9.3 pH (ESCPB)

The pH range (ESCPC)

The concentration of certain ions in solution determines whether the solution is acidic or fundamental. Acids and bases can be described as substances that either rise or decrease the concentration of hydrogen (( extH^+)) or hydronium (( extH_3 extO^+)) ions in a solution. An acid increases the hydrogen ion concentration in a solution, while a base decreases the hydrogen ion concentration.

You are watching: The ph scale is a mathematical indicator of the

pH is used to measure the concentration of ( extH^+) ions ((< extH^+>)) and also therefore, whether a substance is acidic or fundamental (alkaline). Solutions via a pH of much less than salso are acidic, while those via a pH better than salso are fundamental (alkaline). The pH scale varieties from ( ext0) to ( ext14) and a pH of ( ext7) is considered neutral.

The global indicator alters colour from (colorred extbfred) in (colorred extbfstrongly acidic) solutions via to (colorpurple extbfpurple) in (colorpurple extbfstrongly basic) remedies.


The term pH was initially used by in 1909 by Søren Peter Lauritz Sørensen (a Danish biochemist). The p stood for potenz and also the H for hydrogen. This converts to power of hydrogen.


pH is a meacertain of the acidity or alkalinity of a solution.

At the beginning of the chapter we stated that we enrespond to many type of examples of acids and bases in our day-to-day stays. The pH of services of some family acids and bases are offered in Table 9.6.

Fizzy cooldrinks often have actually incredibly low pH (are acidic).




( ext2,5)

Diet coke

( ext3,3)


( ext2,5)

Diet pepsi

( ext3,0)


( ext3,2)

7 Up

( ext3,2)

Diet 7 Up

( ext3,7)


Found in



phosphoric acid

fizzy drinks

( ext2,15)


tartaric acid


( ext2,95)


citric acid

lemon juice

( ext3,14)


acetic acid


( ext4,76)


carbonic acid

fizzy drinks

( ext6,37)



cleaning products

( ext11,5)


ammonium hydroxide

cleaning products

( ext11,63)


sodium hydroxide

caustic soda

( ext13)


Table 9.6: The pH of solutions of acids and bases as found in prevalent family items.

pH calculations (ESCPD)

The pH range is a log scale. Remember from mathematics that a difference of one on a base ( ext10) log scale (the one on your calculator) is tantamount to a multiplication by 10. That is:

1 = log(10)

2 = log(100)

3 = log(( ext1 000))

4 = log(( ext10 000))

So a readjust from a pH of 2 to a pH of 6 represents a very big change in the ( extH^+) concentration.

pH deserve to be calculated making use of the complying with equation:

pH = -log<( extH^+)>

<( extH^+)> and also <( extH_3 extO^+)> deserve to be substituted for one another:

pH = -log<( extH_3 extO^+)>

The brackets in the above equation are used to display concentration in ( extmol.dm$^-3$).

Worked example 11: pH calculations

Calculate the pH of a solution where the concentration of hydrogen ions is

( ext1 imes ext10^- ext7) ( extmol.dm$^-3$).

Determine the concentration of hydrogen ions

In this instance, the concentration has actually been given: ( ext1 imes ext10^- ext7) ( extmol.dm$^-3$)

Substitute this value right into the pH equation and also calculate the pH value

pH = -log<( extH^+)>

= -log(( ext1 imes ext10^- ext7))

= 7

Important: It may be helpful to recognize that for calculations involving the pH range, the adhering to equations can additionally be used:

<( extH^+)><( extOH^-)> = ( ext1 imes ext10^- ext14)

<( extH_3 extO^+)><( extOH^-)> = ( ext1 imes ext10^- ext14)

pH = 14 - p<( extOH^-)>

pH = 14 - (-log<( extOH^-)>)

Worked example 12: pH calculations

In a ( ext162) ( extcm$^3$) solution of ethanoic acid, the complying with equilibrium is established:

( extCH_3 extCOOH( extaq) + extH_2 extO( extl)) (leftrightharpoons) ( extCH_3 extCOO^-( extaq) + extH_3 extO^+( extaq))

The number of moles of ( extCH_3 extCOO^-) is uncovered to be ( ext0,001) ( extmol). Calculate the pH of the solution.

Determine the number of moles of hydronium ions in the solution

According to the balanced equation for this reaction, the mole ratio of ( extCH_3 extCOO^-) ions to ( extH_3 extO^+) ions is ( ext1):( ext1), therefore the number of moles of these 2 ions in the solution will be the same.

So, n(( extH_3 extO^+)) = ( ext0,001) ( extmol).

Determine the concentration of hydronium ions in the solution

( extC (mol.dm^-3 ext) = dfrac extn (mol) extV (dm^3 ext))

V = ( ext162) ( extcm$^3$) ( imes dfrac ext0,001 ext dm^3 ext1 ext cm^3) = ( ext0,162) ( extdm$^3$)

<( extH_3 extO^+)> (= dfrac ext0,001 ext mol ext0,162 ext dm^3 =) ( ext0,0062) ( extmol.dm$^-3$)

Substitute this worth right into the pH equation and also calculate the pH value

pH = -log<( extH^+)> = -log<( extH_3 extO^+)>

= -log(( ext0,0062))

= 2.21

Understanding pH is incredibly important. In living organisms, it is essential to keep a continuous pH in the optimal selection for that organism, so that chemical reactions have the right to happen.


( ext1)

( ext6)

( ext7)

( ext8)

( ext13)

(< extH^+>)

( ext1 imes ext10^- ext1)

( ext1 imes ext10^- ext6)

( ext1 imes ext10^- ext7)

( ext1 imes ext10^- ext8)

( ext1 imes ext10^- ext13)

(< extOH^->)

( ext1 imes ext10^- ext13)

( ext1 imes ext10^- ext8)

( ext1 imes ext10^- ext7)

( ext1 imes ext10^- ext6)

( ext1 imes ext10^- ext1)


(colorred extbfstrongly acidic)

(colorred extweakly acidic)


(colorblue extweakly basic)

(colorblue extbfstrongly basic)


Table 9.7: The concentration of (< extH^+>) and also (< extOH^->) ions in services via various pH.

A construct up of acid in the humale body have the right to be incredibly dangerous. Lactic acidosis is a condition led to by the buildup of lactic acid in the body. It leads to acidification of the blood (acidosis) and can make a perchild extremely ill. Some of the symptoms of lactic acidosis are deep and rapid breapoint, vomiting, and abdominal pain. In the fight against HIV, lactic acidosis is a difficulty. One of the antiretrovirals (ARV"s) that is offered in anti-HIV treatment is Stavudine (also known as Zerit or d4T). One of the side results of Stavudine is lactic acidosis, especially in overweight women. If it is not treated conveniently, it have the right to bring about fatality.

Litmus paper can be used as a pH indicator. It is marketed in strips. Purple litmus paper will end up being red in acidic problems and blue in standard conditions. Blue litmus paper is offered to detect acidic conditions, while red litmus paper is offered to detect basic problems.


In agriculture, it is essential for farmers to recognize the pH of their soils so that they are able to plant the best kinds of plants. The pH of soils have the right to differ relying on a number of determinants, such as rainwater, the kinds of rocks and also materials from which the soil was developed and likewise human impacts such as air pollution and fertilisers. The pH of rain water deserve to likewise differ, and also this too has an effect on farming, buildings, water courses, animals and plants. Rainwater is naturally acidic because carbon dioxide in the setting combines via water to develop carbonic acid. Unpolluted rainwater has actually a pH of about ( ext5,6). However, huguy tasks can alter the acidity of rain and also this can cause severe difficulties such as acid rain.

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Calculating pH

Textbook Exercise 9.5

Calculate the pH of each of the following solutions:

(Tip: (< extH_3 extO^+>< extOH^->) = ( ext1 imes ext10^- ext14) can be supplied to identify (< extH_3 extO^+>))

A ( extKOH) solution with a ( ext0,2) ( extmol.dm$^-3$) concentration of ( extOH^-).

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Remember that (< extOH^->< extH_3 extO^+>) = ( ext1 imes ext10^- ext14).

Because of this (< extH_3 extO^+>) = (dfrac ext1 imes ext10^- ext14 ext)

(< extH_3 extO^+>) = (dfrac ext1 imes ext10^- ext14 ext ext0,2)

(< extH_3 extO^+>) = ( ext5 imes ext10^- ext14) ( extmol.dm$^-3$)

pH = -log<( extH_3 extO^+)>

pH = -log(( ext5 imes ext10^- ext14))

pH = 13.3


p<( extOH^-)> = -log<( ext0,2) ( extmol.dm$^-3$)>

p<( extOH^-)> = 0.699

pH = 14 - p<( extOH^-)>

pH = 14 - 0.699

pH = 13.3

An aqueous solution through a ( ext1,83 imes ext10^- ext7) ( extmol.dm$^-3$) concentration of ( extHCl) molecules at equilibrium (( extK_ exta) = ( ext1,3 imes ext10^ ext6))

To recognize the concentration of ( extH_3 extO^+) ions in solution we need to initially compose the expression for ( extK_ exta), and to perform that we need the balanced equation:

( extHCl( extaq) + extH_2 extO( extl)) ( o) ( extH_3 extO^+( extaq) + extCl^-( extaq))

( extK_ exta = dfrac ext ext)

(< extH_3 extO^+>) = (dfrac extK_ exta imes ext ext)

The concentration of ( extH_3 extO^+) will certainly be the exact same as the concentration of ( extCl^-), therefore we deserve to say:

(< extH_3 extO^+>^2) = ( extK_ exta) x (< extHCl>) = ( ext1,3 imes ext10^ ext6) x ( ext1,83 imes ext10^- ext7) = ( ext0,24)