Calculating and measuring pH

Baking soda

0.1M pH 8.4

Universal Indicators

A universal indicator is a solution comprised of several different indicators so as to represent a colour change across all pHs. pH < 3, red pH 3-6, orange/yellow pH 7, green pH 8-11, blue pH >11, purple.

Indicators

An indicator (HIn) is an acid or base that undergoes dissociation in a known pH range. Consequently, we can use indicators to measure pH because the acid form and base form of the indicator have different colours in solution: HIn(aq) ↔ H+ (aq) + In- (aq). Q. According to Brønsted-Lowry theory, which form is the acid form and which form is the base form? For each indicator, this colour change occurs over a small range of approximately two pH units.

pH

At 25°C, the pH of a solution is the negative logarithm of the hydrogen-ion concentration, i.e.  pH=−log⁡[H+ ] or [H+] = 10^-pH

Common Indicators

At pH values below the range where the colour change occurs, only the colour of the acid form is evident. Conversely, at pH values above this range, you would observe the colour of the base form.Indicators may take the form of a solution or a paper indicator strip.

Historically....

Historically, scientists used to taste and smell their discoveries in order to characterise certain properties. Fortunately, we now have safer techniques for measuring the acidity of different chemicals. eg. Carl Scheele

pH meters

If an accurate measure of pH is required, a pH meter can be used instead of an indicator. These measurements are not affected by the colour or cloudiness of the solution that is being measured.

pH (and pOH)

In aqueous solutions, the product of the hydrogen-ion concentration ([H+ ]) and the hydroxide-ion concentration ([OH-]) equals 1.0×10^−14, i.e. [H+ ]×([OH-])=1.0×10^−14. An acidic solution, is one in which the hydrogen ion concentration is greater than the hydroxide ion concentration 1.0×10^−7. It can be cumbersome to talk about such small concentrations, hence we often about the acidity or alkalinity of a solution in terms of pH.

HCl

Strong acid 1.0M pH 0.0

NaOH

Strong base 1.0M pH 14.0

Acidic, neutral, basic

Using pH and pOH conversions gives us a more workable scale of concentration. Using pH we can categorise solutions as follows: Acidic: pH < 7.0 Neutral: pH = 7.0 Basic: pH > 7.0

Logarithmic Scale

each change in pH by a factor of 1, represents a change in H+ concentration by a factor of 10.

Stomach acid

pH 1.0-3.0

Milk of magnesia

pH 10.5

Household ammonia

pH 11.9

Lemon juice

pH 2.2-2.4

Vinegar

pH 2.4-3.4

Carbonated water

pH 3.9

Beer

pH 4.0-4.5

Milk

pH 6.4

Sea water

pH 7.0-8.3

Blood

pH 7.4

pOH

pOH considers the negative logarithm of the hydroxide-ion concentration, pOH=−log⁡[OH-]

Litmus Paper

turns red in acid, and blue in alkaline solutions.