reaction of these

two bases produces carbonate and water. The procedure requires additional steps to determine

contributions of each component.

This experiment would make use of this single titration using two indicators for the qualitative

analysis of the mixture while a modified procedure will be done for the quantitative analysis part.

The procedure is modified since the single titration technique is not precise in actual titrations for

quantitative analysis. This is because combination of these bases results to a buffer system that may

interfere with the phenolphthalein endpoint. The modified procedure adds the step wherein the

carbonate that is present in the sample will be precipitated with BaCl2 leaving the NaOH component

in solution to be titrated with HCl until the phenolphthalein endpoint while the total basic strength

of the mixture is determined by titration with the same standard acid until the methyl orange

endpoint.


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Pre-Laboratory Questions

1. Define dilution, dilution factor, and concentration factor. Calculate the volume of the original sample needed if you would use a 50 mL volumetric flask, a 100 mL volumetric flask, and a 250 mL volumetric flask to prepare a dilution of 1:25. Answer: Dilution – defined as the weakening of concentration of a solution by adding a similar

solvent, i. e. water.

Dilution factor – A factor that tells by how diluted the solution is from its original

concentrations.

Concentration factor – its original concentration derived off from the dilution factor, cf = 1-df.

a. 50-mL

b. 100 mL

c. 250 mL

2. Write the precipitation reaction involved when BaCl2 is added to a mixture containing sodium carbonate. Answer:

3. Suggest a method to test whether a precipitation reaction is already complete.

Answer: By putting a small amount of the unknown solution in a watchglass and putting barium chloride in it. If there was no precipitate that formed into the sample, then we can say that the precipitation is already complete.


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Application Double indicator titrations mainly work out for mixtures with sodium carbonate, or hence referred to as soda ash. Soda ash is a versatile substance in the industry because of its many uses. Some of its practical applications would be glass making, petroleum refining, and many among others. This way, quality control can determine the sodium carbonate composition of a certain raw material to know and control the overall yield from it, and with this, double indicator titrations is used.

IV. METHODOLOGY

A. Sample Preparation

B. Qualitative analysis of the components of the mixture

1. Methyl orange as indicator

A diluted sample was prepared from the original stock with

a dilution factor of 1:25

The solution was further diluted to the mark with a

previously boiled and cooled distilled water.

All important data were recorded (dilution factor, volume of

sample, total volume)

From the prepared sample, a 10-mL aliquot was transferred

into an Erlenmeyer flask.


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2. Phenolphthalein as indicator

3. Determination of components:

The volume relationships (Vph and ½ Vmo) were determined and based on

these, the components of the unknown mixture were identified

4 drops of methyl orange was added. The solution was

titrated with the standard HCl solution until the peach

endpoint.

The volume of the HCl used was recorded as the Vmo.

From the prepared sample, a 10-mL aliquot was transferred

into an Erlenmeyer flask.

2 drops of phenolphthalein was added. The solution was

titrated with the standard HCl solution until the faint pink

endpoint.

The volume of the HCl used was recorded as the Vph.


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C. Quantitative analysis of NaHCO3-Na2CO3



10-mL aliquot was transferred into an Erlenmeyer flask.

4 drops methyl orange was added. The solution was titrated

with the HCl solution until 2 mL less than what was obtained

in part B.1.

The solution was boiled for 2-3 minutes, cooled to room

temperature and was again titrated until the peach

endpoint. Volume used was recorded as Vmo.


Using the base buret, 25 mL of the standard NaOH was

added to the solution.

10 mL of 10% BaCL was immediately added. A precipitate

formed and was let to settle


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3. Blank preparation

D. Quantitative analysis of NaOH-Na2CO3



immediately titrated to the faint pink endpoint with HCl

A blank consisting of 10 mL of the boiled and cooled distilled

water, 10 mL of 10% BaCl, and the same volume of standard

NaOH solution from part 2 was put in an Erlenmeyer flask. 2

drops phenolphthalein was added and was then titrated

with standard HCl solution until faint pink endpoint.

Difference between HCl used from parts 2 and 3 was

determined as the measure of NaHCO3 present (Vmo)


4 drops methyl orange were added and the solution was

titrated with the standard HCl solution until 2 mL less than

B. 1.

Solution was boiled for 2-3 minutes, cooled, and was

titrated again until the peach endpoint. Volume used was

recorded as Vmo.


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V. DESCRIPTION OF THE APPARATUS/SET-UP

Figure 2. Burette Figure 2.1 Burette with labels


10% BaCL was immediately slowly until no further

precipitation occurred.


immediately titrated with the standard HCl solution until the

faint pink endpoint. Volume used was recorded as Vph.


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A burette (commonly spelled as buret), is a laboratory apparatus that is mainly used for

quantitative chemical analyses of liquids. It consists of a long, graduated glass tube with a stopcock

(in a liquid burette’s case, on the bottom) that is handled by a burette clamp, which is connected to

an iron stand. The volume that the burette dispenses is controlled by the stopcock, and is accurately

measured by the graduations of the glass tube.

VI. DATA SHEET Table 1. Qualitative analysis

SAMPLE A SAMPLE B

Vmo (mL) 35.6 34.3 Vph (mL) 10.6 25.7

A. Since ½ Vmo > Vph, components of mixture are NaHCO3 – Na2CO3.

B. Since ½ Vmo > Vph, components are NaOH –Na2CO3.

Table 2. Quantitative analysis

Sample A (NaHCO3-Na2CO3) — Vol. aliquot = 20.0 mL

Vmo Initial volume 33.6 Volume after boling 1.2 Total volume (Vmo) 34.8 Vph Volume if BaCl2 10.0 Volume of NaOH 25.0 Vph 19.0 Sample B (NaOH-Na2CO3) — Vol. aliquot = 20.0 mL Vmo Initial volume 32.3 Volume after boiling 2.9 Total volume (Vmo) 35.3 Vph Volume of BaCl2 0.5 Vph 19.0

Table 3. Blank analysis

Vol NaOH 10.0

Vol BaCl2 25.0 Vol from titration 10.0 Vol aliquot 20.0


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VII. SAMPLE COMPUTATIONS

( )

( )( )

( ) ( )( )

A.

%Na2CO3 = ( )( )

%NaHCO3 = ( )( )

B.

%NaOH = ( )( )

%Na2CO3 = ( )( )

Average:

A.

B.

VIII. RESULTS AND DISCUSSION

This experiment widely focuses on the analysis of sodium carbonate solutions and mixtures,

which is commonly known as soda ash. It is used in a widespread variety of industrial work, such as

glass making, petroleum refining, detergent manufacture, water treatment, etc. On a regular basis,

one can find a raw sample of pure sodium carbonate, or what would be a mixture of soda ash,

sodium bicarbonate and sodium hydroxide. The three can form a mixture ideally with its respective

percentage compositions. These compositions are determined using acid-base titrations, which are

upped to another level, which we will call as double indicator titrations.

We can say that there should be a mixture of the three, but it is implausible to have a mixture of

NaOH and sodium bicarbonate. This is because they will react to form more carbonate, by the

reaction:


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This reaction is undesirable for it will decrease the hydroxide and bicarbonate ion

concentrations, ultimately altering the overall result of the experiment.

The reaction between soda ash and hydrochloric acid (HCl) occurs in two stages for

neutralization, with the formation of the bicarbonate ion as an intermediate product, as follows:

(1)

(2)

The first equation shows the production of bicarbonate ion intermediate which will then be used

for the second equation, which shows the bicarbonate ion, is consumed in the second titration using

excess HCl from the first titration.

Since there is a possibility that sodium hydroxide is in the solution, the reaction goes on like a

neutralization reaction by the equation:

(1)

Stoichiometry confines each of the above reactions to react according to a mole ratio of 1:1. This

means, from the second equation, the number of mole of hydrochloric acid determined from the

methyl orange titration is equal to the number of moles of sodium bicarbonate. Likewise, the total

number of moles of sodium hydroxide and soda ash in the solution can then be computed according

to the volumes of HCl added accordingly to form the phenolphthalein and methyl orange endpoints,

respectively.

From the graph showing the relation

between the pH of soda ash vs. the

volume titrant (HCl) added, we can point

out a specification that soda ash

contains two endpoints, which is

characterized by two indicators,

phenolphthalein first, and then methyl

orange. The phenolphthalein endpoint

designates the visualization of the

neutralized carbonate ion as it was in a

basic medium, and the methyl orange

endpoint indicates the neutralization of

the bicarbonate ion. Figure 1. Titration curve of soda ash.


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In this experiment, the purity of soda ash would be determined by double indicator titration

using HCl as the titrant, with soda ash being the primary standard, and using two indicators. The

question remains if the sample has inert impurities, and in doing so the titration process is done in

two steps, differentiated by the indicator used.

The experiment begins by diluting two different samples from its original stock using a 100 mL

volumetric flask with previously boiled and cooled distilled water with a ratio of 1:25. The distilled

water that is used must be first boiled and cooled to eliminate most impurities. Also, it removes the

alkaline earth carbonates that reside in the water solution, and as well as the dissolved carbon

dioxide gas in the water which may then produce carbonic acid when dissolved in water, as an

equilibrium process defined by the second equation. Carbonic acid build-up that was not originally

in the analyte might react to the components of soda ash, and will cause alterations in the

percentage components. By getting the dilution factor, the volume of the sample (actual) and the

total volume, the samples will now go through qualitative analysis. This is the most tedious part of

the experiment, because if this step fails, then the next steps that would follow would invalidate

everything up. The qualitative analysis of the sample should be done separately (one for each

indicator used), and the two samples should be labelled accordingly to avoid mix-up.

Using methyl orange as the indicator, transfer 10.0 mL aliquot of the sample into an Erlenmeyer

flask. Then add 4 drops of methyl orange into the aliquot and titrate it with the standard HCl

solution until the peach endpoint. The volume used in this will be labelled as Vmo. Using

phenolphthalein as the indicator, transfer 10.0 mL aliquot of the sample into an Erlenmeyer flask.

Then add 2-3 drops of phenolphthalein indicator on the aliquot and titrate it with the standard HCl

solution until the purple solution reaches its faint pink endpoint. The volume that is used will be

labelled as Vph. By using relations of Vmo and Vph using Table 1 (in the Body, Part II), the components

of the two samples will be determined and is checked by our instructor. When this process is already

done, we can now proceed to the quantitative analysis of the components. This analysis will now be

different in approach for the different components, and if the two samples are swapped of their

procedures, then grave errors would be committed, rendering a full repetition of the whole process.

The quantitative analysis goes on two parts: one being for the sample containing sodium

bicarbonate + sodium carbonate, and the one for the sample that contains sodium hydroxide + soda

ash.

For the quantitative analysis with the sample containing sodium bicarbonate + sodium

carbonate, it begins with the test using methyl orange as an indicator. An exact 10 mL aliquot of the

sample is transferred into an Erlenmeyer flask. Then 4 drops of methyl orange is added into the


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sample and is set to be titrated by the standard HCl solution with 2 mL less from what is determined

from the qualitative analysis part. The under-titrated sample is then heated to a boil for 2-3 minutes

and is cooled to room temperature. After cooling, the titration process is continued until the peach

endpoint. The volume used will be labeled as Vmo of the sample. After that, by using the

phenolphthalein indicator, another 10 mL aliquot is transferred into an Erlenmeyer flask. Using

another buret, an exact 25.0 mL of standard 0.05N NaOH is added to the solution. Thereinafter, an

exact 10 mL of 10% barium chloride is immediately added to the solution, and is swirled and is

settled to let the precipitate stand. After that, 2 drops of phenolphthalein indicator is added to the

sample and is titrated with the standard HCl solution until the faint pink endpoint. Since this sample

contains sodium bicarbonate, a blank should be prepared so that the carbon dioxide dissolved in the

distilled water can be accounted for. By mixing 10 mL of the boiled and cooled distilled water, 10 mL

of 10% barium chloride and 25 mL 0.05N NaOH, it is titrated with the standard HCl solution (with the

addition of 2 drops phenolphthalein indicator) to the faint pink endpoint. The difference between

the volumes of HCl used in the blank and the sample is recorded as the sodium bicarbonate that is

present and is labeled as Vph.

For the quantitative analysis with the sample containing sodium hydroxide + soda ash, it begins

with the test using methyl orange as an indicator. An exact 10 mL aliquot of the sample is

transferred into an Erlenmeyer flask. Then 4 drops of methyl orange is added into the sample and is

set to be titrated by the standard HCl solution with 2 mL less from what is determined from the

qualitative analysis part. The under-titrated sample is then heated to a boil for 2-3 minutes and is

cooled to room temperature. After cooling, the titration process is continued until the peach

endpoint. The volume used will be labeled as Vmo of the sample. It is as the same as the first sample,

since they both contain sodium carbonate, which is determined by the methyl orange titration

process. For the phenolphthalein indicator, the same process is used, except that there would be no

blank preparation, there would be no sodium hydroxide that will be added (since it is already there,

and there would be no need to add one for aesthetic purposes), and the barium chloride solution is

only added drop by drop until no further precipitation occurs (which can be tested by ordinary

means).

After getting all data, the percentage composition of the components for each of the sample is

expressed in %w/v, and is done in two trials for better accuracy.


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IX. SUMMARY AND CONCLUSION

This experiment proved upon the concepts involving one type of titration—Double indicator

titration. Throughout the experiment, we determined the qualitative components of two solutions.

The common wall between the two is that both contain Na2CO3, which is the counterpart of CO32-,

and will be the reason why the double indicator titration is performed. By using comparative

analysis between the volume used to reach phenolphthalein endpoint and the volume used to

reached the methyl orange endpoint, that will determine if the solution is mixed with NaOH or

NaHCO3, Since the two mixtures have different components, it is also a given that their respective

methods for determining the percentage composition on the solution is also different, Though, their

difference is pointed out on preparing a blank for the NaHCO3 + Na2CO3 solution, and by only using a

small amount of 10% BaCl2 solution (drop by drop) on the NaOH-Na2CO3 solution using a relatively

bigger (fixed) amount for the NaHCO3-Na2CO3. By getting all necessary volumes, the percentage of

the components is determined by w/v relatively, and multiplying it by the dilution factor (since it is

diluted). Therefore water is the major component, not the ones stated above.

Various errors might occur from the whole process on throughout. It may come as an

uncertainty in measurement and pretty much similar to that. The uncertainties of the mass of the

samples on beforehand can also contribute to the errors in the solution as they are mixed. The

uncertainties in measurements in the solutions can cause increase or decrease in volume (as it is an

uncertainty) that will be needed for the solutions that are prepared, and can change the results in

the computation process. Errors in reading the volume of the titrant can cause errors in the

calculated HCl concentrations (additionally, if the standardization process of HCl is not carried out

accurately, then chances are it will ultimately deter the quality of the experiment’s final results), as

inversely proportional error relating volume and concentration will result. Another possible source

of error is the determination of the endpoint of the titration since it was only based on color change

of the solution and there is no real basis as to what should be the perfect shade in telling the

endpoint (and since it was only dependent on an indicator, there would be a titration error

differentiating endpoint and equivalence point). This was always the case, and we students are left

behind with the possible explanations as to when the endpoint was actually reached, as some would

decide that the endpoint was already reached earlier (since the color should only persist for about

25 seconds and it would be debatable enough if it is just estimated), or rather some would already

see the phantom pink color that is not seen by others. With this, this can increase uncertainties in

the determination of the volume of HCl used. Lastly, another plausible source of error is the

dissolved carbon dioxide that is in the solution, and can interfere with the methyl orange color, and

thus altering the visual endpoint.


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It is highly recommended the use of well calibrated instrument and glassware for

measurements. One must make sure that the glassware used in solution preparation was clean and

free of chemical contamination. Careful and correct burette reading of the volume of the titrant

used should also be observed. The observation of the color change which signifies the endpoint of

the titration for both indicators should be signified, and can be verified by doing multiple trials of

accurate dispositions

X. REFERENCES

Christian, Gary D. 2004. Analytical chemistry (6th ed.). John Wiley and Sons Inc.

Hage, David S. and James D. Carr. 2011. Analytical chemistry and quantitative analysis. New

Jersey: Pearson Prentice Hall.

Harris, Daniel C. 2003. Quantitative chemical analysis. (6th ed). New York: W. H. Freeman and

Company.

Madamba, Lilia S.P. 1995. Chemistry 32 Laboratory Instruction Manual (3rd rev). Los Baños:

Analytical and Environmental Chemistry Division, Institute of Chemistry, University of the Philippines

Los Baños.

Skoog, Douglas et. al. 2004. Fundamentals of Analytical Chemistry (8th ed.). Singapore:

Thomson Learning.

Experiment 5 - Double Indicator Titration

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