pHWhat is it?

What is pH?

Power of Hydrogen

What is pH?

• A measure/scale that allows us to determine if a solution is acidic (H+), neutral or basic (OH-).

Acidic Basic (alkaline)

weaker strongerstronger

Neutral

pH of common substances

Should know: pH of rain is 5

Scale used to measure the concentration of hydrogen ions, , in a solution.H

+

Scale used to measure the concentration of hydrogen ions, , in a solution.H

+

Scale generally runs from 0 - 14

Scale generally runs from 0 - 14

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

In pure water a few of the

water, H2O, molecules will

split up into hydrogen, H ,

and hydroxide, OH , ions.

+_

H2O → H+

OH-

+

OH- H

+

OH-

H+

0 1 2 3 4 5 6 8 9 10 11 12 13 147

In pure water the number of

H and OH ions are equal.+ -

OH- H

+

OH-

H+

0 1 2 3 4 5 6 8 9 10 11 12 13 14

H and OH are balanced.+ -

This corresponds to a pH of 7.

7

pH 7 = Neutral

In pure water the number of

H and OH ions are equal.+ -

0 1 2 3 4 5 6 8 9 10 11 12 13 14

H and OH are balanced.+ -

This corresponds to a pH of 7.

7

pH 7 = Neutral

Neutral

OH- H

+

OH-

H+

0 1 2 3 4 5 6 8 9 10 11 12 13 14

If an acid is added to the water, H+

the quantity of will increase.

0 1 2 3 4 5 6 8 9 10 11 12 13 14

pH value dropsH

+Acids release more H

+

If an acid is added to the water, H+

the quantity of will increase.

Acids

0 1 2 3 4 5 6 8 9 10 11 12 13 14

H+

Acids release more H+

pH value drops

Acids

0 1 2 3 4 5 6 8 9 10 11 12 13 14

❖ The stronger the acid, the lower the pH.

If a base is added to the water, OH-the quantity of will increase.

0 1 2 3 4 5 6 8 9 10 11 12 13 14

pH value risesBases release OH

- less H+

If a base is added to the water, OH-the quantity of will increase.

Bases release

0 1 2 3 4 5 6 8 9 10 11 12 13 14

❖ The stronger the base, the higher the pH.

OH-

Bases

pH value risesless H

+

Dissolving a salt in the water (normally) does not affect the balance between 𝐇+ and 𝐎𝐇− ions.

Saline (salt) solutions are usually neutral: pH = 7

0 1 2 3 4 5 6 8 9 10 11 12 13 14

Calculating the Strength of pH

For every unit on the pH scale there is a 10x difference between strengths.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

x10 x10 x10

How much weaker is an acid of 4 vs 1?How much stronger is a base of 13 vs 8?How much stronger is a base of 9 vs an acid of 5?

10x10x10 = 1000 or 103

100 000 or 105

10 000 or 104

Strength to Neutralize

• Each specific unit has its opposite on the pH scale

• To neutralize you must have the same amount of solution of the opposite strength

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Strength to Neutralize

• 1- What would you add to neutralize 30 mL of a pH of 6?

30 mL of pH 8• 2- What would you add to neutralize 60 mL of a

pH of 10?60 mL of pH 4

• 3- You want to neutralize 50 mL of a pH of 3. You only have pH 8 available. What do you do?

Add more than 50 mL of pH 8

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

We’ll figure out the calculations next class!

How antacids work

Identifying unknowns using indicators and buffers

• Buffer solutions

– Clear liquids (chemicals) which have the strengths of specific pH levels. ex: buffer 8 = pH 8 buffer 4 = pH 4

• Indicators

– Liquids (chemicals) which will produce various colours depending on the pH of the solution it is mixed with.

pH Indicators

pH indicators can be in the form of a paper or a solution.

Indicators

• Sometimes the colour change gives a lot of info, sometimes very little info…

• That’s why it’s often best to use them in combination

Many pH indicators have 2 main colours.

Some can be colourless for certain pH values.

Some pH indicators change colour several times.

Some natural substances can act as pH indicators.

pH

0pH

1pH

2pH

3pH

4pH

5pH

6pH

7pH

8pH

9pH

10pH

11pH

12pH

13pH

14

Get a sample of each of the different pH values (these are called buffer solutions).

pH

0pH

1pH

2pH

3pH

4pH

5pH

6pH

7pH

8pH

9pH

10pH

11pH

12pH

13pH

14

Add a drop of the indicator (for example… litmus) to each of the buffer solutions.

pH

0pH

1pH

2pH

3pH

4pH

5pH

6pH

7pH

8pH

9pH

10pH

11pH

12pH

13pH

14

pH

0pH

1pH

2pH

3pH

4pH

5pH

6pH

7pH

8pH

9pH

10pH

11pH

12pH

13pH

14

Now we can make a reference chart of the colour for each of the pH values.

Red BluePurpleLitmus:

pH

0pH

1pH

2pH

3pH

4pH

5pH

6pH

7pH

8pH

9pH

10pH

11pH

12pH

13pH

14

Now we can use the indicator, with the chart, to narrow down the pH of an unknown solution.

Red BluePurpleLitmus:

pH range: 9 - 14

If we use a few different indicators, we can narrow down the pH value

even more.

Methyl orange indicator Phenolphthalein indicator

Methyl orange indicator Phenolphthalein indicator

For example: If one indicator

gives a pH range of 4 - 9

And different indicator shows a pH range of 1 - 5

Then we know the solution being tested

has a pH of4 - 5

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Red Cabbage Indicator

TitrationWhat is it?

Acid-Base Titration

• A technique whereby a certain reactant (whose nature and concentration are known) is reacted with another reactant (whose concentration is unknown) in order to determine the unknown’s concentration

– An indicator is used in order to determine the end point (neutralization)

Acid-Base Titration

• Last year, all you had to do was determine the volume needed to neutralize your unknown acid or base

• This year we will go one step further and you will need to determine the unknown’s concentration

Example (Volume)

You are given an unknown solution, and after testing it with litmus paper these are your results:

What is the nature of your unknown solution?

Base!

Titration procedure

1) Obtain a 25ml beaker2) Obtain a 25ml burette3) Using a graduated cylinder, pour 10.0 ml

of unknown base into the beaker4) Add 5 drops of Bromothymol blue5) Fill burette with 1.0 M solution of HCl6) Record starting volume

3.7ml

Example (Volume)7) Slowly add acid from burette into beaker, swirling after each addition8) Stop adding when solution in beaker turns green9) Record end volume on burette10) Calculate volume of acid needed to neutralize unknown base.11) Repeat steps 1-10 two more times12) Determine average volume needed to neutralize.

3.7ml

7.3ml

7.3ml – 3.7ml = 3.6 ml

TITRATION CALCULATIONS

Titration Calculations

• In order to be able to do the calculations necessary we have to be sure we fully understand neutralization.

– Review: in order to have full neutralization, there needs to be the same number of hydrogen ions (H+) as hydroxide ions (OH-)

Titration Calculations

• Review: – Acids release H+ ions in solution

– Bases release OH- ions in solution

– When there are more H+ ions than OH- ions, the solution is acidic

– When there are more OH- ions than H+ ions, the solution is alkaline (basic)

– Therefore: in order to have full neutralization, there needs to be the same number of hydrogen ions (H+) as hydroxide ions (OH-)

Titration Calculations

• Review:

– Molarity is:

𝐶 =𝑛

𝑉

C : concentration in mol/L

n: number of moles

V: volume of solution in L

Titration Calculations

• If there has to be the same number of H+ and OH- ions then that means they have to have the same number of moles (n)

𝐶𝐴 =𝑛𝐴𝑉𝐴

𝐶𝐵 =𝑛𝐵𝑉𝐵

𝑛𝐴 = 𝐶𝐴𝑉𝐴 𝑛𝐵 = 𝐶𝐵𝑉𝐵

Titration Calculations

• If there has to be the same number of H+ and OH- ions then that means they have to have the same number of moles (n)

𝑛𝐴 = 𝑛𝐵𝑛𝐴 = 𝐶𝐴𝑉𝐴 𝑛𝐵 = 𝐶𝐵𝑉𝐵

𝐶𝐴𝑉𝐴 = 𝐶𝐵𝑉𝐵

Note: this equation only works for acids and bases that are in a 1:1 mole

ratio

Example• Let’s revisit the example from last class

• What was the volume of 1.0M HCl needed to neutralize 10.0ml of the unknown base?

3.6 𝑚𝑙

𝐶𝐴𝑉𝐴 = 𝐶𝐵𝑉𝐵(1.0mol/L)(0.0036L) = 𝐶𝐵(0.010L)

0.0036 𝑚𝑙

0.0036mol = 𝐶𝐵(0.010L)0.36 𝑚𝑜𝑙/𝐿 = 𝐶𝐵