pH

Potential Hydrogen

pH

A measure of hydronium ion concentration

pH

A measure of

[

H 3 O +

]

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pH of some solutions

HCl - 0 Stomach acid - 2 Cola drinks - 3 Tomatoes - 4 Coffee - 5 Milk - 6.5

pH of some solutions

HCl - 0 Stomach acid - 2 Cola drinks - 3 Tomatoes - 4 Coffee - 5 Milk - 6.5

Pure water - 7 Blood - 7.5

Sea water - 8 Detergent - 10 Household cleaners - 11 Oven cleaners - 14

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pH Scale - 17 min

Measuring pH Meters Indicators strips liquids

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Measuring pH

Most professional laboratories use a digital pH meter to read the pH of QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

solutions directly.

Measuring pH

These instruments provide a very quick and precise reading.

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Measuring pH

Bench-type pH meters have an electrode on an adjustable arm and are capable of large-scale use, with results to five QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

significant figures.

Measuring pH

Portable pH meters are battery-powered for use in the field.

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Measuring pH

They are obviously less expensive, but still provide readings with two or three significant figures.

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Measuring pH

These are common in high school chem labs.

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Measuring pH

Before pH meters, chemical

indicators

were used to determine pH.

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Measuring pH

Indicators are weak organic acids and bases whose colors differ from the colors of their conjugate QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see t his picture.

acids or bases.

Measuring pH

Indicators have two

major

weaknesses: QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see t his picture.

Measuring pH

1. They only provide a "ballpark" determination of a solution's pH.

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Measuring pH

2. They only work well with solutions that begin colorless.

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pH Indicators

Litmus paper

is the simplest test strip, indicating only if a solution is an

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acid or base.

pH Indicators

Red litmus turns blue in a base.

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pH > 8.5

pH Indicators

Blue litmus turns red in an acid.

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pH < 5.5

pH Indicators DO NOT

dip test papers into solutions.

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pH Indicators

This

MAY

contaminate your sample.

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pH Indicators

Touch a clean stirring rod to the sample, then touch the rod to the test paper.

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Measuring pH

Hydrion paper

(pH paper) is similar to litmus paper, but provides a range of values.

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Measuring pH

The color changes are shown on the container.

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Measuring pH

pH paper is available in several ranges, for use in different types of solutions.

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pH Indicators Phenolphthalein

solution Colorless in acid Faintly pink at neutral Dark pink in base

pH Indicators Universal Indicator

solution A solution that changes to these colors

Ionization of Water 2 H

2

O H

3

O

+ (aq)

+ OH

(aq)

The Ion Product Constant of Water K

w

= [H

3

O

+

] [OH

-

] = 1 X 10

-14

Calculating pH

pH = - log [ H 3 O + ] [H 3 O + ] is expressed in powers of 10 from 10 -14 to 10 0

Sample Problem #1 What is the pH of a solution with a [H 3 O + ] of 1 x 10 -4 M?

Problem Solution p H = - log [H 3 O + ] p H = - log (1 x 10 -4 )

Problem Solution p H = - log [H 3 O + ] p H = - log (1 x 10 -4 ) Do the following on your calculator: Enter 1x10 -4 Press "LOG" PRESS "+/-" P H = ?

Problem Solution p H = - log [H 3 O + ] p H = - log (1 x 10 -4 ) Do the following on your calculator: Enter 1x10 -4 Press "LOG" PRESS "+/-" P H = 4

Sample Problem #2 0.01 moles of HCl is added to water to make 1 dm 3 of solution. Assuming the HCl is completely ionized, what is the pH of the solution?

Problem Solution p H = - log [H 3 O + ] What is the [H 3 O + ]?

Problem Solution p H = - log [H 3 O + ] p H = - log (1 x 10 -2 )

Problem Solution p H = - log [H 3 O + ] p H = - log (1 x 10 -2 ) Do the following on your calculator: Enter 1x10 -2 Press "LOG" PRESS "+/-" P H = ?

Problem Solution p H = - log [H 3 O + ] p H = - log (1 x 10 -2 ) Do the following on your calculator: Enter 1x10 -2 Press "LOG" PRESS "+/-" P H = 2

Have you noticed anything about these two problems?

If this # is 1

1 X 10

-7

If this # is 1

1 X 10

-7

The pH is this as a positive #

Find the pH of a solution whose [H 3 O + ] is 9.5 x 10 -8 M.

Sample Problem #3 pH = - log [H 3 O + ]

Find the pH of a solution whose [H 3 O + ] is 9.5 x 10 -8 M.

Sample Problem #3 pH = - log [H 3 O + ] pH = - log (9.5 x 10 -8 )

Find the pH of a solution whose [H 3 O + ] is 9.5 x 10 -8 M.

Sample Problem #3 pH = - log [H 3 O + ] pH = - log (9.5 x 10 -8 ) pH = - (log 9.5 + log 10 -8 )

Find the pH of a solution whose [H 3 O + ] is 9.5 x 10 -8 M.

Sample Problem #3 pH = - log [H 3 O + ] pH = - log (9.5 x 10 -8 ) pH = - (log 9.5 + log 10 -8 ) pH = - (.98 + (-8))

Find the pH of a solution whose [H 3 O + ] is 9.5 x 10 -8 M.

Sample Problem #3 pH = - log [H 3 O + ] pH = - log (9.5 x 10 -8 ) pH = - (log 9.5 + log 10 -8 ) pH = - (.98 + (-8)) pH = - (-7.02 )

Find the pH of a solution whose [H 3 O + ] is 9.5 x 10 -8 M.

Sample Problem #3 pH = - log [H 3 O + ] pH = - log (9.5 x 10 -8 ) pH = - (log 9.5 + log 10 -8 ) pH = - (.98 + (-8)) pH = - (-7.02 )

pH = 7.02

If this # is not 1

1.5 X 10

-7

If this # is not 1

1.5 X 10

-7

The pH is a positive decimal number one less than this #

Sample Problem #4 What is the [H 3 O + ] of a solution with a pH of 12?

pH = - log [H 3 O + ]

pH = - log [H 3 O + ] 12 = - log [H 3 O + ]

pH = - log [H 3 O + ] 12 = - log [H 3 O + ] - 12 = log [H 3 O + ]

pH = - log [H 3 O + ] 12 = - log [H 3 O + ] - 12 = log [H 3 O + ] [H 3 O + ] = antilog - 12

pH = - log [H 3 O + ] 12 = - log [H 3 O + ] - 12 = log [H 3 O + ] [H 3 O + ] = antilog - 12 antilog is "INV" + "log" on most calculators

pH = - log [H 3 O + ] 12 = - log [H 3 O + ] - 12 = log [H 3 O + ] [H 3 O + ] = antilog - 12 antilog is "INV" + "log" on most calculators

[H 3 O + ] = 1 x 10 -12

Sample Problem #5 What is the [H 3 O + ] of a solution with a pH of 5.45?

pH = - log [H 3 O + ]

pH = - log [H 3 O + ] 5.45 = - log [H 3 O + ]

pH = - log [H 3 O + ] 5.45 = - log [H 3 O + ] - 5.45 = log [H 3 O + ]

pH = - log [H 3 O + ] 5.45 = - log [H 3 O + ] - 5.45 = log [H 3 O + ] [H 3 O + ] = antilog - 5.45

pH = - log [H 3 O + ] 5.45 = - log [H 3 O + ] - 5.45 = log [H 3 O + ] [H 3 O + ] = antilog - 5.45

antilog is "INV" + "log" on most calculators

pH = - log [H 3 O + ] 5.45 = - log [H 3 O + ] - 5.45 = log [H 3 O + ] [H 3 O + ] = antilog - 5.45

antilog is "INV" + "log" on most calculators

[H 3 O + ] = 3.5 x 10 -6

pOH

•

pOH = - log [OH ]

pOH

• •

pOH = - log [OH ] [OH ] is expressed in powers of 10 from 10 -14 to 10 0

pOH

• • •

pOH = - log [OH ] [OH ] is expressed in powers of 10 from 10 -14 to 10 0 [H 3 O + ] [OH ] = 1 x 10 -14

pOH

• • • •

pOH = - log [OH ] [OH ] is expressed in powers of 10 from 10 -14 to 10 0 [H 3 O + ] [OH ] = 1 x 10 -14 pH + pOH = 14

Sample Problem #6 What is the pH of a solution with a pOH of 1.36?

Sample Problem #6 What is the pH of a solution with a pOH of 1.36?

pH + pOH = 14

Sample Problem #6 What is the pH of a solution with a pOH of 1.36?

pH + pOH = 14 pH = 14 - pOH

Sample Problem #6 What is the pH of a solution with a pOH of 1.36?

pH + pOH = 14 pH = 14 - pOH pH = 14 - 1.36

Sample Problem #6 What is the pH of a solution with a pOH of 1.36?

pH + pOH = 14 pH = 14 - pOH pH = 14 - 1.36

pH = 12.64

Sample Problem #7 What is the pOH of a solution with a [OH ] of 2.97 x 10 -10 ?

Sample Problem #7 pOH = - log [OH ]

Sample Problem #7 pOH = - log [OH ] pOH = - log (2.97 x 10 -10 )

Sample Problem #7 pOH = - log [OH ] pOH = - log (2.97 x 10 -10 ) pOH = - (log 2.97 + log 10 -10 )

Sample Problem #7 pOH = - log [OH ] pOH = - log (2.97 x 10 -10 ) pOH = - (log 2.97 + log 10 -10 ) pHO = - (.47 + (-10))

Sample Problem #7 pOH = - log [OH ] pOH = - log (2.97 x 10 -10 ) pOH = - (log 2.97 + log 10 -10 ) pHO = - (.47 + (-10)) pOH = - (-9.53)

Sample Problem #7 pOH = - log [OH ] pOH = - log (2.97 x 10 -10 ) pOH = - (log 2.97 + log 10 -10 ) pHO = - (.47 + (-10)) pOH = - (-9.53)

pOH = 9.53

Solutions that can absorb moderate amounts of acid or base without a significant change in pH.

Buffers provide ions that react with H 3 O + or OH if they are introduced into the solution.

As either ion is neutralized, the pH of the system remains nearly constant.

Buffer solutions are prepared using a weak acid or base with one of its salts.

The weak acid or base reacts with one of the added ions, the salt reacts with the other ion.

The combination keeps the pH fairly constant, up to a point.

By choosing the correct weak acid or base, a buffer solution can be prepared that will maintain almost any pH.

This buffer pH range is between 6.5 and 7.5

Measuring pH

Why does Alka-Seltzer make your upset stomach better?