Sem 1 2013 pH Indikator and Buffer

7/27/2019 Sem 1 2013 pH Indikator and Buffer

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pH, INDICATOR &

BUFFER


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Takrifan pH

Formula pH dan pOH dan cara pengiraan

Penerangan terhadap indikator dan kegunaannya

Senarai jenis jenis indikator dalam pentitratan

asid bes

Penerangan terhadap ciri ciri penting bagiindikator yang baik

Penerangan mengenai larutan buffer dan

kegunaannyaPenerangan mengenai larutan buffer

berasid,beralkali dan biologikal


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pH

pH is a unit of measure which describes the degree of acidity or

alkalinity (basic) of a solution.

It is measured on a scale of 0 to 14.

Mathematically:pH = -log[H+]

pH= power of hydrogenlog = logarithm (math function)[H+] = concentration of H+ expressedin Molarity (moles/liter)


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So this means

log 1000 = log 103 = 3 or that log 0.0000001 = log 10-7= 7So this means that some pH problems, the ones that are powers of 10 are vey easy. Let's look

at some of these

log10100000 = log 105 = w log100.001 = log 10

-3 = a

[H+] pH calculation pH

10-1 pH = -log[H+] = - log 10-1= - -1 =

1

10-2 pH = -log[H+] = - log 10-2= --2 =

2

10-3 pH = -log [H+] = -log 10-3= --3 =

3

[H+] = 10-pH


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pOH

p_ is just a short hand way of describing large

concentration differences that exists for manyother particles. pAg describes the concentration

of silver ions. pCl describes the concentration of

Cl ions. pOH descries the concentration ofhydroxide ions. pOH is calculated in exactly the

same way that pH is calculated.

pOH = -log[OH-]


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[OH-] = 0.00000000845 M Convert this first to a

number in scientific notation.

[OH-] = 8.45 x 10 -9 M (immediately you can see that

the pOH should be close to 8 or 9)

pOH = - log (8.45 x 10 -9) = -(-8.07) = 8.07

[OH] pH calculation pH

10-1 pH = -log[OH-] = - log 10-1= - -1 =

1

10-2

pH = -log[OH-

] = - log 10-2

= --2 = 2

10-3 pH = -log [OH-] = -log 10-3= --3 =

3

[OH-

] = 10-pOH


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Calculating pH

What is the pH if the hydrogen ion concentration in avinegar solution is 0.001M?

pH = log[H+]

pH = log(0.001)

pH = (3) = 3

The pH of the vinegar is 3, so the vinegar is acidic.


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The pH Scale

Most solutions have a pH between 0 and 14.

Acidic solutions have a pH less than 7.

As a solution becomes more acidic, the pH decreases.

Basic solutions have a pH greater than 7. As a solution becomes more basic, the pH increases.


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Kw: the ion-product constant of waterQ: will pure distilled water conduct electricity

A: As the demo shows, it will (slightly). If water conducts electricity, ions must exist Water exists as an equilibrium, which is

referred to as the self-ionization of water:

H2O + H2O H3O+(aq) + OH

(aq)

Simplified reaction: H2O H+(aq) + OH(aq)

Kc =[H3O

+] [OH]

[H2O]2

Kw =or [H3O+] [OH]

Note: H+ is just shorthand for H3O+

Kc =[H+] [OH]

[H2O]

Kw =or [H+] [OH]


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pH testing There are several ways to test pH

Blue litmus paper (red = acid)

Red litmus paper (blue = basic) pH paper (multi-colored)

pH meter (7 is neutral, 7base)

Universal indicator (multi-colored)

Indicators like phenolphthalein

Natural indicators like red cabbage,radishes


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Paper testingPaper tests like litmus paper

and pH paper Put a stirring rod into the solution and

stir.

Take the stirring rod out, and place a

drop of the solution from the end ofthe stirring rod onto a piece of thepaper

Read and record the color change.

Note what the color indicates. You should only use a small portion of

the paper. You can use one piece ofpaper for several tests.


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20

Acid/Base Classifications of Solutions

A solution can be classified according to its pH:

Strongly acidic solutions havea pH less than 2.

Weakly acidic solutions havea pH between 2 and 7.

Weakly basic solutions have apH between 7 and 12.

Strongly basic solutions have

a pH greater than 12. Neutral solutions have a pH

of 7.


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The pH Concept

Recall that pH is a measure of the acidity of a solution.

A neutral solution has a pH of 7, an acidic solution hasa pH less than 7, and a basic solution has a pH greaterthan 7.

The pH scale uses powers of ten to express thehydrogen ion concentration.

Mathematically: pH = log[H+]

[H+] is the molar hydrogen ion concentration


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Calculating pH

What is the pH if the hydrogen ion concentration in avinegar solution is 0.001M?

pH = log[H+]

pH = log(0.001)

pH = (3) = 3

The pH of the vinegar is 3, so the vinegar is acidic.


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Chapter 15 23

Calculating [H+] from pH

If we rearrange the pH equation for [H

+

], we get:[H+] = 10pH

Milk has a pH of 6. What is the concentration of

hydrogen ion in milk?

[H+] = 10pH = 106 = 0.000001M

[H+

] = 1

106

M.


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Chapter 15 24

Advanced pH Calculations

What is the pH of blood with [H

+

] = 4.8

10

8

M?pH = log[H+] = log(4.8 108) = (7.32)

pH = 7.32

What is the [H+] in orange juice with a pH of 2.75?

[H+] = 10pH = 102.75 = 0. 0018M

[H+] = 2.75 103M


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Manipulating pH

Algebraic manipulation of:

pH = - log [H3O+]allows for:

[H3O+] = 10-pH

If pH is a measure of the hydronium ionconcentration then the same equations could be

used to describe the hydroxide (base)

concentration.

[OH-] = 10-pOH pOH = - log [OH-]

thus:

pH + pOH = 14 ; the entire pH range!

pH Equations


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pH Equations

You must know the following equations, which are all based on the ionization of

water at 250 C!

H2O H+ + OH-

Kw = [H+][ OH-] = 1.00 x 10-14

pH = -Log[H+] pOH = -Log[OH-]

[H+] = 10-pH [OH-] = 10-pOH

pH + pOH = pKw = 14.000

Kepekatan Ion H+ dalam air tulen adalah 10-7M sama seperti ion OH-

Kepekatan H+ dan OH- adalah sama, maka air tulen adalah neutral


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pH, INDICATOR &

BUFFER


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Chapter 8 Introducing Acids & Bases

Water pH (Acid rain) in the USA in 2001


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Conjugate acids & bases

R l ti b t [H+] [OH ] d H?


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Relation between [H+], [OH-], and pH?

8 3 St th f id & b


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8-3 Strengths of acids & bases

Strong Acids and Bases

Common strong acids and bases are listed in Table 8-1.

A strong acid or strong base is completely dissociated

in aqueous solution.

(8-4)

(8-5)

P.175


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Carboxylic Acids Are Weak Acids and

Amines Are Weak Bases

(8-6)

P.175


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8-3 Strengths of acids & bases

Ask yourself atp.178

Carboxylic Acids are Weak Acids and Amines are Weak Bases


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Metal Ions with Charge 2 Are Weak Acids

A proton can dissociate from M(H2O)wn+ to reduce

the positive charge on the metal complex.

P.177

Relation Between Ka and Kb


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8-4 pH of strong Acids & Bases

Example atp.180

The pH of 4.2 x 10-3 M HClO4 ?

The pH of 4.2 x 10-3 M KOH?

Can we dissolve base in water and

obtain an acidic pH (


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8-5 Tools for Dealing with Weak Acids and Bases

pK the negative logarithm of an equilibrium

constant

Weak Is Conjugate to Weak

The conjugate base of a weak acid is a

weak base. The conjugate acid of a weak

base is a weak acid.

P.181


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P.182

Using Appendix B

Acid dissociation constants appear inAppendix B. Each compound is shown

in its fully protonated form.

Pyridoxal phosphate is given in its fully

protonated form as follows:


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P.182


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8-6 Weak-Acid Equilibrium

P.182


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Fraction of Dissociation

Figure 8-4 compares the fraction

of dissociation of two weak acidsas a function of formalconcentration.

acid increase as it is diluted.

P.184

Ch t 9 B ff


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Chapter 9 Buffers

Buffered solution resists changes in pH when small

amounts of acids or base are added or when dilutionoccurs.

pH dependence of the rate of aparticular enzyme-catalyzed

reaction.

The rate near pH 8 is twice as the

rate at pH 7 or 9


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9.2 The Henderson-Hasselbalch eqn

H AAnw hepKpHacid

basel ogpKpH

H A

Al ogpHpK

H A

Al ogHl oglogK

H A

AHKAHH A

-

a

a

a

a

a(aq)(aq)

(aq)

If pH = pKa [HA] = [A-]


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If pH = pKa, [HA] = [A ]

If pH < pKa, [HA] > [A-]

If pH > pKa, [HA] < [A

-

]

9 3 A B ff i A i


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9-3 A Buffer in Action Example: find the pH of a buffer solution atp. 198

Effect of adding acid to a buffer


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9-4 Preparing Buffers

Example atp. 202

In the real lifep. 203

Preparing a Buffer in Real Life


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Preparing a Buffer in Real Life

Suppose you wish to prepare 1.00 L of buffer

containing 0.100 M tris at pH 7.60. When we say

0.100 M tris, we mean that the total concentration oftris plus tris H+ will be 0.100M.

Procedure:

1. Weigh out 0.100 mol tris hydrochloride and dissolve it in abeaker containing about 800 mL water and a stirring bar.

2. Place a pH electrode in the solution and monitor the pH.

3. Add NaOH solution until the pH is exactly 7.60. The electrode

does not respond instantly.4. Transfer the solution to a volumetric flask and wash the beaker

and stirring bar a few times. Add the washings to the

volumetric flask.

5. Dilute to the mark and mix. P.202


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9.5 Buffer capacity -1

The amount of H

+

or OH

-

that buffered solution can

absorb without a significant

change in pH

Buffer capacity measures how well a solution resists changesin pH when acid or base is added.

The greater the buffer capacity, the less the pH changes.

9 5 Buffer capacity 2


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0.670.06

0.04

H A

A1

0.05

0.05

H A

A

:B

0.9965.01

4.99

H A

A1

5.00

5.00

H A

A

:A

Hmol0.01

Hmol0.01

2) Magnitudes of [HA] and [A-]

the capacity of a buffered soln.Ex : soln A : 5.00 M HOAc + 5.00 M NaOAc

soln B : 0.05 M HOAc + 0.05 M NaOAc

pH change when 0.01 mol of HCl(g) is added


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3) [A-] / [HA] ratio the pH of a buffered soln.

solnnew50.5%solnoriginal

49.5H AA100

0.011.00

H AA

0.98H A

A1.00

1.00

1.00

H A

A

2%:C

Hmol0.01

Hmol0.01

Table 9-2 Structures and pKa values for common buffers


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Table 9 2Structures and pKa values for common buffers

P.205

9 6 H i di t k 1


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9.6 How indicators work -1

1) Usually a weak organic acid or basethat has distinctly different colors in its

nonionized & ionized forms.

HIn(aq) H+

(aq) + In-(aq) pKHIn

nonionized ionized

form form


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1)

2)

3)

-

HIn

-

H In

I n&H I nofcolortheofncombinatio

I nH I n

1pKpH

I nofcolortheshow10

H I n

I n

1-pKpH

H I nofcolortheshow10

I n

H I n

9 6 H i di t k 3


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2) The useful pH range for indicator is

pKHIn 1

(Fig 10.3)

encompass the pH at equivalence point

(titration curve)

3) Not all indicators change color at the samepH. (Table 9.3)

T bl 9 3


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Table 9-3


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Two different sets of colors

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Sem 1 2013 pH Indikator and Buffer