Lab4Analysis of Soda Ash

8/11/2019 Lab4Analysis of Soda Ash

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LEARNING OBJECTIVES

The objectives of this experiment are to ...

- to understand the titration curve for a diprotic base.

- to understand the difference between the equivalence point and end point of titration.

- to use the end point to calculate the percent of Na2CO3in an unnown sample

of soda ash.

BACKGROUND

!odium carbonate is an important industrial chemical. "t is used in the manufacture of soap#

$lass# paper and as a source of alalinit%# that is# as a base. &bout half the sodium carbonate used

in the 'nited !tates is manufactured b% the !olva% process. "n this process# which is carried outat ()C# carbon dioxide is bubbled throu$h a concentrated sodium chloride solution which is

saturated with ammonia. !odium h%dro$en carbonate precipitates from the solution and is

isolated*

CO2+$, N3+$, 2O+l, Na+aq, Cl-+aq,NaCO3+s, N/

+aq, Cl-+aq,

0hen heated to 3((oC# the sodium h%dro$en carbonate decomposes to form sodium carbonate#carbon dioxide# and water.

2NaCO3Na2CO3 CO2 2O

The crude product obtained from this thermal decomposition is called soda ash# which is

primaril% Na2CO3# althou$h it also contains unreacted NaCO3and other impurities.

The total acid neutrali1in$ capacit% of a soda ash sample# its alalinit% value# can be stated in

terms of percent sodium carbonate. "n doin$ so# an% sodium h%dro$en carbonate present in the

sample is converted to its equivalent neutrali1in$ capacit% in terms of sodium carbonate. That is#a (( NaCO3sample is represented as a 4( Na2CO3sample in terms of its acid

neutrali1ation capacit%. "t is not uncommon to $roup compounds of similar behavior to$ether

and state the composition in terms of one of them. 5or example# the potassium content offertili1er is stated as percent 62O# althou$h the potassium compounds actuall% present ma% be

phosphates# nitrates or carbonates. "n this case the relatin$ factor is the amount of potassium in a

sample of fertili1er.

ExperimentChemistry 21

Experiment !

&N&78!"! O5 !O9& &!


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Carbonate ion and h%dro$en carbonate ion are both sufficientl% basic to be quantitativel%

determined b% reaction with stron$ acid in a titration procedure. The neutrali1ation of carbonate

ion proceeds in two steps*

CO32- :-; CO3

- +h%dro$en carbonate or bicarbonate,

andCO3

- :-; 2CO3 +carbonic acid,

Note that althou$h we t%picall% represent carbonic acid as 2CO3# in aqueous solution there isvirtuall% no 2CO3present. To be accurate we should represent 2CO3as CO2 2O. Thus# the

titration of CO3-with is most accuratel% represented b% the equation*

CO3- CO2 2O

5or convenience# we will adopt the normal convention of representin$ carbonic acid as 2CO3.

The titration curve shown above has two breas as evidence of the two-step neutrali1ation

process. Note that the breas are not as sharp as those obtained when a stron$ base is titratedwith a stron$ acid. The first equivalence point# occurrin$ at a p of about /.? x (-

2CO3 :-; CO3

- 6a> /.4 x (-?

Notice that the two 6avalues differ b% about four orders of ma$nitude. "t is this relativel% lar$e

difference that allows us to see both equivalence points on the titration curve. =ecause CO3-is

a much weaer acid than 2CO3+or one could sa%# because CO32-is a much stron$er conju$ate

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base than CO3-, nearl% all the carbonate ion in the sample will be converted to CO3

-+at the

first equivalence point, before the second step of the neutrali1ation be$ins formin$ 2CO3

SA"ET# $RECAUTIONS

!afet% $o$$les must be worn in the lab at all times. &n% sin contacted b% chemicals should bewashed immediatel%.

BE"ORE $ER"OR%ING T&IS E'$ERI%ENT

. !tandardi1ed Cl has been provided.

2. The unnown soda ash sample has also been dried for two hours at (oC.

&s stated above# althou$h soda ash is mostl% Na2CO3# it contains some NaCO3. @iven the fact

that the two equivalence points show up on the titration curve# it should be possible to anal%1efor the percent Na2CO3in the sample b% usin$ the first equivalence point +the reaction is

CO32- CO3

-,. "n $oin$ from the first to the second equivalence point the h%dro$en

carbonate reacts to form carbonic acid +CO3- 2CO3,. There are two sources of CO3

-*

the CO3-formed from the CO3

2-in the first step of the titration and the CO3-from an%

NaCO3present in the ori$inal sample. "n principle we can calculate the amount of CO3-

formed from the CO32-b% nowin$ the moles of required to reach the first equivalence point.

=% subtractin$ this amount of CO3-from the total CO3

- determined in the second step of thetitration we could then calculate the amount of CO3

- ori$inall% present in the sample.

owever# we cannot carr% out these calculations ver% accuratel% because the first equivalence

point is not ver% distinct. That is# because the p chan$e at the first equivalence point is not ver%dramatic# we have trouble accuratel% specif%in$ the exact volume of acid required to reach the

first equivalence point. To avoid this problem we will use the more distinct second equivalencepoint and report the acid neutrali1in$ abilit% of the soda ash sample in terms of percent Na2CO3

+even thou$h a small part of the neutrali1in$ abilit% of the sample reall% comes from NaCO3,.

"n other words we will assume that the number of moles of Na2CO3in the soda ash sample

equals one-half the number of moles of stron$ acid required to reach the second equivalencepoint.

&lthou$h less precise# acidAbase indicators can be used to measure p. "nstead of measurin$ thep and determinin$ the equivalence point# we can use acidAbase indicators and determine the end

point of a titration where the indicators chan$e color. These are a $roup of wea acids and basesthat are hi$hl% colored or$anic d%es. Their chan$e in color depends on the p and serves as theend point of a titration# provided that their chan$e in color occurs at the equivalence point of the

titration. 0e will use phenolphthalein# to detect the first end point. Bhenolphthalein is pin

above p (.( and colorless below p


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E'$ERI%ENTAL $ROCEDURE

Titr(ti)n )* (n Un+n),n S)-( Ash S(mp.e

. =oil distilled water to expel an% dissolved carbon dioxide. Cool to room temperature.

9issolve approximatel% (.2( $ +record the exact mass, of the unnown soda ash in about


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5or example# assume %ou have dissolved (.24( $ sample of soda ash into


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ANAL#SIS O" SODA AS&

D(t( Sheet

%).(rity )* the st(n-(r-ie- &C. 333333333%

An(.ysis )* Un+n),n S)-( Ash

'nnown "dentification I JJJJJJJJJ

Color Chan$e at the stEnd Boint > JJJJJJJJJJtoJJJJJJJJJJJJColor Chan$e at the 2ndEnd point > JJJJJJJJJJtoJJJJJJJJJJJJ

Trial Trial 2 Trial 3 Trial / Trial 4

"nitial mass of soda ash +$,

Golume of titrant +C7,added. +2ndEnd Boint,

+m7,

oles of Na2CO3 in sample +mole,

ass of Na2CO3 in sample +$,

Na2CO3 in sample +,

&vera$e Bercent Na2CO3 JJJJJJJJJJJJ

&ctual Bercent Na2CO3 JJJJJJJJJJJJ +instructor use onl%,

error JJJJJJJJJJJJJ +instructor use onl%,

F


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&nal%sis of !oda &shBre 7aborator% &ssi$nment

NameJJJJJJJJJJJJJJJJJJJJJJJJ!ectionJJJJJJJJJJJJJJJJJJJJJJJ9ateJJJJJJJJJJJJJJJJJJJJJJJJJ

. 0rite an outline of the procedure for this experiment.

2. "n one experiment# %ou have dissolved (.2H< $ of soda ash into

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Lab4Analysis of Soda Ash