pH

Chemistry Project

pH scale

pH scale In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at 25 °C (77 °F). Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline. pH measurements are important in medicine, biology, chemistry, agriculture, forestry, food science, environmental science, oceanography, civil engineering and many other applications.  The pH scale is the concentration of hydrogen ions in a given substance. pH= -log [ H+ ]

Let’s dig the history

Why pH?

 The concept of p[H] was first introduced by Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg Laboratory in 1909 and revised to the modern pH in 1924 after it became apparent that electromotive force in cells depends on activity rather than concentration of  hydrogen ions. In the first papers, the notation had the H as a subscript to the lowercase p, like so: pH. It is unknown what the exact definition of 'p' in pH is. A common definition often

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However some references suggest the p stands for “Power”,others refer to the German word “Potenz” (meaning power in German),still others refer to “potential”.  Jens Norby published a paper in 2000 arguing tha p is a constant and stands for “negative logarithm”; H then stands for Hydrogen. According to the Carlsberg Foundation pH stands for "power of hydrogen". Other suggestions that have surfaced over the years are that the p stands for puissance (also meaning power , based on the fact that the Carlsberg Laboratory was French-speaking), or that pH stands for the Latin terms pondus Hydrogenii or potentia hydrogenii. It is also suggested that Sørensen used the letter p and q (commonly paired letters in mathematics simply to label the test solution (p) and the

If pH scale records 7 in its reading then it is water  (neutralizer)

Universal indicator compound Indicator 

Low pH color 

Transition pH range

High pH color 

Thymol blue (first transition)

Red

1.2 – 2.8

Yellow

Methyl red

Red

4.4 – 6.2

Yellow

Bromothymol blue

Yellow

6.0 – 7.6

Blue

Thymol blue (second transition)

Yellow

8.0 – 9.6

Blue

Phenolphthalein

Colorless

8.3 – 10.0

Fuchsia

Questions A solution has a pH of 8.5. What is the Molarity of hydrogen ions in the solution? pH = - log [H+] 

8.5 = - log [H+] -8.5 = log [H+] Antilog -8.5 = antilog (log [H+]) 10-8.5 = [H+] 3.16 X 10-9 = [H+]

 Test Your Knowledge What is the range of an ACID on the pH scale? Ans: 0-7 •

What is the range of a BASE and what is another name for a BASE? Ans: 7-14, Alkaline •

pOH 

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Since acids and bases are opposites, pH and pOH are opposites! pOH does not really exist, but it is useful for changing bases to pH. pOH looks at the perspective of  a base pOH = - log [OH-] Since pH and pOH are on opposite ends, pH + pOH = 14  –

pH

[H+]

[OH-]

pOH