PREPARATION OF A STANDARD ACID SOLUTION Name: Eghan Kojo Index N o : 6138811 Experiment N 0 : A 2.2.3 Graduate Assistant: udith !dei "ate: 2 nd Apri#$ 2013
AIMS 1. To be able able to prepar preparee standa standard rd solut solution ions. s. 2. To determine determine the strength strength of a given solution solution of Hydrochl Hydrochloric oric acid acid (Hl!. (Hl!. ". To analy#e analy#e errors errors that occurs occurs during during standardi standardi#ation #ation e$perim e$periments. ents. I%T&')TI'% In analytical chemistry* a standard solution is a solution containing a precisely +no,n concentration of an element or a substance i.e.* a +no,n ,eight of solute is dissolved to ma+ e a specific volume. It is prepared using a standard substance* such as a primary standard. Standard solutions are used to determine the concentrations of other substances* such as solutions in titrations. The concentrations of standard solutions are normally e$pressed in units of moles per litre (mol-* often abbreviated to M for molarity!* moles per cubic decimetre (mol-dm"!* +ilomoles per cubic metre (+mol-m"! or in terms related to those used in particular titrations (such as titres!. Standardi#ation is doing a titration to ,or+ out the e$act concentration of the solution you ,ant to use to determine the concentration of an un+no,n un+ no,n solution. To do a titration titration you must to +no, the e$act number of moles of one of the reagents (titrant! you are using so that you can then determine the number of moles of the un+no,n reagent. A simple simple standard is obtained by the dilution of a single element or a substance in a soluble solvent ,ith ,hich it reacts. A solution of acid can be standardi#ed by titrating it against a solution of al+ali of +no,n concentration. 'nce this has been calculated* ca lculated* it can in turn be used as a standard solution to find the concentration of a solution of al+ali. Standard solutions are also commonly used to de termine the concentration of an analyte species. /y comparing the absorbance of the sample sa mple solution at a specific ,avelength to a series of standard solutions at differing +no,n concentrations of the analyte species* the concentration of the sample solution can be found via /eer0s a,. Any form of spectroscopy can be used in this ,ay so long as the analyte species has substantial absorbance in the spectra. The standard solution is a reference guide to discover the molarity of un+no,n species. Titration Titration methods can be used to acuire the concentration of a standard solution. These involve using euipment such as a burette.
ood standardi#ing reagents should have the follo,ing characteristics3 They should have a high degree of purity. They should be stable and unaffected by the atmosphere* i.e. they should not be efflorescent or deliuescent* for easy ,eighing. The process used to determine the concentration conce ntration of a solution ,ith very high accuracy accur acy is called standardi#ing a solution. To standardi#e an un+no,n solution* you react that solution ,ith another solution ,hose concentration is already +no,n very accurately. In a standardi#ation e$periment* the solution being standardi#ed is compared to a +no,n standard. This +no,n standard can be either a solution that is already a standard solution or an accurately ,eighed solid material. The accuracy of standardi#ation depends on the glass,are and uality of the reagent used to prepare the standard. &eagents &eagen ts used as standards are divided into primary reagent an d secondary reagent. A primary reagent can be used to prepare a standard containing an accurately +no,n amount of analyte. an alyte. A primary primary reagent must have a +no,n stoichiometry* a +no,n purity (or assay! and be stable during a long term storage both in solid and solution form. The purity of a secondary reagent in a solid form or the concentration of a standard prepared from a secondary reagent must be determined relative to a primary reagent There are commercially available acids as concen trated solutions. T,o T,o of these acids ,idely used in the preparation of standard solutions of acids are hydrochloric acid and sulphuric acid. oncentered hydrochloric acid is about 14.5 6 12M and sulphuric acid is about 17M. 8rom these concentrated solutions* solutions of any desired appro$imate concentration can be prepared. p repared. If a solution of an e$act e$ac t concentration is reuired* a solution of an appro$imate concentration is first prepared by diluting appropriate volume of the concentrated acid ,ith distilled ,ater and this is standardi#ed against some al+aline substance such as an hydrous sodium carbonate. 8or e$ample* to standardi#e the hydrochloric acid solution ,e made up in a preceding lab* ,e might very carefully measure a +no,n uantity of that solution (called an aliuot! and neutrali#e that aliuot ,ith a solution of sodium carbonate ,hose concentration is already +no,n very accurately. Adding Adding a fe, drops of an indicator* such as phenolphthalein or methyl orange* to the solution provides a visual indication (a color change! ,hen an euivalence point is reached* ,hen 9ust enough of the standard solution has been added to the un+no,n solution to neutrali#e it e$actly. /y determining ho, much of the sodium carbonate solution is reuired to neutrali#e the hydrochloric acid* ,e can calculate a very accurate value for the concentration of the hydrochloric acid. This procedure is called titration.
H:MIAS 1. 2. ". ;.
Anhy Anhydr drous ous sod sodium ium car carbo bonat natee Methy Methyll oran orange ge indi indicat cator or oncent oncentrat rated ed hydroc hydrochlo hloric ric acid acid ist istil ille led d ,ate ,ater r
A<
1444m 1444mll volu volume metr tric ic fla flas+ s+ 254m 254mll coni conica call fla flas+ s+ 8unnel /urette
5. Meas Measur urin ing g cyli cylind nder er =. :lec :lectr tron onic ic bal balan ance ce
<&':)&: 1. 4.1M 4.1M hydro hydrochl chlori oricc acid acid ,as ,as prep prepare ared d Specific gravity > 1.17g-ml "=.5? Molar mass of Hl > "=.5g-mol density x precentage precentage purity purity x 10 concentration= Molar mass concentration =
1.18 x 36.5 x 10 36.5
concentration > 11.7mol-dm" 8inal concentration > 4.1M 8inal volume > 254ml Initial concentration > 11.7M Initial volume > $ 0.1 x 250 x = 11.8
x =2.12 ml 2. 2.12ml of of the stoc+ stoc+ solution solution ,as measure measured d and diluted diluted to 254ml 254ml in a volumetri volumetricc flas+. ". 4.2g of sodium sodium carbonate carbonate ,ere ,eighed ,eighed into a 254ml conical conical flas+ flas+ and dissolved dissolved in 144ml 144ml of distilled distilled ,ater and top up to the mar+ ;. Methyl Methyl orange indicat indicator or ,as added added and titrated titrated against against the the prepared prepared acid soluti solution. on. 5. T,o T,o more more titrat titration ionss ,ere ,ere done done
TA/: '8 &:S)TS olour change > @ello, @ello, to orange Indicator used > Methyl orange Burette reading/ml 8inal reading-ml Initial reading-ml Titre value-ml Average Average titre =
1 ;;.54 4.44 ;;.54 44.50 + 44.40 + 44.30 3
2 ;;.;4 4 .4 4 ;;.;4
3 ;;."4 4.44 ;;."4
Average Average titre titre =44.40 ml A)ATI'%S The reaction euation of the reaction that occurred bet,een %a2'" and Hl is %a2'" 2Hl 2%al '2 H2' from the above reaction euation n(%a2'"! >1 n(Hl! 2 this implies that n(Hl!> 2Bn(%a 2'"! n(%a2'"!> m-M > 4.2g-14=gmol61> 1.77=7B146"mol hence n(Hl! > 2B1.77=7B146"> ".CC"=B146"mol for ;;.54ml DHlE> DHl E> ".CC"=B ".CC "=B14 146"mol > 4.47;7M ;;.54 B146" for ;;.;4ml DHlE> DHl E> ".CC"=B ".CC "=B14 146"mol > 4.4754M ;;.;4B146" for ;;."4ml DHlE> DHl E> ".CC"=B ".CC "=B14 146"mol > 4.4752M ;;."4B146" mean concentration concentration =
0.0848+ 0.0850 + 0.0852 3
mean concentration concentration =0.085 M
IS)SSI'% In this e$periment a standard 4.1M Hl solution ,as prepared by diluting a stoc+ solution of the Hl. A stoc+ solution is a solution from ,hich a dilute solution can be prepared. The prepared solution ,as standardi#ed ,ith sodium carbonate pellets ,hich is hygroscopic ,hich absorb ,ater from the surroundings but do not dissolve. %a2'" is FF? pure thus* only the FF? of %a2'" ,ould give the accurate concentration of Hl. That is only 75? of the e$pected concentration ,ould be determined hence the concentration of Hl after the e$periment is 4.475M ,hich is 75? of the e$pected concentration. So the 75? of the e$pected concentration ,hich is less than the e$pected concentration of 144? can be due to some errors occurring during the measurement of the solution also the sodium carbona te might have absorb some ,ater from the surroundings.
<&:A)TI'%S
1. The elect electron ronic ic balance balance ,as ,as calibra calibrated ted before before use. use. 2. Accurate volume measurements ,ere ensured. ". Sodium carbonate carbonate ,as used used for the standard standardi#ati i#ation on because because it is is FF? pure. pure. S')&:S '8 :&&'& 1. Since %a2'" is hygroscopic* it could have absorbed ,ater molecules from the atmosphere during ,eighing thereby increasing the ,eight.
'%)SI'% The concentration of Hl after the preparation ,as found to be 4.475M ,hich is 75? of the e$pected concentration. Also* the concentration of the analyte ,as found to depend on the purity of the standard solution used.
&:8:&:%:S 1. 8reiser* 8reiser* HenryG HenryG %ancollas* %ancollas* eorge eorge H (1F7C!. (1F7C!. ompendium of Analyti Analytical cal %omenclature3 %omenclature3 efiniti efinitive ve &ules 1F7C. '$ford3 /lac+,ell Scientific
