Acidity and Alkalinity

Definitions

Alkalinity Measurement

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Acidity Measurement

Importance

What are Acidity and Alkalinity

Acidity The capacity of a water sample to neutralise added alkali.

Two Types Mineral Acidity from Strong Acids, and from Salts of Trivalent Metal Ions

H2SO4 2 H+ + SO42-

FeCl3 + 3 H2O Fe(OH)3 + 3 H+ + 3 Cl -

Carbon Dioxide Acidity (Weak Acidity)

CO2 + H2O H2CO3* H+ + HCO3

-

Alkalinity The capacity of a water sample to neutralise added acid.

Mainly from:

1. The Carbonate - Bicarbonate buffering system.

2. The salts of weak acids CO2 + CaCO3 + H2O Ca2+ + HCO3

-

Na Acetate, Na Propionate

3. Hydroxide

Ammonia

What are Acidity and Alkalinity

Equilibrium of CO2 , HCO3- , CO3

2-

CO2 (aq) + H2O H2CO3 H+ + HCO3-

2H+ + CO3

2-

CO2 (g)

pH 4 4.5 5 6 7 8 8.3 9 10

pKa = 6.3 pKa = 10.3

350ppm

10-5M

pH = pKa + log ([A-] / [AH])

When reaction is 99% to the left then pH = 6.3 + log (1/100)

= 6.3 - 2 pH = 4.3

air

water

How is Acidity Determined

By titration with a Standard Alkali solution to a specific Endpoint.

Mineral Acidity Indicator used is Methyl Orange (endpoint at pH 4.5 )

Carbon Dioxide Acidity Indicator used is Phenolphthalein (endpoint at pH 8.3 )

In practice CO2 is the major weak acid. Others are possible, eg. Acetic

Acidity

Strong Acids

Weak Acids (CO2 Acidity)

P endpoint (pH 8.3)

M endpoint (pH 4.5)

P

M

M = Methyl OrangeP = Phenolphthalein

Titration with N/50 NaOH

pH

High

Low

starting pH

starting pH

How is the Acidity Calculated and Reported

Titration Volume, Alkali Strength, Sample Volume

Usually Report Acidity as Calcium Carbonate Equivalent

“ The Acidity to pH X = Y mg CaCO3 / litre “

or Methyl Orange Acidity / Mineral Acidity =

Phenolphthalein Acidity / CO2 Acidity =

If N/50 NaOH is used

Acidity (mg CaCO3) = Vt x 1000/ Vs

Vt = Titre volume (ml) Vs = Sample volume (ml)

How is Alkalinity Determined

By titration with a Standard Acid solution to a specific Endpoint.

Sample pH Values > pH 8.3 Two Step Titration Step 1. Indicator used is Phenolphthalein (endpoint at pH 8.3 )* Step 2. Indicator used is Methyl Orange (endpoint at pH 4.5 )**

Step 1. neutralises alkalinity due to strong bases (OH- , CO32-)

Step2. neutralises alkalinity due to weaker bases (HCO3-)

* chosen as CO32- HCO3

- reaction is complete.

** chosen as HCO3- H2CO3 reaction is complete.

How is Alkalinity Determined

Sample pH Values < pH 8.3 One Step Titration Step 1. Indicator used is Methyl Orange (endpoint at pH 4.5 )

only Bicarbonate present when pH < 8.3 at start

Interpretation of Data When Sample pH value is > pH 8.3 a number of base species can

potentially exist together. 1. Carbonate and Bicarbonate pH > 8.3 2. Carbonate only pH > 10 3. Hydroxide and Carbonate pH > 10 4. Hydroxide only pH >>11

Alkalinity (5 cases)

Hydroxide

Carbonate

Bicarbonate

P endpoint (pH 8.3)

M endpoint (pH 4.5)

P

M

A

B

B

X

Y

X

M = Methyl OrangeP = Phenolphthalein

P = a + bM = b + P

Hydroxide = a = 2P - MCarbonate = 2b = 2M - 2P

P = xM = 2x + y

Carbonate = 2x = 2PBicarbonate = y = M - 2P

Titration with N/50 H2SO4

P=1/2M

P>1/2M

P<1/2M

pH

High

Low

starting pH

starting pH

How is the Alkalinity Calculated and Reported

Titration Volume, Acid Strength, Sample Volume

Usually Report Alkalinity as Calcium Carbonate Equivalent

ie. mg CaCO3 / litre

For each of the 3 Classes Hydroxide Alkalinity

Carbonate Alkalinity

Bicarbonate Alkalinity

If N/50 H2SO4 is used

Alkalinity (mg CaCO3) = Vt x 1000/ Vs

Vt = Titre volume (ml) Vs = Sample volume (ml)

Application of Acidity and Alkalinity Data

Acidity Selection of new Water Supplies

minimise treatment costs (Lime, NaOH)

Industrial Wastewater Metal Pickling Liquor (phosphoric acid) Consent to Discharge

Alkalinity Calculate safe levels of Ferric and Alum Biological WWT Plants - gives buffering capacity Potable - range 50 - 300 mg/l Industrial - consent to discharge