ABSTRACT / SUMMARY

An exper iment i s conduc ted to de te rmine the

concen t ra t ion o f ace t i c ac id (HC 2 H 3 O 2 ) i n v inegar bes ides

p repar ing a s tock so lu t ion o f s tandard ized so lu t ion o f sod ium

hydrox ide (NaOH) . The NaOH so lu t ion i s t i t ra ted aga ins t the

s tandard po tass ium hydrogen ph tha la te (KHP) so lu t ion . The

end po in t o f the t i t ra t ion i s de te rmined by us ing pH e lec t rode to

measure the shape o f the pH curve . The same techn iques a re

app l ied in o rder to de te rmine the concen t ra t ion as we l l as

percen tage by mass o f ace t i c ac id in commerc ia l v inegar . The

vo lumes o f NaOH so lu t ion used fo r bo th t i t ra t ions a re 12 .05

mi l l i l i t e rs and 17 .57 mi l l i l i t e rs respec t i ve ly . The percen tage by

mass o f ace t i c ac id in v inegar i s 6 .373%. The who le exper iment

i s comple ted and success fu l l y conduc ted .

INTRODUCTION

A t i t ra t ion i s an ana ly t i ca l p rocedure used to de te rmine

the concen t ra t ion o f a sample by reac t ing i t w i th a s tandard

so lu t ion .

In a t i t ra t ion , the s tandard so lu t ion , wh ich appears to be

NaOH so lu t ion , goes in a bure t te , wh ich i s a p iece o f g lassware

used to measure the vo lume o f so lven t to approx imate ly 0 .1

m i l l i l i t e rs o f accuracy . The so lu t ion tha t undergoes t i t ra t ion

goes in an Er lenmeyer f l ask , wh ich shou ld be la rge enough to

accommodate bo th the sample and the s tandard so lu t ion tha t i s

be ing added.

1

A pH meter o r pH e lec t rode i s used in th is exper iment

ins tead o f us ing pheno lph tha le in as an ind ica to r so lu t ion in

o rder to de te rmine the pH va lue a t each 1 mi l l i l i t e r and 0 .5

m i l l i l i t e r inc rements a t a t ime dur ing the t i t ra t ion p rocess . A pH

meter uses an e lec t rode to measure the pH o f a so lu t ion . The

e lec t ron i s s to red in d is t i l l ed wate r in o rder to keep i t a t a

neu t ra l pH.

AIMS / OBJECTIVES

The exper iment i s conduc ted to ach ieve the ob jec t i ves

wh ich inc lude the de te rmina t ion o f the concen t ra t ion o f ace t i c

ac id in v inegar by t i t ra t ion w i th a s tandard ized so lu t ion o f

sod ium hydrox ide , the mo la r i t y o f sod ium hydrox ide needed fo r

neu t ra l i z ing the v inegar in t i t ra t ion .

THEORY

I n the f i r s t se t o f t i t ra t ions , a p rocedure ca l led

s tandard iza t ion i s conduc ted to de te rmine the concen t ra t ion o f

the sod ium hydrox ide so lu t ion , wh ich i s no to r ious ly d i f f i cu l t to

p repare to an accura te concen t ra t ion . In th is exper iment , the

neu t ra l i za t ion reac t ion occurs be tween an ac id and a base ,

wh ich p roduce a sa l t and wate r .

C 8 H 4 O 4 H – (aq) + OH – (aq) → C 8 H 4 O 2 - (aq) + H 2 O ( l )

2

Potass ium hydrogen ph tha la te o r genera l l y known as KHP

is a po tass ium sa l t o f the ph tha la te ion , C 8 H 5 O 4 - , wh ich i s a

weak ac id . The reac t ion be tween C 8 H 5 O 4 – (aq) and NaOH is a

t yp ica l ac id -base neu t ra l i za t ion reac t ion :

C 8 H 5 O 4 – (aq) + NaOH(aq) → C 8 H 4 O 4

2 - (aq) + Na + (aq) + H 2 O ( l )

In the s tandard iza t ion p rocedure , the NaOH so lu t ion i s

added to the KHP so lu t ion v ia a bure t te . A t the endpo in t , the

number o f mo les o f NaOH con ta ined in the added so lu t ion

equa ls the number o f mo les o f KHP neu t ra l i zed . S ince the

number o f mo les o f NaOH as we l l as the vo lume o f so lu t ion i s

known, thus the concen t ra t ion o f NaOH can be de te rmined .

The po in t a t wh ich s to ich iomet r i c amounts o f each reagent

have reac ted i s ca l led the equ iva lence po in t . The po in t a t wh ich

the reac t ion i s observed to be comple te i s ca l led the end po in t .

The end po in t shou ld co inc ide w i th the equ iva lence po in t .

Once the NaOH so lu t ion i s s tandard ized , i t can be used to

ana lyze v inegar and de te rmine the concen t ra t ion o f ace t i c ac id

con ten t .

3

APPARATUS

-spa tu la

-beakers

-g radua ted cy l inder

-ana ly t i ca l ba lance

- f i l t e r funne l

-bure t te

-pH meter w i th e lec t rode

-magnet i c s t i r re r

- re to r t s tand w i th bure t te c lamp

CHEMICAL REAGENTS

-NaOH so lu t ion

-d is t i l l ed wate r

-1 .5 g rams o f d ry po tass ium hydrogen ph tha la te , KHP

-10 mi l l i l i t e rs a l iquo ts o f v inegar

EXPERIMENTAL PROCEDURE

A. S tandard iza t ion o f NaOH so lu t ion .

1 . An approx imate ly 1 .5 g rams o f d r ied po tass ium hydrogen

ph tha la te , KHC 8 H 4 O 4 , o r KHP is we ighed us ing the ana ly t i ca l

ba lance to the neares t fou r s ign i f i can t f i gu res .

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2. The KHP is then t rans fe r red in to an Er lenmeyer f l ask and

d isso lved in 30 mi l l i l i t e rs o f d is t i l l ed wate r .

3 . The amount o f KHP and wate r used a re recorded .

4 . A bure t te i s c leaned , r insed , and f i l l ed w i th NaOH so lu t ion .

5 . A pH meter i s ca l ib ra ted us ing bu f fe r so lu t ion be fo re the

e lec t rodes be ing r insed we l l w i th d is t i l l ed wate r and b lo t ted

d ry .

6 . The con ica l f l ask con ta in ing KHP so lu t ion i s then p laced on

a s t i r p la te and a magnet i c s t i r re r i s inser ted in to the f lask and

le t to s t i r the so lu t ion .

7 . The pH meter i s inser ted in to the f lask and the pos i t ion o f

e lec t rode i s ad jus ted so tha t the magnet i c s t i r re r does no t h i t

i t .

8 . The so lu t ion i s a l lowed to equ i l i b ra te w i th s t i r r ing fo r 1

m inu te . The pH is then recorded .

9 . The ac id i s t i t ra ted and the pH read ing as recorded w i th the

fo l low ing inc rements :

A t pH 1 un t i l 5 .5 : 1 m i l l i l i t e r o f NaOH a t a t ime

At pH 5 .5 un t i l 10 .5 : 0 .5 m i l l i l i t e rs o f NaOH a t a t ime

At pH 10 .5 un t i l 12 .5 : 1 m i l l i l i t e r o f NaOH a t a t ime

10. The t i t ra t ion i s con t inued to a t leas t a pH o f 12 .

5

11. The da ta o f pH read ings and

the vo lume o f NaOH so lu t ion

added and t i t ra ted a re recorded

and tabu la ted .

B . De te rmine the concen t ra t ion o f ace t i c ac id in v inegar .

1 . 10 mi l l i l i t e rs o f v inegar i s measured in to a beaker .

2 . The so lu t ion i s then t rans fe r red to an Er lenmeyer f l ask and

the f lask i s then p laced on a s t i r ba r w i th a magnet i c s t i r re r pu t

in to the f lask .

3 . A pH meter i s inser ted and the pos i t ion o f e lec t rode i s

ad jus ted so tha t the magnet i c s t i r re r does no t h i t i t . The pH

read ing i s recorded .

4 . The s tandard ized NaOH so lu t ion i s t i t ra ted w i th the fo l low ing

inc rements :

A t pH 1 un t i l 5 .5 : 1 m i l l i l i t e r o f NaOH a t a t ime

At pH 5 .5 un t i l 10 .5 : 0 .5 m i l l i l i t e rs o f NaOH a t a t ime

At pH 10 .5 un t i l 12 .5 : 1 m i l l i l i t e r o f NaOH a t a t ime

5. The t i t ra t ion i s con t inued to a t leas t a pH o f 12 .

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6. The da tas fo r the vo lume o f NaOH be ing t i t ra ted and pH

read ings a re recorded and tabu la ted .

RESULTS

A. S tandard iza t ion o f NaOH so lu t ion

Data tab le :

Mass o f po tass ium hydrogen ph tha la te used : 1 .5 g rams

Vol . NaOH

(mL)

pH Vol . NaOH

(mL)

pH

0.0 4 .186 11.0 6 .062

1 .0 4 .377 11.5 6 .278

2 .0 4 .502 12.0 6 .656

3 .0 4 .750 12.5 10 .475

4 .0 4 .889 13.0 11 .637

5 .0 5 .024 14.0 12 .247

6 .0 5 .154 15.0 12 .435

7 .0 5 .281 16.0 12 .627

8 .0 5 .420 17.0 12 .702

9 .0 5 .578 18.0 12 .706

9 .5 5 .673 19.0 12 .802

10.0 5 .778 20.0 12 .836

10.5 5 .918

Vo lume o f NaOH a t end po in t : 12 .05mi l l i l i t e rs

7

0 2 4 6 8 10 12 14 16 18 20 220

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Graph of Volume of NaOH (mL) against pH

NaOH (mL)

pH

F igure A .1 S tandard iza t ion o f NaOH so lu t ion

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B. Dete rmina t ion o f concen t ra t ion o f ace t i c ac id in v inegar

Data tab le :

Vo lume o f v inegar : 10 mi l l i l i t e rs

Vol . NaOH

(mL)

pH Vol . NaOH

(mL)

pH

0.0 2 .762 15.0 5 .547

1 .0 3 .458 15.5 5 .647

2 .0 3 .874 16.0 5 .761

3 .0 4 .092 16.5 5 .909

4 .0 4 .261 17.0 6 .090

5 .0 4 .417 17.5 6 .459

6 .0 4 .546 18.0 10 .261

7 .0 4 .643 19.0 12 .141

8 .0 4 .730 20.0 12 .480

9 .0 4 .841 21.0 12 .625

10.0 4 .927 22.0 12 .709

11.0 5 .028 23.0 12 .764

12.0 5 .133 24.0 12 .816

13.0 5 .267 25.0 12 .854

14.0 5 .383

Vo lume o f NaOH a t end po in t :17 .57mi l l i l i t e rs

Mo les o f NaOH a t end po in t :0 .0107177mol

Mo les o f v inegar neu t ra l i zed : 0.1681652mol

Mo les o f ace t i c ac id in v inegar : 0 .0107177mol

Mo la r i t y o f ace t i c ac id in v inegar : 1.07177M

Percen tage by mass o f ace t i c ac id in v inegar :6 .373%

9

0 2 4 6 8 10 12 14 16 18 20 22 240

1

2

3

4

5

6

7

8

9

10

11

12

13

14

pH

NaOH (mL)

Graph of Volume of NaOH (mL) against pH

F igure B .1 Dete rmina t ion o f the concen t ra t ion o f

ace t i c ac id in v inegar

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SAMPLE CALCULATIONS (A)

1. F rom the g raph in F igure A .1 , the vo lume o f NaOH so lu t ion

a t equ iva lence po in t i s de te rmined as 12 .05 mi l l i l i t e rs .

2 . The number o f mo les o f po tass ium hydrogen ph tha la te , KHP

used i s ca lcu la ted .

1 .5 g o f KHP x x 1 mo l o f KHP = 0 .00734502mol o f KHP

204 .22g o f KHP

3. The number o f mo les o f NaOH needed to reach the

equ iva lence po in t i s ca lcu la ted .

0 .00734502mol KHP x 1mo l KHP = 0 .00734502mol NaOH

1mo l NaOH

4. The concen t ra t ion o f NaOH so lu t ion i s ca lcu la ted .

0 .00734502mol NaOH = 0 .61 M o f NaOH

0 .01205L o f NaOH

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SAMPLE ERROR CALCULATIONS (A)

1. F rom the g raph , the vo lume o f NaOH so lu t ion a t equ iva lence

po in t i s de te rmined as 12 .1 m i l l i l i t e rs (on ly 3 s ign i f i can t

f i gu res ) .

2 . The number o f mo les o f po tass ium hydrogen ph tha la te , KHP

used i s ca lcu la ted .

1 .5 g o f KHP x x 1 mo l o f KHP = 0 .007 mo l o f KHP

204 g o f KHP

Note tha t the va lue i s taken a t on ly 1 s ign i f i can t f i gu re .

3 . The number o f mo les o f NaOH needed to reach the

equ iva lence po in t i s ca lcu la ted .

0 .007mol KHP x 1mo l KHP = 0 .007mol NaOH

1mol NaOH

4. The concen t ra t ion o f NaOH so lu t ion i s ca lcu la ted .

0 .007mol NaOH = 0 .58M o f NaOH

0.0121L NaOH

5. The percen tage o f e r ro r i s ca lcu la ted .

0 .61M – 0 .58M x 100% = 4 .92 %

0 .61M

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SAMPLE CALCULATIONS (B)

1. F rom the g raph in F igure B .1 , the vo lume o f NaOH so lu t ion

a t equ iva lence po in t i s de te rmined as 17 .57 mi l l i l i t e rs .

2 . The ba lanced equa t ion fo r the neu t ra l i za t ion reac t ion i s as

be low:

NaOH (aq) + HC2H3O2 (aq ) → H2O (l) + NaC2H3O2 (aq)

The number of moles of NaOH solution used is calculated.

0.61M NaOH x 17.57 mL NaOH = 0.0107177mol NaOH

1000

3. The number of moles of acetic acid in vinegar solution is calculated.

0.0107177mol NaOH x 1mol HC2H3O2 = 0.0107177mol HC2H3O2

1mol NaOH

4. The molarity of acetic acid in vinegar solution is calculated.

1000 x 0.0107177mol HC2H3O2 = 1.07177M of HC2H3O2

10 mL of vinegar

5. The mass of vinegar used is calculated.

1.01 g/mL density of vinegar x 10mL of vinegar = 10.1grams of vinegar

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6. The number of moles of vinegar used is calculated.

10.1g of vinegar = 0.1681652mol of vinegar

60.06g/mol vinegar

7. The percentage by mass of acetic acid in vinegar solution is calculated.

0.0107177mol HC2H3O2 x 100% = 6.373%

0.1681652mol vinegar

SAMPLE ERROR CALCULATIONS (B)

1. The theoretical value of percentage by mass of acetic acid in vinegar is

between 4 percents until 8 percents. In comparison of the theoretical value with

the calculated value, let the theoretical value be the mean value that is 6

percents.

2. The percentage of error for the calculated value is calculated.

│6.000 – 6.373 │ x 100% = 6.217%

6.000

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DISCUSSION

The ma in ob jec t i ve o f the exper iment i s to de te rmine the

concen t ra t ion as we l l as the percen tage by mass o f ace t i c ac id

in v inegar . However , an e r ro r o f 6 .217 percen ts a f te r compar ing

the theore t i ca l va lue w i th the exper imenta l va lue i s ob ta ined .

There fo re , a few mis takes o r lack o f awareness o f the

p recau t ions tha t mus t be cons idered when conduc t ing the

exper iments may be the reasons tha t lead to the e r roneous

ca lcu la t ions .

F i rs t l y , the d ry po tass ium hydrogen ph tha la te may no t be

we ighed as 1 .5 g rams accura te ly to 4 s ign i f i can t f i gu res . In

add i t ion , the d i lu ted KHP so lu t ion may a lso con ta in impur i t i es

and there fo re lead to the p resence o f in te r fe r ing subs tances in

the s tandard so lu t ion fo r t i t ra t ion p rocess . As a resu l t , the pH

va lues o f the t i t ra ted so lu t ion a re a lso in te r rup ted by the

impur i t i es .

A pH meter i s a more accura te means o f measur ing pH

va lue because i t can be ca l ib ra ted to measure one ten th o f a

pH un i t , whereas the ind ica to r so lu t ion may on ly measures to

one pH un i t . Thus , pH meter i s used to conduc t th is exper iment

ins tead o f pheno lph tha le in as ind ica to r so lu t ion . However , the

e r ro rs when us ing the pH meter shou ld be cons idered too . The

cor rec t ways to ca l ib ra te the pH meter be fo re i t can be used to

measure any pH are as be low:

1 . The e lec t rode mus t be removed f rom the d is t i l l ed wate r

and p laced in pH 4 bu f fe r so lu t ion .

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2. The pH meter i s se t to pH 4 and the e lec t rode i s r insed

w i th d is t i l l ed wate r to remove any excess so lu t ion .

3 . The e lec t rode i s then p laced in pH 10 bu f fe r so lu t ion .

4 . The pH meter i s se t to pH 10 and the e lec t rode i s

r insed , and p laced in d is t i l l ed wate r .

5 . The pH meter shou ld now be ca l ib ra ted and i s ready to

be used to measure any pH accura te ly .

There fo re , the ca l ib ra t ion o f the pH meter may no t be done

thorough ly be fo re i t i s used to measure the pH va lue o f the

t i t ra ted so lu t ions . In add i t ion , the pH e lec t rode may have come

in to con tac t w i th the magnet i c s t i r re r ins ide the Er lenmeyer

f l ask , wh ich may a lso in te r rup t the pH read ings .

Bes ides tha t , the who le exper iments a re conduc ted on ly

once , wh ich means tha t no average va lue w i l l be ob ta ined in

o rder to ge t more accura te resu l t s . Thus , a t leas t 3 t r ia l s

shou ld be done in o rder to ge t an average va lue o f the

percen tage by mass o f ace t i c ac id in v inegar and there fo re , a

more accura te resu l t o f i t can be ob ta ined .

16

CONCLUSION

The exper imenta l va lue o f the percen tage by mass o f

ace t i c ac id in v inegar so lu t ion i s 6 .373%, wh ich leads to

6 .217% o f e r ro r compared to the theore t i ca l va lue , wh ich i s

6 .000%. Th is p rove tha t the concen t ra t ion o f ace t i c ac id in

common househo ld v inegar does no t exceeds the l im i t tha t has

been s tandard ized by hea l th sa fe ty regu la to ry and there fo re i s

sa fe to be used by pub l i c consumers .

RECOMMENDATIONS

There a re a few recommendat ions , and p recau t ions tha t have to

be cons idered dur ing the exper iments in o rder to ge t an

accura te va lue and read ings o f da ta .

F i rs t l y , the s tandard so lu t ion tha t i s used shou ld be a

hundred percen t pure and s tab le a t room tempera tu res . Thus , i t

i s more p re fe rab le to use a d r ied s tandard mate r ia l be fo re

we igh ing and d i lu ted .

Second ly , i n o rder to reduce exper imenta l e r ro r when

conduc t ing an exper iment , the s tandard so lu t ion tha t i s chosen

mus t have a h igh mo lecu la r we igh t . In th is exper iment , KHP

so lu t ion i s used as a s tandard so lu t ion and s ince i t has a la rge

mo lecu la r we igh t , wh ich i s 204 .22 g ram/mo l , the re fo re a la rge

mo le can be ob ta ined to per fo rm the t i t ra t ion . The re la t i ve e r ro r

17

i n we igh ing a g rea te r amount o f mate r ia l i s sma l le r than fo r a

sma l l amount .

Nex t , t i t ra t ion p rocess requ i res a cer ta in c r i te r ia wh ich

inc lude the s to ich iomet ry o f the unknown and s tandard so lu t ion .

There shou ld be a we l l de f ined and known reac t ion be tween the

two mater ia ls .

Bes ides tha t , the reac t ion be tween the ac id and base

shou ld be rap id and w i thou t any s ide reac t ions . Thus , i f t he re

a re any in te r fe r ing subs tances , they shou ld be removed.

Nex t , the neu t ra l i za t ion reac t ion shou ld be quan t i ta t i ve

wh ich means tha t the reac t ion shou ld p roceed to comple t ion . A

marked change in some proper ty o f the so lu t ion ind ica tes tha t

the reac t ion has come to comple t ion .

Apar t f rom tha t , sod ium hydrox ide so lu t ion w i l l reac t w i th

ca rbon d iox ide in the a tmosphere o r su r round ings over t ime.

Th is p rob lem w i l l resu l t i n the decrease o f the concen t ra t ion o f

sod ium hydrox ide so lu t ion . There fo re , so lu t ion o f sod ium

hydrox ide needs to be s tandard ized .

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REFERENCES

1. Steven L. Murov, 2004, Experiments in General Chemistry, 4th Edition, United States:

Thomson/Brooks/Cole.

2. http://en.wikipedia.org/wiki/Acid-base_titration

3. http://www.ph-meter.info/

4. http://www.measurementuncertainty.org/mu/guide.html

5. http://alpha.chem.umb.edu/chemistry/ch313/Experiment%204%20vinegar.pdf

APPENDICES

* Re fe r to the a t tachment p rov ided in the nex t page .

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