Hardness, Calcium, Magnesium, and Scaling

Hardness: What It Is and Why It Matters

Water hardness is the total concentration of dissolved divalent metal ions that react with soap and deposit scale. In drinking water and industrial systems, calcium (Ca²⁺) and magnesium (Mg²⁺) are the ions that matter almost every time. Hard water forms mineral deposits inside pipes, boilers, heat exchangers, and water heaters, reducing efficiency and eventually blocking flow. Very soft water, on the other hand, tends to be corrosive—it can leach metals such as lead and copper from plumbing.

Hardness is expressed as mg/L as CaCO₃ (milligrams per liter as calcium carbonate equivalent). This standard unit allows calcium hardness, magnesium hardness, and total hardness to be added directly. A secondary unit still seen in practice is grains per gallon (gpg); one gpg equals 17.12 mg/L as CaCO₃.

• Total Hardness (TH) = Calcium Hardness + Magnesium Hardness
• Calcium Hardness (CH) – measured directly by titration
• Magnesium Hardness (MH) = Total Hardness − Calcium Hardness (calculated by difference)

General hardness classification ranges (approximate, used widely in the field):

| Classification | mg/L as CaCO₃ |

|---|---|

| Soft | 0–75 |

| Moderately hard | 75–150 |

| Hard | 150–300 |

| Very hard | >300 |

EDTA Titration Method

The standard laboratory method for measuring hardness is EDTA (ethylenediaminetetraacetic acid) titration, described in Standard Methods for the Examination of Water and Wastewater. EDTA is a chelating agent: it forms stable, colorless complexes with calcium and magnesium ions one-for-one on a molar basis.

Total Hardness Procedure

1. Pipette a measured sample volume (commonly 100 mL) into an Erlenmeyer flask.

2. Add the pH 10 buffer to hold conditions that favor the titration.

3. Add a few drops of Eriochrome Black T (EBT) indicator. The sample turns wine-red because EBT binds to the metal ions.

4. Titrate with standard 0.01 M EDTA from a burette. EDTA preferentially takes the metal ions away from EBT. At the endpoint, the last metal ion is captured and the solution turns a clear blue.

5. Record the volume of EDTA used (mL).

Calcium Hardness Procedure

1. Use a separate 100 mL aliquot.

2. Raise pH to about 12–13 with sodium hydroxide; at this pH magnesium precipitates as Mg(OH)₂ and does not interfere.

3. Add murexide indicator; solution turns pink-red.

4. Titrate with 0.01 M EDTA to a purple endpoint.

5. Calculate calcium hardness from the EDTA volume used.

Calculation

For a 100 mL sample with 0.01 M EDTA:

> Hardness (mg/L as CaCO₃) = mL EDTA used × 10

This simplified factor applies when the sample volume is exactly 100 mL and the EDTA concentration is exactly 0.01 M. Always verify with your formula sheet when conditions differ.

Magnesium hardness is found by subtraction: MH = TH − CH.

The Langelier Saturation Index (LSI)

The Langelier Saturation Index predicts whether a water will deposit calcium carbonate scale, dissolve it (corrosive tendency), or be in balance. It is widely used in distribution system management and is relevant to both water and wastewater treatment contexts.

> LSI = pH(measured) − pH(saturation)

pH(saturation) (also called pHs) is the theoretical pH at which the water is exactly saturated with CaCO₃. It is calculated from:

> pHs = pK₂ − pKsp + pCa + pAlk

where pCa and pAlk are the negative logs of the calcium and alkalinity concentrations (expressed as equivalents), and the constants account for temperature and ionic strength (total dissolved solids, TDS). Operators typically read correction factors for temperature and TDS from a provided table rather than computing them from first principles.

Interpreting LSI:

| LSI Value | Tendency |

|---|---|

| Positive (> 0) | Scale-forming (supersaturated with CaCO₃) |

| Zero (≈ 0) | Balanced |

| Negative (< 0) | Corrosive (undersaturated; may dissolve pipe material) |

A slightly positive LSI is sometimes managed intentionally in distribution systems to maintain a thin protective CaCO₃ film inside pipes.

Safety in the Hardness Lab

Personal protective equipment (PPE) is required at all times: safety glasses, chemical-resistant gloves, and a lab apron. EDTA solution at the concentrations used is low-hazard but must not be ingested. The high-pH sodium hydroxide solution used in the calcium hardness procedure is corrosive—handle carefully, avoid skin and eye contact, and know the location of the eyewash station before you begin. Buffer solutions and indicators should be treated as chemical waste; dispose of all solutions according to your facility's chemical waste procedures. When using glassware, inspect burettes for cracks before filling. Never pipette by mouth. Follow OSHA Hazard Communication requirements: review the Safety Data Sheets (SDS) for each chemical before the lab.