ABSTRACT / SUMMARY

An exper iment i s conduc ted to iden t i f y an unknown

monopro t i c ac id by ob ta in ing the pKa va lue fo rm t i t ra t ion curves . 10

mi l l i l i t res o f the unknown ac id i s t i t ra ted w i th approx imate ly 0 .1 M o f

sod ium hydrox ide so lu t ion . Three t r ia l s a re conduc ted and there fo re

th ree t i t ra t ion curves a re p lo t ted . The va lues o f pKa and Ka o f the

unknown ac id f rom the t i t ra t ion p rocess a re 4 .537 and 2 .90 x 10 - 5

respec t i ve ly . Whereas , the pKa and Ka va lues ob ta ined f rom us ing

the in i t i a l pH o f the unknown ac id a re 4 .81 and 1 .53 x 10 - 5

respec t i ve ly . Theore t i ca l l y , the pKa and Ka va lues o f the unknown

monopro t i c ac id , wh ich i s be l ieved to be o f ace t i c ac id a re 4 .75 and

1 .76 x 10 - 5 . There fo re , an e r ro r o f 4 .48 percen ts o f the pKa va lue

and 64 .77 percen ts o f the Ka va lue f rom the t i t ra t ion p rocess a re

ca lcu la ted . Meanwh i le , an e r ro r o f 1 .26% and 13 .1% o f pKa and Ka

va lues ob ta ined f rom us ing the in i t i a l pH o f the unknown ac id i s

ca lcu la ted . There fo re , the re i s on ly a s l igh t d i f fe rence f rom the

theore t i ca l va lue compared to the va lues f rom th e t i t ra t ion p rocess .

Hence , the unknown monopro t i c ac id i s iden t i f i ed as ace t i c ac id and

the va lue de te rmined f rom us ing the in i t i a l pH va lue o f the ac id i s a

more accura te method . The exper iment i s comple ted and

success fu l l y conduc ted .

1 | P a g e

INTRODUCTION

Ac id -base t i t ra t ion i s a method o f neu t ra l i za t ion p rocess wh ich

p rov ides in fo rmat ion regard ing the p roper t ies as we l l as na tu re o f

e i the r ac id o r base app l ied dur ing the p rocess . L ikew ise , i t i s use fu l

to de te rmine the mo lecu la r mass and pKa va lues o f the subs tances .

The end-po in t o f such t i t ra t ion can be mon i to red by us ing ind ica to rs

o r as what i s used in th is exper iment , a pH meter w i th e lec t rodes .

A t i t ra t ion curve i s a g raph o f measured pH va lues ob ta ined

f rom pH meter read ings versus vo lume o f t i t ran t be ing added in

m i l l ime t res . Bas ica l l y , the mos t impor tan t in fo rmat ion tha t i s

supposed to be ob ta ined f rom ac id -base t i t ra t ion i s the equ iva lence

po in t . The equ iva lence po in t i s theore t i ca l l y reached when the

number o f mo les o f base be ing added in to the Er lenmeyer f l ask i s

equa l to the number o f mo les o f ac id . I t occurs in the t i t ra t ion curve

in the reg ion where there i s a la rge and no t i ceab le change in pH w i th

a re la t i ve ly sma l l change in vo lume o f t i t ran t .

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AIMS / OBJECTIVES

The exper iment i s conduc ted to ach ieve ma in ob jec t i ve , wh ich

i s to iden t i f y the unknown monopro t i c ac id by compar ing the

ca lcu la ted average Ka va lues f rom the da ta ob ta ined f rom th is

exper iment w i th the Ka va lues o f some common ac ids found in a

genera l chemis t ry book . In o rder to ge t the Ka va lues fo r the

unknown ac id , i t can be ob ta ined by per fo rming t i t ra t ion p rocess

based on the pH va lue . Bes ides tha t , the Ka va lue can a lso be

de te rmined by us ing the in i t i a l pH o f the unknown monopro t i c ac id .

THEORY

I n th is exper iment , we w i l l be dea l ing w i th monopro t i c ac id .

Based on Brøns ted and Lowry , an ac id i s a p ro ton donor whereas a

base i s a p ro ton accep to r . Th is por t rays a very impor tan t idea to

unders tand ing monopro t i c and po lypro t i c ac ids and bases s ince

monopro t i c , as a mat te r o f fac t , i s bas ica l l y re fe r red to the t rans fe r

o f one p ro ton . On the con t ra ry , po lyp ro t i c co r responds to the

t rans fe r o f more than one p ro ton . There fo re , monopro t i c ac id i s an

ac id tha t can dona te one p ro ton wh i le po lypro t i c ac id i s an ac id tha t

can dona te more than one p ro ton . To be more p rec ise , monopro t i c

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ac id can re lease one p ro ton per mo lecu le and hence , have on ly one

equ iva lence po in t .

When a weak monopro t i c ac id (HA) i s d isso lved in wa te r , on ly

ce r ta in o f i t s mo lecu les w i l l d i ssoc ia te to y ie ld hydron ium ions ,

H3O + , and A- ions . A t th i s po in t , the reac t ion has reached dynamic

equ i l i b r ium. Cons ider the fo l low ing reac t ion :

HA (aq) + H 2 O ( l iq . ) <−> H 3 O + (aq ) + A - (aq)

Under such equ i l i b r ium cond i t ions , the to ta l concen t ra t ions o f

each spec ies rema in cons tan t , even though the spec ies in so lu t ion

a re cons tan t l y d issoc ia t ing and recombin ing . The ion iza t ion cons tan t

o f the weak monopro t i c ac id i s used to charac te r i ze the ac id , and i s

ca lcu la ted by us ing the fo l low ing equa t ion :

Ka = [H3O + ] [A - ] . . . . . . . . . . . . . . . . . . . . . . . . . (1 )

[HA]

In th is express ion , Ka re fe rs to ac id ion iza t ion cons tan t . S t rong

ac ids t yp ica l l y d issoc ia te comple te ly , and there fo re wou ld have a Ka

va lue o f g rea te r than 1 . Weak ac ids have Ka va lues much sma l le r

than 1 , t yp ica l l y less than 10 - 4 t o be more p rec ise . For ins tance , the

Ka va lue o f p ropano ic ac id i s 1 .3 x 10 - 5 and i t s pKa va lue i s 4 .874 .

In p r io r i t i z ing the conven ience , sc ien t i s ts o f ten use the pKa va lues

o f weak ac id , as i t a l lows them to dea l w i th who le numbers .

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By assuming a 1 :1 mo la r ra t io o f ac id : base , the vo lume as

we l l as concen t ra t ion o f sod ium hydrox ide , wh ich i s the base can be

used to de te rmine the number o f mo les o f ac id p resen t :

Ƞ b a s e = mass b a s e . . . . . . . . . . . . . . . . . . . . . . (2 )

M r b a s e

Ƞ a c i d = Ƞ b a s e x 1 mo l o f ac id . . . . . . . . . . . . . . . . . . (3 )

1 mo l o f base

The pH o f a so lu t ion i s re la ted to the hydron ium ions , H3O+,

concen t ra t ion by the equa t ion :

pH = − log [H3O + ] . . . . . . . . . . . . . . . . . . . . (4 )

and the pKa o f an ac id i s s imp ly :

pKa = − log Ka . . . . . . . . . . . . . . . . . . . . . . . . . . . (5 )

By cons ider ing equa t ion (1 ) , take – log on bo th s ides :

- log Ka = - log [H3O + ] – log [A - ] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (6 )

[HA]

Thus , by subs t i tu t ing equa t ion (6 ) in to equa t ion (5 ) :

pKa = - l og [H3O + ] – log [A - ] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (7 )

[HA]

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and there fo re , by subs t i tu t ing equa t ion (4 ) in to equa t ion (7 ) :

pKa = pH – log [A - ] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (8 )

[HA]

However , s ince pKa i s cons tan t and pH var ies ,hence :

pH = pKa + log [A - ] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (9 )

[HA]

There fo re , when [A - ] = [HA] , thus :

pH a t ha l f -equ iva lence po in t = pKa . . . . . . . . . . . . . . . . . . . . . . . . (10 )

Th is i s the midway po in t to the equ iva lence po in t . A t i t ra t ion

curve i s needed to ana lyze the ac id ion iza t ion cons tan t , Ka va lues o f

the unknown monopro t i c ac id to be iden t i f i ed in th is exper iment . The

t i t ra t ion curve represen ts the pH va lues a t par t i cu la r vo lume o f

sod ium hydrox ide be ing added. Thus , pKa va lues can be read

d i rec t l y f rom t i t ra t ion curve . F rom the t i t ra t ion da ta co l lec ted f rom

the exper iment conduc ted , th ree t i t ra t ion curves o f pH versus vo lume

o f base added in m i l l i l i t res a re p lo t ted . On each curve , the vo lume o f

base a t equ iva lence po in t i s c lear l y marked . Nex t , the vo lume o f

base a t ha l f -equ iva lence po in t a t each curves a re de te rmine and

c lear l y marked . The pH va lues a t tha t par t i cu la r ha l f -equ iva lence

po in t a re then ex t rapo la ted . The pH va lues can now be used to

ca lcu la te the ac id ion iza t ion cons tan t and the average va lue i s

de te rmined . In add i t ion , the vo lume o f base t i t ra ted a t the

equ iva lence po in t i s used to ca lcu la te the concen t ra t ion o f the

6 | P a g e

unknown monopro t i c ac id . The average i s de te rmined as we l l . The

average ac id ion iza t ion cons tan t i s then used to iden t i f y the unknown

monopro t i c ac id .

APPARATUS / REAGENTS

~ 50-mi l l i l i t re bure t te , 10-mi l l i l i t re g radua ted cy l inders , 50 and 250-

mi l l i l i t re beakers , E r lenmeyer f l ask , re to r t s tand and bure t te c lamp,

pH meter , spa tu la , ana ly t i ca l ba lance , f i l t e r funne l , magnet i c s t i r re r ,

pe l le ts o f sod ium hydrox ide , d is t i l l ed wate r , 10 mi l l i l i t res o f unknown

monopro t i c ac id .

EXPERIMENTAL PROCEDURE

1. An approx imate ly 2 .0 g rams o f pe l le ts o f sod ium hydrox ide , NaOH

is we igh ted to the neares t fou r dec ima l po in ts and d isso lved in 500

mi l l i l i t res o f d is t i l l ed wate r .

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

7 | P a g e

3. 10 mi l l i l i t res o f unknown monopro t i c ac id i s p repared and then

t rans fe r red in to the Er lenmeyer f l ask .

4 . The f lask 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 .

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 rode i s 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 pH e lec t rode i s inser ted in to the f lask and the pos i t ion o f the

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 .

7 . 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 re 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 re 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 re o f NaOH a t a t ime

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

9 . 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 .

10 . A t i t ra t ion curve o f pH versus vo lume o f NaOH so lu t ion t i t ra ted i s

p lo t ted . Based on the t i t ra t ion curve , pKa va lue o f the ac id i s

ca lcu la ted and the ac id i s iden t i f i ed .

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RESULTS

Mass o f sod ium hydrox ide pe l le ts d isso lved : 2 .0823 g rams

Vol . NaOH

(mL)

pH

(Tr ia l 1 )

pH

(Tr ia l 2 )

pH

(Tr ia l 3 )

Average

pH

0.0 2 .89 2 .87 2 .98 2 .91

1 .0 4 .08 3 .70 3 .67 3 .82

2 .0 4 .23 4 .09 4 .05 4 .12

3 .0 4 .42 4 .32 4 .26 4 .33

4 .0 4 .68 4 .55 4 .53 4 .59

5 .0 4 .85 4 .79 4 .72 4 .79

6 .0 5 .17 5 .04 5 .02 5 .08

7 .0 5 .89 5 .46 5 .43 5 .59

7 .5 6 .33 6 .11 5 .90 6 .11

8 .0 11 .17 10 .63 10 .56 10 .79

9 .0 11 .55 11 .34 11 .26 11 .38

10 .0 11 .73 11 .52 11 .52 11 .63

11 .0 11 .83 11 .64 11 .67 11 .71

12 .0 11 .90 11 .72 11 .77 11 .80

13 .0 11 .93 11 .77 11 .82 11 .84

14 .0 12 .00 11 .80 11 .87 11 .89

15 .0 12 .04 11 .84 11 .90 11 .93

Volume of NaOH at equiva lence point (mL) 7.5951

pH at ha l f -equiva lence point ≈ pKa 4.537

Acid ion izat ion constant , Ka 2.90 x 10 - 5

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0 2 4 6 8 10 12 14 160123456789

10111213

Titration curve of pH versus Volume of NaOH (mL)pH

Vol. of NaOH (mL)

←equivalence point

←half-equivalence point

SAMPLE CALCULATION

By us ing equa t ion (2 ) :

Ƞ b a s e = mass b a s e

M r b a s e

= 2 .0823 g

39 .997 g /mo l

= 0 .0521 mo l

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From the reac t ion equa t ion , number o f mo les o f base i s equ iva len t to

the number o f mo les o f ac id . Thus ,

Ƞ a c i d = Ƞ b a s e

= 0 .0521 mo l

By us ing equa t ion (10) , pH a t ha l f -equ iva lence po in t = pKa = 4 .535

To ca lcu la te the ac id ion iza t ion cons tan t , by us ing equa t ion (5 ) :

pKa = − log Ka

4 .537 = - log Ka

Ka = a rc log -4 .535

= 2 .90 x 10 - 5

L ikewise , f rom the reac t ion equa t ion ,

HA (aq) + H 2 O ( l iq . ) <−> H 3 O + (aq ) + A - (aq)

by cons ider ing the fo l low ing “ ICE” tab le :

components HA H 3 O + A -

I n i t i a l

c o n c e n t r a t i o n

0.1 0 0

C h a n g e i n

c o n c e n t r a t i o n

0.1 – x + x + x

C o n c e n t r a t i o n a t

E q u i l i b r i u m

0.1 - x x x

the concen t ra t ion o f H 3 O + i s ca lcu la ted us ing the average va lue o f

pH read ing f rom the t i t ra t ion conduc ted in th ear l ie r exper iment .

11 | P a g e

pH = - log [H 3 O + ]

2 .91 = - log [H 3 O + ]

[H 3 O + ] = a rc log ( -2 .91)

= 1 .23 x 10 - 3

Thus , the ac id ion iza t ion cons tan t , Ka o f the unknown monopro t i c

ac id i s d te rmine by us ing equa t ion (1 ) :

Ka = x .x__

0 .1 – x

= x 2 ___

0 .1 – x

= (1 .23 x 10 - 3 ) 2

0 .1 – 1 .23x10 - 3

= 1 .53 x 10 - 5

Thus , the pKa i s equa l to :

pKa = - log Ka

= - log (1 .53 x 10 - 5 )

= 4 .81

SAMPLE ERROR CALCULATION

The unknown monopro t i c ac id i s iden t i f i ed to be ace t i c ac id .

However , theore t i ca l l y , the pKa va lue fo r ace t i c ac id i s 4 .75 ,

whereas the Ka va lue cor responds to the unknwon monopro t i c ac id

f rom the t i t ra t ion i s 1 .76 x 10 - 5 . Thus , the percen tage e r ro rs fo r each

theore t i ca l va lue to the ca lcu la ted va lue a re :

Percen tage e r ro r (pKa) = │4 .75 – 4 .537 │ x 100%

4 .75

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= 4 .48 %

Percen tage e r ro r (Ka) = │ 1 .76 x 10 - 5 - 2 .90 x 10 - 5 │x100%

1 .76 x 10 - 5

= 64 .77 %

For the iden t i f y ing the unknown ac id us ing the pH va lue in o rder to

de te rmine the Ka va lue , the va lue i s s l igh t l y d i f fe ren t f rom the one

ob ta ined f rom the t i t ra t ion p rocess . The percen tage e r ro r i s

ca lcu la ted to be :

Percen tage e r ro r (pKa) = │4 .75 – 4 .81│ x 100%

4 .75

= 1 .26%

Percen tage e r ro r (Ka) = │1 .76x10 - 5 – 1 .53x10 - 5 │ x100%

1.76x10 - 5

= 13 .1%

DISCUSSION

A lo t o f i n fo rmat ion i s requ i red in o rder to assure tha t the

iden t i t y o f the unknown ac id i s conc lus ive . No t i ce tha t one

equ iva lence po in t i s ob ta ined , there fo re one pKa va lue . A f te r

ana lyz ing i t as we l l the Ka va lue , i t i s conc luded to be remarkab ly

13 | P a g e

s im i la r to ace t i c ac id . The unknown ’s Ka va lue i s 2 .90 x 10 - 5 whereas

ace t i c ac id ’s Ka i s mere ly d i f fe ren t wh ich va lue i s 1 .76 x 10 - 5 .

As fo r the pKa va lue o f the unknown ac id i s ca lcu la ted to be 4 .537

whereas ace t i c ac id ’s pKa va lue i s 4 .75 .

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

va lue o f the unknown monopro t i c ac id so as to iden t i f y the ac id .

However , an e r ro r o f 64 .77 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 e r ro r tha t m igh t a f fec t the ca lcu la ted va lues i s done

dur ing we igh ing the pe l le ts o f sod ium hydrox ide . The we igh t o f an

empty beaker shou ld have been cons idered as we l l . Then , i t mus t be

sub t rac ted f rom the we igh t o f the beaker con ta in ing the pe l le ts .

Hence , the t i t ra t ion curves migh t no t have the accura te va lues as the

concen t ra t ion o f sod ium hydrox ide i s no t per fec t l y 0 .1 M.

Second, the pH meter tha t has been used has never g iven the

de f in i te read ings , as the va lues tha t a re shown are a lways chang ing

rap id ly . Thus , one can never te l l t he accura te read ings o f pH va lues .

There fo re , these w i l l a l so a f fec t the t i t ra t ion curves as we l l as pKa

va lues wh ich cor respond to the pH va lues a t ha l f equ iva lence po in t .

14 | P a g e

Bes ides tha t , the equ iva lence po in t i s no t necessar i l y be ing a t

pH o f 7 as i t occurs jus t when the concen t ra t ion o f ac id i s equa l to

the concen t ra t ion o f base reac ted in so lu t ion . There fo re , the f ina l pH

depends on the ma jo r spec ies o f ions le f t i n the so lu t ion a f te r the

reac t ion .

In add i t ion , the pH e lec t rode migh t have come in to con tac t w i th

the magnet i c s t i r re r . There fo re , a p rob lem migh t be encounte red

dur ing the read ings o f pH va lues on the pH meter .

CONCLUSION

The exper imenta l va lues o f pKa and Ka o f the unknown ac id a re

4 .537 and 2 .90 x 10 - 5 respec t i ve ly . Whereas , the pKa and Ka va lues

ob ta ined f rom us ing the in i t i a l pH o f the unknown ac id a re 4 .81 and

1 .53 x 10 - 5 respec t i ve ly . Theore t i ca l l y , the pKa and Ka va lues o f the

unknown monopro t i c ac id , wh ich i s be l ieved to be o f ace t i c ac id a re

4 .75 and 1 .76 x 10 - 5 . There fo re , an e r ro r o f 4 .48 percen ts o f the pKa

va lue and 64 .77 percen ts o f the Ka va lue f rom the t i t ra t ion p rocess

a re ca lcu la ted . Meanwh i le , an e r ro r o f 1 .26% and 13 .1% o f pKa and

Ka va lues ob ta ined f rom us ing the in i t i a l pH o f the unknown ac id i s

ca lcu la ted . There fo re , the re i s on ly a s l igh t d i f fe rence f rom the

theore t i ca l va lue compared to the va lues f rom th e t i t ra t ion p rocess .

Hence , the unknown monopro t i c ac id i s iden t i f i ed as ace t i c ac id and

15 | P a g e

t he va lue de te rmined f rom us ing the in i t i a l pH va lue o f the ac id i s a

more accura te method .

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 be more conc lus ive in iden t i f y ing the

unknown monopro t i c ac id , the mo lecu la r we igh t o f the ac id shou ld be

cons idered as we l l . Th is then can be used to compare i t w i th the

theore t i ca l va lue o f mo lecu la r we igh t o f ace t i c ac id . There fo re , i t i s

more p re fe rab le to ob ta in the mass o f so l id ac id and then on ly i t i s

d i lu ted and t i t ra ted .

16 | P a g e

REFERENCES

~ S teven L . Murov , 2004 , Exper iments in Genera l Chemis t ry , 4 t h

Ed i t ion , Un i ted S ta tes : Thomson/Brooks /Co le .

~ h t tp : / /en .w ik iped ia .o rg /w ik i /Ac id -base_ t i t ra t ion

~ h t tp : / / chemlab .com/Chemis t ry _3_5_-

_Monopro t i c_and_Po lypro t i c_Ac ids_-_Chemis t ry

~ h t tp : / /www. t i t ra t ionexper iment .com/h tml /

APPENDICES

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Table 1: Weak Acids , K a , and pK a   va lues

Ac i d HA A - K a pKa

A c e t i c C H 3 C O O H C H 3 C O O - 1 . 7 6 x 1 0 - 5 4 . 7 5

A m m o n i u m N H 4+ N H 3 5 . 6 x 1 0 - 1 0 9 . 2 5

B e n z o i c C 6 H 5 C O O H C 6 H 5 C O O - 6 . 4 6 x 1 0 - 5 4 . 1 9

C a r b o n i c H 2 C O 3 H C O 3- 4 . 3 x 1 0 - 7 6 . 3 7

H C O 3- C O 3

2 - 4 . 8 x 1 0 - 1 1 1 0 . 3 2

C h l o r o a c e t i c C H 2 C l C O O H C H 2 C l C O O - 1 . 4 x 1 0 - 3 2 . 8 5

C i t r i c C 6 O 7 H 8 C 6 O 7 H 7- 7 . 4 1 x 1 0 - 4 3 . 1 3

C 6 O 7 H 7- C 6 O 7 H 6

2 - 1 . 7 4 x 1 0 - 5 4 . 7 6

C 6 O 7 H 62 - C 6 O 7 H 5

3 - 3 . 9 8 x 1 0 - 7 6 . 4 0

F o r m i c H C O O H H C O O - 1 . 7 7 x 1 0 - 4 3 . 7 5

P h o s p h o r i c H 3 P O 4 H 2 P O 4- 7 . 5 2 x 1 0 - 3 2 . 1 2

H 2 P O 4- H P O 4

- 2 6 . 2 3 x 1 0 - 8 7 . 2 1

H P O 42 - P O 4

3 - 2 . 2 x 1 0 - 1 3 1 2 . 6 7

Refer to the a t tachment p rov ided on the nex t page .

18 | P a g e