Hardness
Precise rapid tests for water and waste water samples
General hardness or water hardness is defined as the sum of the concentrations of all alkaline earth ions dissolved in an aqueous sample.
However, since, strontium and barium ions are contained only in traces and therefore play only a minor role as hardeners, the definition according to the German norm DIN 38409-H 6 was adopted. According to this standard, the hardness is characterized only by the molar concentrations of calcium and magnesium ions in mmol/L. The amount of calcium ions (Ca2+) is generally higher (about 70–85 %) than that of magnesium ions (Mg2+). The “harder” water is, the higher the total amount of dissolved alkaline earth ions.
Previously, hardness was specified in German hardness degrees rather than in mmol/L (SI unit). This unit referred to calcium oxide (CaO), where 1 °d formally corresponds to 10 mg calciumoxide in 1 liter of water. Other hardness factors such as magnesium were defined equivalently (1 °d = 7.19 mg/L). The conversion of German hardness degrees into SI and other units is listed in the following table:
| °d | °e | °f | mg/L CaO | mg/L CaCO3 | mmol/L CaCO3 |
|---|---|---|---|---|---|
| 1 | 1.3 | 1.8 | 10 | 18 | 0.18 |
| 2 | 2.5 | 3.6 | 20 | 36 | 0.36 |
| 3 | 3.8 | 5.4 | 30 | 54 | 0.54 |
| 4 | 5.0 | 7.1 | 40 | 71 | 0.71 |
| 5 | 6.3 | 8.9 | 50 | 89 | 0.89 |
| 6 | 7.5 | 10.7 | 60 | 107 | 1.07 |
| 7 | 8.8 | 12.5 | 70 | 125 | 1.25 |
| 8 | 10.0 | 14.3 | 80 | 143 | 1.43 |
| 9 | 11.3 | 16.1 | 90 | 161 | 1.61 |
| 10 | 12.5 | 17.8 | 100 | 178 | 1.78 |
Table 9: Conversion from degrees German hardness into SI and other units
Generally, water with a total hardness of up to 1.25 mmol/L CaCO3 is described as soft, up to 1.98 mmol/L CaCO3 as of intermediate hardness and above 2.16 mmol/L CaCO3 as hard.
Total hardness consists of two differently defined areas:
Our Hardness Products
| Product | Platform | Range | REF | Order | |||||
|---|---|---|---|---|---|---|---|---|---|
| AQUADUR® Sensitive test strips | HT | 0 · 5.4 · 10.8 · 18.8 ppm CaCO3 | 91243 |
|
|||||
| AQUADUR® Water Hardness test strips | HT | < 54 · > 90 · > 180 · > 270 · > 360 · > 450 ppm CaCO3 | 91201 |
|
|||||
| AQUADUR® test strips | HT | < 54 · > 72 · > 126 · > 252 · > 378 ppm CaCO3 | 91220 |
|
|||||
| AQUADUR® test strips | HT | < 54 · > 90 · > 180 · > 270 · > 360 · > 450 ppm CaCO3 | 91223 |
|
|||||
| AQUADUR® test strips - 1000 pack | HT | < 54 · > 72 · > 126 · > 252 · > 378 ppm CaCO3 | 91224 |
|
|||||
| AQUADUR® test strips - 1000 pack | HT | < 54 · > 72 · > 151.2 · > 252 · > 378 ppm CaCO3 | 91240 |
|
|||||
| AQUADUR® TH 400 test strips | HT | 0 · 25 · 50 · 120 · 250 · 425 ppm CaCO3 | 91202.1 |
|
|||||
| AQUADUR® test strips | HT | < 54 · > 90 · > 180 · > 270 · > 360 · > 450 ppm CaCO3 | 912902 |
|
|||||
| AQUADUR® test strips | HT | < 54 · > 90 · > 180 · > 270 · > 360 · > 450 ppm CaCO3 | 91221 |
|
|||||
| AQUADUR® test strips | HT | < 54 · > 72 · > 126 · > 252 · > 378 ppm CaCO3 | 91222 |
|
|||||
| QUANTOFIX® Carbonate Hardness test strips | HT | 0 · 3.75 · 7.5 · 12.5 · 18.75 · 25 °e | 91323 |
|
|||||
| VISOCOLOR® ECO Carbonate Hardness test kit | TI | 1 drop corresponds to 1.25 °e | 931014 |
|
|||||
| VISOCOLOR® ECO Total Hardness test kit | TI | 1 drop corresponds to 1.25 °e | 931029 |
|
|||||
| VISOCOLOR® HE Carbonate Hardness C 20 test kit | TI | 0.6 – 25.0 °e | 915003 |
|
|||||
| VISOCOLOR® HE Total Hardness H 20 F test kit | TI | 0.6 – 25.0 °e | 915005 |
|
|||||
| VISOCOLOR® HE Total Hardness H 2 test kit | TI | 0.6 – 25.0 °e | 915002 |
|
|||||
| NANOCOLOR® Carbonate Hardness 15 tube test | TT | 1.25 - 18.75 °e | 985015 |
|
|||||
| NANOCOLOR® Hardness 20 tube test | TT | 1.25 - 25.0 °e | 985043 |
|
|||||
| NANOCOLOR® Hardness Ca/Mg tube test | TT | 1.25 - 25.0 °e | 985044 |
|
|||||
| NANOCOLOR® Residual Hardness 1 tube test | TT | 0.03 - 1.25 °e | 985084 |
|
Carbonate vs Noncarbonate Hardness
Carbonate hardness (temporary hardness)
The term carbonate hardness describes the amount of magnesium and calcium ions for which equivalent molar amounts of hydrogen carbonate are available in the solution. In contrast to permanent hardness, the carbonate hardness can be removed simply by heating. This is based on a temperature-dependent equilibrium reaction (carbonate equilibrium):
Limestone (CaCO3) is dissolved, forming calcium hydrogen carbonate, in the presence of carbonated water, which is formed by CO2 and water. By heating, however, the balance is shifted in the opposite direction, favoring the formation of so-called “boiler scale”, calcium carbonate precipitates.
The higher the proportion of dissolved carbonates in the water, the greater the buffer capacity (pH stability) of the water. Very soft waters (e.g. distilled water), however, have a very low buffering capacity, which can affect measurements. By contrast, very hard water leads to a formation of lime and lime soaps, as found frequently in everyday life (kettle, coffee maker). These deposits are formed by the calcium carbonate produced.
The hydrogen carbonates contained in aqueous samples and the good buffer capacity stabilize the pH, even upon introduction of acids and bases:
The term carbonate hardness is synonymous with the terms acid capacity or acid neutralizing capacity and alkalinity. The determination is performed by titration with hydrochloric acid against the so-called p value (p = phenolphthalein) and the m value (m = methyl orange).
In the original sense, carbonate hardness was expressed (often in °d) as acid capacity with the unit [mmol/L], or in the fish farming sector as acid binding capacity (ABC) with the unit [meq/L]. This terminology originates from the buffer capacity of the water to acids and the associated pH stability over a certain pH range.
Thus, carbonate hardness, acid capacity and acid neutralizing capacity are homonyms that are merely used in different fields of application.
In the first titration stage, the carbonates are converted to hydrogen carbonates. In case of purplish discoloration of the sample after addition of the indicator phenolphthalein, carbonates are present, and titration with hydrochloric acid up to complete decoloration of the sample solution is required. The consumption of hydrochloric acid up to decoloration is referred to as the p value.
In the second titration stage, the m value is measured directly after determination of the p value. If after addition of the indicator methyl orange there is a blue color, titration is performed until the color changes to red. All hydrogen carbonates are detected.
Experience shows that in most water samples the p value is very small or zero, since carbonates are poorly soluble. In this case, the carbonate hardness corresponds to the m value.
Figure 26: Overview: Acid and base capacity
The carbonate hardness is lower, equal at maximum to the total hardness. However, in exceptional cases it can be greater, namely if more carbonate or hydrogen carbonate ions are present in the sample solution than alkaline earth ions (e.g. in alkali NaHCO3).
| Designation | Description | Unit |
|---|---|---|
| Total hardness (water hardness) | Concentration of all alkaline earth ions. Keywords: Soft water, hard water → lime scale, lime soap | SI: mmol/L |
| Calcium hardness (calcium carbonate hardness) | Concentration of all dissolved calcium ions | mmol/L often: mg/l CaCO3 or ppm rarely: meq/L |
| Magnesium hardness | Concentration of all dissolved magnesium ions | mmol/L often: ppm rarely: meq/L |
| Carbonate hardness (temporary hardness, transient hardness) | Concentration of all alkaline earth ions bound as carbonate or hydrogen carbonate | SI: mmol/L often: ppm rarely: meq/L |
| Non-carbonate hardness (permanent hardness, sulfate hardness) | Concentration of all alkaline earth ions not bound as carbonate or hydrogen carbonate | SI: mmol/L often: ppm rarely: meq/L |
| Base capacity KB (Acidity) | Base consumption (NaOH, c = 0.1 mol/L) to reach a pH of 8.2; ability to release protons | mmol/L or meq/L |
| Acid capacity KS (acid neutralization capacity from pH 8.3 to pH 4.3) | Acid consumption (HCl, c = 0.1 mol/L) to reach a pH of 4.3, high acid capacity: good buffering capacity, according to: DIN 38409-H7-1-2. | mmol/L or meq/L or mg/L CaCO3 |
| Acid-neutralizing capacity (alkalinity) | Ability to bind oxonium ions (H3O+) and hydrogen ions (H+),respectively, depending on basically acting ions, almost only carbonates → “Alkalinity = carbonate alkalinity = carbonate hardness = acid capacity” | mmol/L often: ppm |
| Total alkalinity | Concentration of all ions that can bind acid (sulfates, phosphates, etc.) | mmol/L often: ppm rarely: meq/L |
| Carbonate alkalinity | Concentration of all ions present as carbonate that can bind acid. Usually referred to as alkalinity, since carbonates are the main component | mmol/L often: ppm rarely: meq/L |
Non-carbonate hardness (permanent hardness)
Non-carbonate hardness consists of all remaining dissolved, non-precipitable anionic alkaline earth metal salts such as sulfates, chlorides or nitrates. The mole fractions of the individual components do not matter in the determination, since residual hardness is defined as a cumulative parameter and determined as such.
Water hardness is important for sewage treatment plants as well, which is why special softening processes are used.
Excessive temporary hardness can lead to deposition of carbonates in the piping and heat exchangers. Too high calcium and magnesium ion concentrations can lead to precipitation of certain anionic parameters.
Reaction Basis
Total hardness and residual hardness
Depending on the product range (VISOCOLOR® or NANOCOLOR®) and test kit, one of three different reactions is underlying:
(a) Complexometric titration: Reaction basis in analog to DIN 38406-3 E3 Except for one test kit in the VISOCOLOR® range (VISOCOLOR® alpha Residual Hardness), total hardness is determined by complexometric titration. The alkaline earth ions are bound by the disodium salt of ethylenediaminetetraacetic acid (EDTA), forming a so-called chelate complex (gr. chele = pincer). While in acid-base titrations (see carbonate hardness) the equivalence point is detected using indicators that respond to a change in pH with a color change, in complexometric titrations metal- specific indicators are used that respond to a change in metal ion concentration. The chelate complexes formed by the indicators with the metal ions differ in color from the free indicators.
The reaction proceeds at a pH of 10. The liquid indicator comprises a buffer substance by which the pH is adjusted and which takes-up the hydrogen ions (protons, +), which are released during titration and which would otherwise lead to lowering of the pH (complex stability decreases with decreasing pH).
In addition to the buffering agent, the indicator composition furthermore comprises Mg-EDTA as complexometrically neutral substance. The calcium complex has a higher stability constant than the corresponding magnesium complex. The result is that calcium ions replace the magnesium ions in the complex and release equivalent amounts of the latter from this complex.
The magnesium ions produce a much sharper color change with the color indicator than the calcium ions. This prevents the problem that in water samples whose hardness is caused exclusively or at least in large part by calcium salts no sharp color change and consequently no accurate endpoint detection of the titration is detected.
Prior to titration, an indicator is added to the water sample. This indicator forms loose complexes of red color with the hardness minerals (alkaline earth ions).
The mixture is then titrated with an EDTA solution, where first the alkaline earth ions not bound to the indicator are converted into chelate complexes. After the binding of these free ions, the ions loosely bound to the indicator are also withdrawn from the complex by EDTA and subsequently chelated.
Due to the decomposition of the indicator/alkaline earth metal ion complexes the indicator changes its color (from red to green) and accordingly indicates the end of the titration. The consumption of titration solution is thus a measure of the concentration of hardness minerals present in solution.
(b) Colorimetry with mixed indicator (only with VISOCOLOR® alpha Residual Hardness)
(c) Photometric determination of total hardness/residual hardness with phthalein purple. The use of a selective masking agent allows the differentiation of calcium and magnesium in the determination of total hardness.
Carbonate hardness
Determination in the VISOCOLOR® range is performed analog to DIN EN ISO 9963- 1 C24: The determination is performed by titration with hydrochloric acid against a mixed indicator that changes color at pH = 4.5.
Photometrically, carbonate hardness can be determined using bromophenol blue.
Sample preservation 🧪
Total hardness: The sample can be stored after pre-treatment with HNO3 to a pH of 1–2 for up to one month.
Carbonate hardness: The measurement must be done within 1 day.
For storage a PE bottle should be used; store and transport at 4 °C in the dark.
Tips & tricks 🧪
- Background information
- For differentiation of calcium and magnesium contents, a selective masking agent is used for photometric total hardness testing.
- The relationship between water hardness and the magnesium and calcium ion concentrations can be calculated as follows:
- Normally, carbonate hardness is lower than total hardness. If the carbonate hardness exceeds the total hardness, there is an abnormal situation that requires clarification (e.g. ingress of alkali hydrogen carbonates or high buffering capacity).
- As a screening test for determination of total hardness, AQUADUR® test strips are suitable.
- Water hardness [mmol/L] ≈ Ca2+ [mg/L] / 40 + Mg2+ [mg/L] / 24.3 or °dH ≈ 0.14 ∙ Ca2+ [mg/L] + 0.23 ∙ Mg2+ [mg/L]
- Sea water suitability
- Almost all VISOCOLOR® and NANOCOLOR® tests are suitable for sea water analysis; some require dilution (1+29). Without dilution, chloride interferences result in incorrect results. For more information, please refer to the respective instruction leaflet. The measurement of residual hardness is not suitable for seawater analysis.
- Interferences
- Copper(II) ions may delay the indicator color change (> 5 mg/L) and even completely block it at higher concentrations. Therefore, e.g. sufficient water must be drained prior to sampling with copper pipes. To eliminate copper interferences, the VISOCOLOR® ECO reagent additive (REF 931 929) can be used for elimination of copper ions in total hardness determination.
- In complexometric titration, the added indicator buffer mixture may be insufficient for a stable pH range around 10 in case of water samples very rich in carbon dioxide or iron. In these cases, dilution of the sample with distilled water (later taking the dilution into account by multiplying the obtained value with the appropriate factor) or addition of additional indicator buffer mixtures must be resorted to. The pH should always be controlled after addition of the indicator buffer mixture for perfect results.
- In the photometric test kits, concentrations above the double measuring range can simulate measured values that lie within the simple measuring range and may thus be misinterpreted. The sample must previously be diluted into the range specified by the test. For water of unknown concentration, studies with strongly differing dilutions should be performed until the last dilution confirms the value previously found.
- Further interfering ions are listed in the package inserts.
