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Gravimetric AnalysisDetermination of Sulfur in Soluble

Sulfate

Hans Del Mundo

 Teresa Dacalanio

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 Types of Gravimetric Methods

!" #recipitation method

•. the analyte is separated from a solution ofthe sample as a precipitate and isconverted to a compound of $noncomposition that can be eighed

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%" &olatili'ation method

• the analyte is separated from otherconstituents of a sample by conversion toa gas of $non chemical composition

• the eight of this gas then serves as ameasure of the analyte concentration

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Steps in Gravimetric Analysis

!" Preparation of the Solution(

) may involve several steps including ad*ustment of

the pH of the solution in order for the precipitate tooccur quantitatively and get a precipitate of desiredproperties+ removing interferences+ ad*usting thevolume of the sample to suit the amount ofprecipitating agent to be added"

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%" Precipitation: 

-  this requires addition of a precipitating agentsolution to the sample solution"

- upon addition of the ,rst drops of the precipitatingagent+ supersaturation  occurs+ then nucleation

-formation of smallest molecules hich can grointo a large precipitate. starts to occur

-Induction period / the time beteen mi0ing andthe appearance of precipitate.

-

 at this point+ addition of e0tra precipitating agentill either form ne nuclei or ill build up one0isting nuclei to give a precipitate"

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-Von eimarn ratio states that the particle si'e isinversely proportional to a quantity called therelative supersaturation 

-  the 1 is the concentration of reactants beforeprecipitation+ S is the solubility of precipitate in the

medium from hich it is being precipitated" Therefore+ in order to get particle groth instead offurther nucleation e need to ma$e the relativesupersaturation ratio as small as possible"

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 The optimum con!itions for precipitation"hich ma#e the supersaturation lo" are:

 

a" #recipitation using !ilute solutions to decrease1

b" Slo" a!!ition  of precipitating agent to $eep 1as lo as possible

c" Stirring the solution during addition ofprecipitating agent to avoid concentration sitesand $eep 1 lo

d" Increase solubility by precipitation from hot

solutione" A!$ust the p% in order to increase S but not a

too much increase as e do not ant to looseprecipitate by dissolution

f" 2sually add a little e&cess of the precipitating

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3" 'igestion of the Precipitate(

-  The precipitate is left hot -belo boiling. for 34min to ! hour in order for the particles to bedigested"

- Digestion involves dissolution of small particlesand reprecipitation on larger ones resulting inparticle groth and better precipitatecharacteristics" This process is called (st"al!ripening"

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•  Adsorption is a major problem in gravimetry in case ofcolloidal precipitate since a precipitate tends to adsorb

its own ions present in excess, therefore forming what is

called a primary ion layer which attracts ions from

solution forming a secondary or counter ion layer• Individual particles repel each other keeping the colloidal

properties of the precipitate.

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•

Particle coagulation can be forced by either digestionor addition of a high concentration of a diverse ions

strong electrolytic solution in order to shield the charges

on colloidal particles and force agglomeration

• Usually, coagulated particles return to the colloidal stateif washed with water, a process called peptization

5 hi ! )il i h

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5" ashing an! )iltering thePrecipitate(

-

  the precipitate should be washed very well in order toremove all adsorbed species which will affect the weight if

the precipitate

- should be careful not to use too much water since part of

the precipitate may be lost

- in case of colloidal precipitates we should not use water

as a washing solution since peptization  would occur

instead, dilute nitric acid, ammonium nitrate, or dilute acetic

acid may be used

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• It is a good practice to check for the presence of

precipitating agent in the filtrate of the final washing

solution because presence of precipitating agent means

that extra washing is required.

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6" 'rying an! Ignition(

- the purpose of drying -heating at about !%4)!64  o7in an oven. or ignition at temperatures ranging from844)!%44 o7 is to get a material ith e0actly $nonchemical structure and remove moisture and

volatile electrolytes

-  this is done until the precipitate reaches aconstant eight

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8" eighing the Precipitate(

) the precipitate is preferred to be eighed in ananalytical balance for a more accurate result

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9" *alculating the Results(

) the result of a gravimetric determination is usuallyreported as a percentage of analyte(

: analyte ; eight of analyte 0 !44 :

  eight of sample

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•  The precipitate is seldom the analyte itself+ but ade,nite relationship beteen the to e0ists"

•  This relationship is called gravimetric factorand is used to convert the $non eight of theprecipitate to the corresponding eight of theanalyte

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Impurities in Precipitates

+, *oprecipitation

•.  This is anything unanted hich precipitates iththe desired precipitate

•.

7oprecipitation occurs to some degree in everygravimetric analysis -especially barium sulfateand those involving hydrous o0ides."

•. 7annot be avoided ) all you can do is minimi'e itby careful precipitation and thorough ashing"

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-, Surface a!sorption

•   hen ion layers are not removed from theprecipitate

• 2nanted material is adsorbed onto the surfaceof the precipitate"

• Digestion of a precipitate reduces the amount ofsurface area and hence the area available forsurface adsorption"

•

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., (cclusion

•  This is a type of coprecipitation in hichimpurities are trapped during particle groth" Thetrapped materials can be ater+ analyte ions+precipitating agent ions+ or other constituents in

the medium"

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/, Postprecipitation

•  There are ions other than analyte ions hich formprecipitates ith the precipitating agent but atmuch sloer rate then analyte+ and if theprecipitate of the analyte is left for a long time

ithout ,ltration+ then the other ions startforming a precipitate over the original precipitateleading to positive error"

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E0PERIME1TA2

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=b*ectives

!. Determine the constant eight of aprecipitate"

%. #recipitate S=3 in a sample as >aS=5"3. 7alculate the : of S=3 in a sample from

the eight of barium sulfate"

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he sample should be dried in an oven at !"" #$ to !%"#$. &eigh out accurately ".'

g of sample and dissolve it in about %"" m( of distilled water and ! m( of

concentrated hydrochloric acid. )repare a * + solution of barium chloride by

dissolving %.* g of a$l%•%%# in *" m( water. $alculate the volume of this solution

required to precipitate the sulfate in each sample, including a !"+ excess. easurethis volume of solution into a clean beaker using a graduated cylinder.

&eight of /ample 01 ".'"23 g

*+ a$l% solution1

g a$l%. %%# 4 %.* g a$l% x ! mol a$l% x ! mol a$l%. %%# x %55.%36 g%"2.%%6 g ! mol a$l%  ! mol a$l%. %%#

4 %.7'%6 g a$l%. %%# in *" m( solution& of /ample 0 89/#5:1 !'%.!5 g;mol 9/#5 < a$l% -= 9$l% < a/#5

g a$l%. %%# 4 ".'"23 g x ! mol 0 x !mol /#5 %- x ! mol a %< x ! mol a$l%  !'%.!5 g ! mol 0 !mol /#5 %- ! mol a %<x ! mol a$l%. %%# x %55.%36 g

! mol a$l%  ! mol a$l%. %%#

  4 ".*6"53 g a$l%. %%#

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m( a$l% solution 4 ".*6"53 g a$l%. %# x *" m( a$l% solution

  %.7'%6 g a$l%. %#4 7.6%*2 m( a$l% solution

!" + excess1 7.6%*2 m( x !.! 4 !".3725 m( a$l% solution

eat both sample solution and the solution of barium chloride nearly to boiling. )our

the hot barium chloride solution quickly but carefully into the hot sample and stir

vigorously. 0llow the precipitate to settle and test the supernatant liquid for

completeness of precipitation by adding a few more drops of barium chloride

solution.

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 0fter precipitation is complete, cover the beaker with a watch glass and allow the

precipitate to digest for '" minutes, keeping the solution hot 82" to 7"   #$: on a

steam bath, on a hot plate, or using a low flame.

$ollect the precipitate using a funnel with filter paper 8the slow type &hatman

>o.5% should be used unless the precipitate has been unusually well digested, in

which case >o. 5" may suffice:.

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$lean, rinse and heat to constant weight three crucibles and lids of the ordinary

porcelain. Use the highest temperature of the irrill burner.

Trial Mass (Crucible w/ cover)

! ''."%** g

% ''."%5! g

' %%."%'% g

5 %%."%%! g

* -----------

he solution must be hot at the

time of filtration. ?ecant the clear

supernatant solution through the

filter. ?iscard the clear filtrate so

that if the precipitate later runs

through the filter paper, only a

small volume need to be refiltered.

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hen rinse the precipitate into the funnel with hot water. @emove any precipitate

from the walls of the beaker and rinse such particles into the filter paper with hot

water. If the filtrate is cloudy, it must be refiltered, in which case, the second

passage generally clears it up. $ontinue to rinse the precipitate in the filter paper

with hot water until a drop of silver nitrate solution added to a test portion of thewashings collected in a test tube shows that chloride is absent.

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 0fter washing is complete, transfer the paper and precipitate carefully to the

previously prepared crucible. ?ry the precipitate slowly over a low flame or oven.

hen, increasing the heat, char the paper carefully and finally burn it off completely.

Ignite the precipitate for about %" min at the highest temperature of the irrill

burner. he crucible should be uncovered and in a slanted position for free accessof air to prevent reduction of barium sulfate to barium sulfide.

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$ool and weigh the crucible and its contentsA then reignite for a second %"-min

period. @epeat this procedure until constant weight is attained.

&eight of precipitate 4 8$onstant &eight of crucible < precipitate: B

8$onstant &eight of empty crucible:

Theoretical weight of precipitate1 ".!3'3 g 8for /ample 0:

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g /#' 4 ".'"23g 0 x !mol 0 x !mol /#' x 2"."3*g /#'

!'%.!5g 0 !mol /#5 !mol /#'

4 ".!26"g /#'

+ 8w;w: /#' 4 g /#'  x !"" +

 g sample

4 8".!26"g ; ".'"23 g: x !"" +

4 !"!# $%&

/ulfur is usually reported as sulfur trioxide. $alculate the percentage of sulfur

trioxide in the unknown sample and report the results.

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Discussion

•

Slightly e0cess precipitating agent ->a7l%

. asused in order to ensure complete precipitation"

•  The precipitate as alloed to digest for 34minutes in order to dissolve small particles andreprecipitate larger ones for better precipitatecharacteristics and easier ,ltration"

• Heating the crucibles to constant eight ensuresthe absence of impurities for a more accuratemeasurement of the eight if the precipitate

later"•

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ecommendations

• anage your time wisely most especially because of the

weighing part.

• e careful in handling the heated crucible and donCt

touch it with your bare hands.

• &hile forming the precipitate, add slowly the precipitating

agent and stir the solution vigorously to have a low

supersaturation ratio, thus to have a good precipitate.

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eferences

• /koog, ?., &est, ?., oller, D.., E $rouch, /. 8%""5:

Dundamentals of 0nalytical $hemistry 2th ed. U/0

• http1;;www.chem.tamu.edu;class;majors;tutorialnotefiles;gra

vimetric.htm

• http1;;www.monFirpal.net;$hem

+%"%'!";$ontents;GravimetricH0nalysis.htmH/tepsHinHa

• http1;;www.csudh.edu;oliver;demos;gravsulf;hhgrvslf.htm