Aquatic Chemistry V - Water Hardness Intro

What does alkaline water mean for other particles present?

• increased pH to a sufficient alkaline condition will cause transition metals to precipitate and form sediment

• Deposition of metals may result in co-precipitation and trace metals are also deposited

When is water considered ‘hard’?

• It is hard due to high concentrations of calcium dn magnesium

• can be seen as white fur/scum on items

• Effects are produced by the presence of polyvalent metal ions in the water from weathering of minerals

What mineral types are often part of water hardness?

• Carbonates - e.g. CaCO₃

• Sulphates - e.g. Gypsum (CaSO₄×2H₂O)

How does temperature affect the solubility of these hard water salts?

• their solubility decreases with a temperature increase

• Alkalinity also supports the sediment formation as metal solubility decreases with a high/alkaline pH

What are the benefits of alkaline and hard water?

• alkalinity in water lower solubility of toxic metals

• the buffering action of the salts producing the hardness lessens the effect of acidic pollutants

• Hard water is not considered a health hazard, it is just a technical hazard

• It provides additional calcium and magnesium in your diet

What is complexation?

A chemical reaction that takes place between a metal ion and a molecular or ionic entity known as a ligand that contains at least one atom with an unshared pair of electrons

What is a complex compound?

• A central metal ion surrounded by ligands

• These ligands can be ions or neutral

How does bioavailability affect complexation?

• complexation increases the total concentration of metal ions in a solution, which then increases bioavailability

• Bioavailability is the external availability of a chemical to an aquatic organism

• This promotes the transport and cycling of metal in our aquatic environments (biogeochemical cycle)

What is the biogeochemical cycle?

the movement of elements and nutrients between biotic (living organisms) and abiotic (non-living environment shapers) factors

What are chelates/chelating agents?

• A polydentate ligand forming ringed structures with metal ions

• A chelate is a chemical compound that consists of a metal ion and a chelating agent

• A chelating agent is a substance whose molecules can form multiple bonds toa single metal ion

What is a polydentate ligand?

A ligand that can donate multiple electron pairs to the central metal ion

What is the polydentate ligand used in reducing water hardness?

• Ethylenediaminetetraacetic acid - EDTA - a hexadentate ligand, forms six bonds with the central metal ion

• Used to reduce water hardness by forming bonds with the metal ions and forming chelating complexes, chelates, therefore removing them from the water

• Prevents new sediment from forming

• low solubility in water

How does EDTA chelation work?

• EDTA forms chelates with transition metals and main group metal ions making it a good remover

• It forms complexes with calcium and magnesium - hard water compounds

• EDTA can be used as a de-scaler in hard water areas

What is complexometric titration?

Also called chelatometry, it is used to determine water hardness levels through measuring levels of metals like calcium and magnesium

What are the three types of complexometric titration?

• Chelation

• Precipitation

• Ion Exchange

What are other applications of complexometric titration?

• Determination of solution pH

• Determination of metal ions in solution

• Determination of water Hardness of a solution

What are the steps for EDTA chelatometry?

• EDTA is the chelate/chelating agent, it also acts as a water hardness sequester so is important for water treatment

1. Sample of water is collected and stored until use

2. The sample undergoes titration using aqueous EDTA solution with a specified molar concentration

3. After completing the titration, you will have a volume of EDTA required for complete complexation of Ca²⁺ or Mg²⁺

4. The data collected can then be used to to calculate the amount of mineral in ppm.

What is the EDTA chemical reaction?

H₂Y^2- + Mⁿ⁺ → MY^(n-4) + 2H⁺

• where H₂Y^2- is the dominant form of EDTA in solution

• Mⁿ⁺ is the metal ions

• MY^(n-4) is the complex

E.g. H₂YY^2- + Caⁿ⁺ → CaY^(2-) + 2H⁺

The ration of EDTA ion : metal ion = 1:1

What are the steps to find the mineral content in ppm after EDTA chelation?

1. Recall the equation and the 1:1 ratio: H₂Y^2- + Mⁿ⁺ → MY^(n-4) + 2H⁺

2. Work out the number of moles of EDTA using the equation: (cm³ of EDTA use x Molar conc of EDTA)/volume = mol

3. As the ratio is 1:1, you have also worked out the mol of metal ion in step 2 and can use this to find the concentration of the sample using mol/volume of sample = conc of mineral in sample

4. Convert to ppm by multiplying by doing conc x mineral molar mass as this makes it g/l

5. x1000 to convert to mg/l which is the same as ppm

How is EDTA removed after water treatment?

• EDTA is removed using bacteria, and about 80% of it is removed through this process

• It is biodegradable but the process is slow

• Due to the number of application for EDTA, there are concerns about its impact on the environment as it produces CO₂