Determination of Hardness by EDTA Method
Definition
Hardness of water is the property due to the presence of dissolved calcium and magnesium ions that prevents water from producing lather readily with soap.
EDTA method is a complexometric titration method used to determine hardness by titrating water with disodium EDTA solution, which reacts quantitatively with calcium and magnesium ions to form stable 1:1 chelate complexes.
Main Content
1. Nature and Causes of Hardness
- Hardness is mainly due to the presence of Ca²⁺ and Mg²⁺ ions in water, though ions like Fe²⁺, Mn²⁺, Sr²⁺, and Al³⁺ may also contribute in small amounts.
- It is classified into:
- Temporary hardness: caused by bicarbonates of calcium and magnesium, such as Ca(HCO₃)₂ and Mg(HCO₃)₂; it can be removed by boiling.
- Permanent hardness: caused by sulfates, chlorides, nitrates, and other non-bicarbonate salts of calcium and magnesium; it cannot be removed by boiling easily.
- Hard water causes:
- wastage of soap due to formation of insoluble scum,
- scale formation in boilers, heaters, and pipes,
- reduced efficiency in laundry and industrial processes.
- The hardness is usually expressed in mg/L (ppm) as CaCO₃, which allows comparison of different water samples on a common basis.
2. Principle of EDTA Titration
- EDTA stands for ethylenediaminetetraacetic acid, a hexadentate ligand that can bind metal ions through six donor atoms.
- In aqueous solution, EDTA forms highly stable 1:1 chelate complexes with calcium and magnesium ions:
- Ca²⁺ + EDTA⁴⁻ → [Ca-EDTA]²⁻
- Mg²⁺ + EDTA⁴⁻ → [Mg-EDTA]²⁻
- Because one EDTA molecule reacts with one metal ion, the titration is highly stoichiometric and suitable for quantitative analysis.
- The titration is carried out in the presence of a buffer solution of pH 10, because EDTA binds Ca²⁺ and Mg²⁺ effectively under alkaline conditions and the indicator works properly in this pH range.
- An indicator such as Eriochrome Black T (EBT) is used. It forms a weak wine-red complex with Mg²⁺. When EDTA is added, it first complexes with free metal ions, and finally displaces the indicator at the end point, causing a color change from wine red to pure blue.
- This color change indicates that all hardness-causing ions have been complexed by EDTA.
3. Types of Hardness Determined by EDTA Method
Total hardness
- is determined by titrating the water sample directly with EDTA using EBT indicator at pH 10. It represents the combined hardness due to calcium and magnesium ions.
Calcium hardness
- can be determined separately by titrating at a higher pH, generally around 12, using murexide indicator. At this pH, magnesium precipitates as Mg(OH)₂, so only calcium ions are titrated.
Magnesium hardness
- is calculated indirectly by subtracting calcium hardness from total hardness:
- Magnesium hardness = Total hardness − Calcium hardness
- This separation is important in water analysis because calcium and magnesium may behave differently in industrial systems.
- Hardness can also be classified as:
- Carbonate hardness: associated with bicarbonates and carbonates.
- Non-carbonate hardness: associated with chlorides, sulfates, and nitrates.
- The EDTA method is especially useful because it can be adapted to determine each of these hardness components through suitable conditions and indicators.
Working / Process
1. Preparation of reagents and standardization
- Prepare a standard EDTA solution of known concentration, usually around 0.01 M.
- Prepare an ammonium buffer solution to maintain the pH at about 10.
- Prepare the indicator solution, commonly Eriochrome Black T.
- Standardize the EDTA solution using a standard hard water sample or a solution of known calcium concentration, because exact concentration is essential for accurate hardness determination.
2. Titration of the water sample
- Measure a known volume of the water sample, commonly 50 mL, into a conical flask.
- Add a few milliliters of buffer solution to bring the pH to 10.
- Add 2–3 drops of Eriochrome Black T indicator; the solution becomes wine red if hardness ions are present.
- Titrate with EDTA from a burette while stirring continuously.
- Continue until the color changes from wine red to clear blue, indicating the end point.
3. Calculation of hardness
- Record the volume of EDTA used for the sample.
- Use the formula:
- Hardness as CaCO₃ (mg/L) =
- If EDTA is standardized in terms of the volume required for a known hard water sample, the hardness can be calculated from the equivalence relationship.
- For calcium hardness, use murexide indicator at pH 12 and perform a similar titration.
- Determine magnesium hardness by subtraction if required.
- Repeat the titration for concordant readings to ensure accuracy.
Advantages / Applications
- The EDTA method is highly accurate and reliable for routine water analysis because it gives a direct quantitative measurement of hardness-causing ions.
- It is simple, rapid, and economical, making it suitable for laboratory work, municipal water testing, and industrial quality control.
- The method can determine total hardness, calcium hardness, and magnesium hardness separately, which is useful in detailed water characterization.
- It is widely used in boiler water analysis, cooling water monitoring, drinking water testing, and treatment plant control to prevent scaling and corrosion problems.
- The method is applicable to many industrial and environmental samples, including groundwater, river water, tap water, and process water.
- Since EDTA forms very stable complexes, the method provides sharp end points and good reproducibility when proper pH and indicator conditions are maintained.
- It helps in deciding suitable water softening methods such as lime-soda treatment, ion exchange, or reverse osmosis.
- It is an important analytical tool in the study of water treatment, water quality control, and industrial applications.
Summary
- EDTA titration is a standard complexometric method used to determine water hardness.
- It works on the principle of stable 1:1 complex formation between EDTA and calcium/magnesium ions.
- The method is carried out at controlled pH using suitable indicators to detect the end point clearly.