EXPERIMENT 2 ANALYSIS OF AN UNKNOWN VINEGAR SAMPLE Objectives
1. To prepare prepare the sodium sodium hydro hydroxide xide solution solution.. 2. To standardise standardise the the base against potassium potassium hydrogen hydrogen phthalate. phthalate. 3. To analyse analyse the unknown unknown vinegar vinegar sample. sample. Intr!"ctin
In this experiment you will learn about primary standard, standardisation and standard solution. By defnition, standard solution is a solution whose solute onentration is aurately known. ! solute whih an be obtained in a very pure, stable, weighable "orm is alled a primary standard. !n aurately weighted primary standard is dissolved in the desired solvent and diluted to an aurately known fnal volume in volumetri #ask. Then the volumetri #ask is inverted a "ew times to obtain homogeneous solution. This solution thus prepared is known as the primary standard solution. $ou $ou will frst prepare prepare an approximate approximate onentration o" sodium sodium hydroxide hydroxide solution. %odium hydroxide is not a primary standard beause it is hygrosopi& meaning that it absorbs moisture rapidly when exposed to air. air. Thus its aurate onentration an only be determined by standardi'ing the solution against a very pure potassium hydrogen phthalate in a series o" repliate titrations. This proess is alled alled standardisation. (otassium (otassium hydrogen hydrogen phthalate )*+( is a monoproti aid with the "ormula *+- +/0/. The standardised sodium hydroxide hydroxide solution is then used to determine the aeti aid ontent o" a vinegar sample. %ometimes, the primary standard "orm o" the desired reagent reagent annot be obtained. Thus a seondary standard solution an be used. !n approximate approximate onentration o" the solution o" the reagent is frst prepared, and then its true onentration is determined by standardisation proess. %tandardisation an be ahieved by titrating the prepared solution against )a a measured mass o" a suitable primary standard substane& )b another reliably known seondary standard solution& or ) by diret analysis "or the reagent o" interest by some suitable nonvolumetri method suh as spetrosopi analysis. In this experiment, we shall use standardisation method )a i.e. using potassium hydrogen phthalate as the primary standard to prepare prepare a seondary standard solution o" sodium hydroxide. The standardised solution o" sodium hydroxide hydroxide is then used to analyse the ontent o" aeti aid in a vinegar sample. Prce!"re
!. (reparation o" the sodium hydroxide hydroxide solution In this (art ! o" the experiment you will prepare m4 o" approximately .25 6 sodium hydroxide solution by diluting a laboratory prepared stok solution o" approximately 57 )w8w 9a0+. The density o" the ommerially prepared prepared stok solution o" 57 )w8w 9a0+ is 1.525 g8m4.
the density o" the 57 )w8w 9a0+ prepared in the laboratory may be slightly di:erent. 1. ;irst, the rough density o" the prepared 57 9a0+ were determined by measuring the mass o" 1o m4 solution. ! dry and empty 5 m4 beaker were weighted. 1 m4 o" the 9a0+ solution were measured in a ylinder and trans"er it to the preweighted beaker. The beaker were reweighted. The di:erene in weight gives the weight o" 1 m4 57 9a0+ prepared in this laboratory. inse the ylinder out a "ew times with "resh distilled water and add all rinses into the ontents o" the plasti bottle. %rew the ap on the plasti bottle and mix the ontents thoroughly by are"ully and vigorously inverting the bottle and swirling it repeatedly. ;inally add the remaining volume o" water in three 1 m4 bathes, mixing the ontents in the bottle thoroughly eah time. The bottle should be shaken at least 2 times a"ter the last addition. B. %tandardisation o" the base against potassium hydrogen phthalate +ere you will standardise your ?ust prepared sodium hydroxide solution against the primary standard, potassium hydrogen phthalate. $ou will also adopt a weighing tehni=ue alled weighing by di:erene. ;irst, weigh the ontainer ontaining the primary standard. Then remove some o" the solute and plae it in a separate ontainer. >eweigh the original ontainer and the residual ontent. The di:erene between the two values gives the mass o" the solute removed. I" the weight o" the solute removed has not yet reahed the desired weight, repeat the proess. This technique is a very important technique to learn and use because it eliminates systematic errors from the balance. The tehni=ue o" weighing by
di:erene is appliable to measuring mass o" hygrosopi samples. 1. !urately, weight about 1 g sample o" dry primarystandard grade potassium hydrogen phthalate )*+( on to a weighing boat. The *+( has been dried earlier, in an oven at 11@ "or 2 hours and stored in a desiator prior to use. 2. inse any drops o" the sample solution on the side walls o" the #ask with distilled water. 3. >inse and fll a 5 m4 burette with the 9a0+ solution you wish to standardise. Be very are"ul with air bubbles espeially at the tip o" the burette. >emove the air bubbles be"ore ad?usting the initial volume and
doing the titration proess. Turn the stopok =uikly 3@ a "ew times until all bubbles are removed. Then ad?ust the initial volume. $ou do not have to start at the 'ero mark. Instead plae the menisus at your eye level. >ead and reord the atual starting volume to the nearest .2 m4. wipe any adhering solution at the sides and the tip o" the burette with the laboratory tissue be"ore you begin the titration. /. (ut three drops o" phenolphthalein indiator into the frst onial #ask. (lae the #ask under the burette and lower the burette tip well into it. It is always advisable to have a Ctitration thie"C"or your frst trial. (lae a piee o" a white tile under the #ask, hold the #ask with your right hand i" you are right handed )and the other way around, i" you are le"t handed, ontrol the stopok o" the burette with your le"t hand and start titration by are"ully turning the stopok to let a gentle and steady stream o" the titrant #ow )without splashing into the aid solution. Dently, swirl the solution in the #ask. The base an be added rapidly "ast, but onentrate on the olour hange. Initially the pink olour disappears as soon as it is "ormed but with time, this olour begins to linger as you swirl the #ask. !t this point, redue the #ow rate o" the titrant "rom the burette. The frst permanent "aint pink olour that persists "or at least 2 seonds is the end point. -. !nalysis o" the unknown vinegar sample 1. ;irst, the density o" the vinegar sample were determined. (ipette 1 m4 vinegar into a dry preweighted 5 m4 beaker. >eweigh the beaker. The di:erene will give the weight "or 1 m4 vinegar sample. Take more vinegar sample into 5 m4 beaker "or titration. 2. (ipette 1 m4 vinegar sample "rom the beaker into a lean 25 m4 onial #ask. (repare two more suh samples. Eash down the sides o" eah #ask with 25 m4 water "rom your wash bottle. 3. !dd 3 drops o" phenolphthalein indiator into one o" the titration vessels and titrate the ontents to end point with the standardised 9a0+ solution "rom B. you may want to adopt the Ftitration thie"C still, at least "or the frst titration. >epeat the proess "or the other two samples. Ideally, the titration volumes should be reproduible. Ans#ers
1. Eeighing by di:erene an redue, but not eliminate, systemi errors in an experiment beause systemi errors do not arise simply "rom errors in measurement, but "rom a variety o" soures. Eeighing by di:erenes is still advised whenever possible. %o that, while made to be as aurate as possible, typial sales have error systematially as a part o" their measurements. I" you plae an ob?et diretly on the sale, it will be e:eted by this error )keeping you "rom obtaining an aurate result. Instead, measure the weight o" ob?et !, then add ob?et B. $ou are able to alulate the di:erene and obtain an exat measurement sine both ob?ets are sub?et to the same error. 2. The titration e=uivalene point ours when the aid present in the sample has been exatly neutrali'ed by the volume o" base added. !dditional
water added to the reation vessel has no e:et on the volume o" base added.
$isc"ssin
Ginegar is a solution o" aeti aid in water. !eti aid, -+ 3-00+, is a weak monoproti aid with a molar mass o" .5 g8mole. The perent by mass o" aeti aid in vinegar an be determined by titrating a known amount o" vinegar with a standardi'ed solution o" sodium hydroxide solution o" aurately known onentration. !eti aid and sodium hydroxide reat as shown belowH -+3-00+ )a= 9a0+ )a= J -+ 3-009a )a= + 20 )l %odium hydroxide is a hygrosopi solid whih means it absorbs water "rom the air. ! weighed =uantity o" sodium hydroxide there"ore ontains an unknown mass o" water. There"ore, a solution o" known molarity annot be made by dissolving a known mass o" solid sodium hydroxide in water. The onentration o" a sodium hydroxide solution must be determined experimentally. This is done by titrating the sodium hydroxide solution against a primary standard. ! primary standard is a substane "rom whih a solution o" known onentration an be prepared. The primary standard used in this experiment is potassium hydrogen phthalate, *+-+/0/, whih is re"erred to by the shorthand notation o" K*+(K. *+( has several advantages suh as it does not absorb moisture readily, it is easily dried, it an be aurately weighed, it an be obtained in very pure "orm, it has a high molar mass o" 2/.23 g8mole and it is very soluble in water. *+( is an aid, whih reats in a=ueous solution to neutrali'e the base, sodium hydroxide, as shown belowH *+-+/0/ )a= 9a0+ )a= J *+- +/0/9a )a= +20 )l
In this experiment, we use the average molarity o" 9a0+ whih is 1.L2x1 1 6 to alulate the weight perent o" the aeti aid in the unknown vinegar solution. !t the end, we get 2.7. %nc&"sin
1. !t the end o" this experiment, we are able to prepare the sodium hydroxide solution whih give us the molarity 1.L2x1 1 6. 2. Ee also get to standardise the base against potassium hydrogen phthalate by titrating with sodium hydroxide solution. 3. 4astly, we suess to analyse the unknown vinegar sample by alulating the weight perent o" the aeti aid in the unknown vinegar solution that give us 2. 7.
