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CO2 and pH

Introduction:the aim of this experiment is to show the influence of the CO 2absorbed by

water on its pH.

Preparation time: 10 minutes

Activity time: 15 minutes

Applications:

- llustration of the production of chemical species durin! a chemical reaction

- "xperiment of pH measurement : use of a pH meter

- Comparison of precision between different measurement

Time of analysis and interpretation: #0 minutes

Previous knowledge required:

- $asic %nowled!e on pH and lo!arithmic calculation

Teorical part:

!"# tis part requires a good knowledge of cemistry and can $e skiped%

A% Here are !i&en:

'he followin! chemical reaction : CO2( H2O ) HCO-#( H

(.

*) +,H(,HCO-#/,CO2

'he aspect of the lo!arithmic function :

'he mathematical expression of pH : pH ) +p*( pCO2 with pCO2) -lo!

,CO2

- raw the aspect of the function representin! the pH in function of CO2

- How does pH chan!e in function of the uantity of CO23

&'periment:

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(aterial: for a !roup of two students:

- 1 250 m4 bea%er

- 1 straw

- 1 pH meter and its manual- 1 timer

- pH indicator paper

- alt to ma%e artificial sea water

Procedure:

- 6ill the bea%er with 2004 of water

- 7dd the uantity of salt indicated on the instructions to ma%e sea water

- Calibrate the pH-meter and put it in the bea%er

- 8easure the solution9s pH at t)0 start the timer. One member of the !roup blows

air with the straw in the water for the number of seconds indicated by thisprocedure. t has to be the same person that blows air e&ery time. ;ote the pH

indicated by the pH meter. +the timer will be stopped between each blowin! period

and the one blowin! will blow e&enly each time

- 0 ?0 120 1@0 2=0

pH !i&en by the pH meter

pH !i&en by indicator paper

)esults analysis:

#% raw the cur&e of the pH &aryin! with time. o these &ariations correspond to your

pre&ious conclusions3

C% Compare your results with those of another !roup

- 7re both cur&es the same3

- Could you explain this3

- Ahy does the procedure indicate that only one person should blow air in the

water3

*% Comparison of measurement de&ices:

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- Ahich one is the most precise3

- o we always need the most precise de&ice3 Bi&e two examples of cases where it

is not necessary.

'o !o further:

'he teacher can explain the conseuences of acidification on or!anisms with limestone shells

li&in! in the ocean:

'o create the limestone of his shell an or!anism needs ions that are present in water. 'hey

react accordin! to the followin! reaction:

Ca2( (CO#2-CaCO#

Howe&er CO2 HCO#-et CO#

2-are distributed the followin! way accordin! to pH le&el:

Ae can obser&e that when pH decreases the uantity of CO#2-

ions decreases. 7s a resultcalcifyin! or!anisms can9t produce as much limestone their shell or structure shrin%s and

they become more and more &ulnerable.

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CO2and pH: answers

)esults presentation:+example

t pH

0 8,14

10 8,2

20 7,7

30 7,2

45 6,85

60 6,68

90 6,5120 6,43

A few teorical results :

A%'he followin! chemical reaction is !i&en : CO2( H2O ) HCO-#( H(. t leads to: pH ) +p*( pCO2 with pCO2 ) -lo! ,CO2.

Ahat does the cur&e of pH &aryin! with pCO2 loo% li%e :

How does pH &ary with the uantity of CO23

pH=f(t)

6

6,5

7

7,5

8

8,5

0 20 40 60 80 100 120 140

t

pH

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Ae %now that pCO2 ) -lo!,CO2

o : when pCO2decreases CO2 uantity increases. o: when CO2increases pH decreases.

'his is what happens with ocean acidification: the CO2absorbed reduces ocean pH.

)esults analysis :

#%raw the cur&e of the pH &aryin! with time. o these &ariations correspond to your

pre&ious conclusions3 Ahy3

7ir is blown it means that CO2 uantity increases as a result pH decreases.

C%Compare your results with those of another !roup

7re both cur&es the same3

Could you explain this3

Ahy does the procedure indicate that only one person should blow air in the

water3

;o both cur&es are not identical. t can easily be explained : CO2 uantity in the air breathed

out depends on who blows that air. 'his is also why only one person per !roup blows air.

*%Comparison of measurement de&ices:

Ahich one is the most precise3

o we always need the most precise de&ice3 Bi&e two examples of cases

where it is not necessary.

'he pH meter is more precise. Ahen one doesn9t need exact measurements of pH indicator

paper can be enou!h. Here indicator paper results cannot be used because &ariations are not

&isible enou!h.