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Extraction · Water

Hardness and alkalinity

What total, carbonate and non-carbonate hardness and alkalinity mean, how the carbonate system ties them to pH, how alkalinity buffers coffee acids, and how both values are measured and reported.

Across the chain: Science

Source-checkedby Coffearium verification pass· Last source-checked 20 Oct 2026· Prepared with AI assistance· 17 sources (14 tier A)· 3 contested· Updated 20 Oct 2026
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Contents
  • Total hardness is the dissolved calcium and magnesium; alkalinity is the water’s capacity to neutralise acid, in most waters mainly bicarbonate.4,5,2
  • Carbonate (temporary) hardness is the lower of total hardness and alkalinity; the rest of the hardness is non-carbonate (permanent) hardness.1,2,12
  • Alkalinity is titrated with strong acid to about pH 4.3–4.5; a second end point near pH 8.2–8.3 marks the carbonate share.1,6,10
  • Bicarbonate is the main form of dissolved carbonate between about pH 6.3 and 10.3 (our calculation);7,8 below pH 8.3 alkalinity is practically all bicarbonate.2
  • The EU drinking-water directive sets no limit for hardness but requires suppliers to inform the public about it.13

Two different measures#

Hardness and alkalinity are often confused, but they count different ions. Hardness counts positive ions; alkalinity counts the capacity of negative ions to take up acid.1,2

Total hardness is the amount of dissolved calcium and magnesium.4 The name comes from washing: these ions make soap precipitate.1,12 Other doubly or triply charged ions, such as iron or strontium, also count in principle, but fresh water rarely holds more than traces of them.1

Alkalinity is the water’s ability to neutralise acids.3,5 It is not a substance but a property, which bicarbonate, carbonate and hydroxide ions provide.3,5 Below pH 8.3, the range of most tap water, almost all of it is bicarbonate, also called hydrogen carbonate (HCO₃⁻).1,2 The SCA handbook therefore also calls it the acid buffer capacity.1

Alkalinity is not the same as an alkaline pH. A pH above 7 only says that the water is basic at that moment.2 Alkalinity says how much acid the water can absorb before its pH falls, whatever its starting pH.2,5 Why pH alone misleads for coffee water is shown with numbers in Water for coffee.

Carbonate and non-carbonate hardness#

Heating hard water makes calcium carbonate settle out as scale.4 Scale needs both partners: calcium or magnesium from the hardness, and carbonate from the alkalinity.1 So at most the smaller of the two can leave the water.

The SCA handbook calls this smaller amount the carbonate hardness, also known as temporary hardness.1 The rest of the hardness is non-carbonate hardness, or permanent hardness: it stays dissolved when the water is boiled.1,12 Calcium and magnesium in this part are balanced by anions such as sulfate and chloride.1,3

Three kinds of water show how the terms work:

  • Limestone water. Water that has dissolved only calcium or magnesium carbonate holds equal amounts of hardness and alkalinity.2 Most tap waters come close, with hardness slightly above alkalinity.1
  • Gypsum water. Calcium sulfate adds hardness without alkalinity. A hardness far above the alkalinity points to much sulfate.1,2
  • Softened or salt-influenced water. A softener swaps calcium and magnesium for sodium or potassium, which lowers hardness but leaves alkalinity.1,2 Sea-water intrusion can do the same.1 Here alkalinity exceeds hardness, and carbonate hardness equals total hardness.1
A Limestone watermost tap waters 1 Ca²⁺ + Mg²⁺Na⁺, K⁺HCO₃⁻Cl⁻, SO₄²⁻, NO₃⁻carbonate hardnessnon-carbonatealkalinityB Gypsum watercalcium sulfate 1,2 Ca²⁺ + Mg²⁺Na⁺, K⁺HCO₃⁻Cl⁻, SO₄²⁻, NO₃⁻carbonate hardnessnon-carbonatealkalinityC Softened orsalt-influenced 1,2 Ca²⁺ + Mg²⁺Na⁺, K⁺HCO₃⁻Cl⁻, SO₄²⁻, NO₃⁻carbonate hardnessalkalinity“not hard” carbonateUpper bar: cations (Ca²⁺ + Mg²⁺ = total hardness); lower bar: anions. Both in equivalents,so they are equal in length. Carbonate hardness = the lower of total hardness and alkalinity 1,2 .Schematic, not to scale.
Hardness and alkalinity in three types of water, drawn as cations and anions in equivalents. Carbonate hardness is the lower of total hardness and alkalinity; in softened water the surplus bicarbonate is “not hard” carbonate.1,2 Schematic, not to scale.

The SCA handbook calls bicarbonate in excess of hardness “not hard” carbonate.1 Carbonate hardness can never exceed alkalinity, by definition.1 Many test kits labelled “carbonate hardness” actually measure alkalinity.1,2 In softened water the label and the result no longer agree.

How alkalinity buffers coffee acids#

Bicarbonate takes up a hydrogen ion from an acid and becomes carbonic acid, which can leave as carbon dioxide.1,2 Each bicarbonate ion removes one hydrogen ion. The acid is neutralised, and the cup tastes less acidic.1,2

A coffee science textbook chapter puts it simply: perceived acidity is the acidity extracted from the coffee minus the water’s alkalinity.2 The more water per gram of coffee, the larger the effect, so filter coffee is more sensitive than espresso.1,2 Water for coffee works this through with brew ratios.

Hardness acts differently. A modelling study suggests that calcium and magnesium ions bind to coffee acids, caffeine and an aroma compound.16 How hardness and alkalinity change flavour is covered in Water minerals and flavour.

Units and reporting#

Hardness and alkalinity are usually expressed as if all the ions were calcium carbonate.1,12 These are equivalent units: they weight each ion by its valence, not by its mass.1 One calcium ion and two bicarbonate ions then show up as equal amounts, just as they react in scale.1,2

  • ppm or mg/L as CaCO₃ is used in the USA and by the SCA.1
  • French degrees (°fH) are also based on calcium carbonate; one is 10 ppm as CaCO₃.1 In Aargau, Switzerland, water suppliers must report hardness in them.14
  • German degrees (°dH) are based on calcium oxide instead.1 One German degree is 17.85 ppm as CaCO₃.1
  • English (Clark) degrees and grains per US gallon are based on a grain of calcium carbonate per imperial or US gallon.1,17
  • mmol/L counts particles directly. Laboratories and standards use it.9,11

Bottle labels list calcium, magnesium and hydrogen carbonate in mg/L instead; these have to be converted before they can be compared.1 Water for coffee gives the factors for converting them.

The carbonate system#

Bicarbonate, carbonate and dissolved carbon dioxide form one linked system:1,2

CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺ ⇌ CO₃²⁻ + 2 H⁺

Water chemistry customarily lumps dissolved CO₂ and true carbonic acid together.1 Two acid constants then set which form dominates at a given pH. The first, pK₁, is about 6.3, according to the USGS alkalinity calculator.7 The second, pK₂, follows from the temperature equation of a 1982 calcite solubility study; by our calculation it is 10.33 at 25 °C.8 Between the two, about pH 6.3 to 10.3, bicarbonate is the main form (our calculation).7,8

SCA water target, pH 6–8 1 0%25%50%75%100%3456789101112pHShare of dissolved carbonateTotal alkalinityend point4.3–4.5 1,6,10 Carbonateend point8.2–8.3 6,10 CO₂ +H₂CO₃HCO₃⁻bicarbonateCO₃²⁻carbonatepK₁ ≈ 6.3 7 pK₂ = 10.33 8 Our calculation at 25 °C from the cited constants, without activity corrections.
Share of dissolved carbonate present as CO₂ and carbonic acid, bicarbonate and carbonate, from pH 3 to 12, with the titration end points and the SCA pH range for brewing water.1,6,10 Our calculation at 25 °C from pK₁ ≈ 6.37 and pK₂ = 10.33,8 without activity corrections.

Our calculation with these constants, for the SCA’s pH range of 6–8 for brewing water:1 at pH 6 about 33% of the dissolved carbonate is bicarbonate, at pH 7 about 83%, at pH 8 about 98%. Carbonate stays below 1% up to pH 8.3. This is why the SCA handbook notes that carbonate forms only in very basic water.1 A tap water left to stand loses excess CO₂ and rises in pH.1 Its alkalinity stays the same, because CO₂ is electrically neutral and does not appear in the equation below.2

Temperature shifts the constants. The 1982 equation for pK₂ holds from 0 to 100 °C at 1 atm.8 By our calculation, pK₂ falls from 10.38 at 20 °C to 10.14 at 90 °C, so hot water holds slightly more carbonate at a given pH. No source opened for this entry gave a temperature equation for pK₁.

Alkalinity as a charge balance#

For fresh water, alkalinity can be written as a sum of concentrations in mol/L:2

Alkalinity = [HCO₃⁻] + 2 [CO₃²⁻] + [OH⁻] − [H⁺]

Carbonate counts twice because it can take up two hydrogen ions. Below pH 8.3 the other terms are several orders of magnitude smaller than bicarbonate, so alkalinity equals the bicarbonate content to a close approximation.2 Our calculation: at pH 8.3 and 25 °C, hydroxide amounts to about 0.1 ppm as CaCO₃.

The USGS describes alkalinity as a deficiency of hydrogen ions relative to a solution of carbon dioxide in water.6 That reference point explains the titration end point. Adding acid until all bicarbonate has become carbonic acid returns the water to a plain CO₂ solution, which is acidic.6

Equivalent units follow from the same balance. One mmol/L of bicarbonate is 50.04 ppm as CaCO₃, half the figure for one mmol/L of calcium plus magnesium, because bicarbonate is singly and calcium doubly charged.1 When calcium carbonate forms scale, the calcium falls by one mole and the bicarbonate by two; in equivalent units both fall by the same amount.1,2

Titration end points and standards#

Alkalinity is titrated with a strong acid such as hydrochloric acid.1 Two end points are in use, and the sources do not quite agree on either (see Contested):

  • Total alkalinity. The SCA handbook defines alkalinity as the acid needed to lower a sample to pH 4.3.1 DIN 38409-7 also uses pH 4.3, as the acid capacity to pH 4.3, known in German as Säurekapazität KS4.3.10 The USGS places the bicarbonate equivalence point of most environmental waters near pH 4.5.6
  • Carbonate alkalinity. A first end point near pH 8.3 (USGS) or at pH 8.2 (DIN) captures carbonate and hydroxide.6,10 The DIN method determines both end points for waters above pH 8.2.10 Below pH 8.3 the carbonate share is negligible, so for most drinking waters this value is zero.2

The DIN validation report notes that the acid capacity to pH 4.3 is often equated with bicarbonate, although it also includes other buffers.10 ISO 9963-1 specifies the titration of total and composite alkalinity for natural, treated and waste water.9 It applies directly up to 20 mmol/L, with a recommended lower limit of 0.4 mmol/L;9 by our calculation, that is about 20–1,000 ppm as CaCO₃. Hardness is titrated with EDTA; ISO 6059 standardises this method for the sum of calcium and magnesium.1,11 Kits, resolution and meters are covered in Measuring water.

Buffering in numbers#

Our own calculation, from the cited definitions. An alkalinity of 50 ppm as CaCO₃ is 1.0 mmol/L of bicarbonate.1 It can neutralise 1.0 mmol of hydrogen ions per litre. That equals about 64 mg of citric acid if all three of its acid groups react, or 60 mg of acetic acid. It releases up to 1.0 mmol, or 44 mg, of CO₂.

The reaction runs nearly to completion in a cup. Our calculation with pK₁ ≈ 6.3:7 at pH 5, about 95% of the original bicarbonate has become CO₂ or carbonic acid. Brew pH values are given in Acids in coffee.

A coffee science textbook chapter, based on earlier titrations of filter and lungo coffees, estimates that water of moderate alkalinity neutralises a noticeable share of the titratable acidity in filter coffee and a smaller share in a lungo.2 It also states that at high alkalinity, the CO₂ formed can add resistance and lengthen espresso extraction.2 In a 2024 study of cold brew made with six commercial waters, calcium, magnesium and bicarbonate correlated negatively with titratable acid.15 Its abstract reports the three ions together and does not separate their effects.15

Rules and reporting classes#

Drinking-water rules treat hardness as information, not as a limit:

  • WHO. The 2009 background document proposes no health-based guideline value for hardness.12 It uses the same bands as the USGS: below 60 mg/L as CaCO₃ soft, 60–120 moderately hard, 120–180 hard, above 180 very hard.4,12
  • European Union. Directive 2020/2184 sets no parametric value for hardness, calcium, magnesium or alkalinity.13 Its Annex IV requires information to the public on hardness, calcium, magnesium and potassium.13 Annex I sets pH 6.5–9.5, asks that water not be aggressive, and allows minimum calcium and magnesium levels for softened or demineralised water.13
  • Switzerland. The Swiss drinking-water ordinance (TBDV, Art. 5) obliges suppliers to inform consumers about water quality at least once a year.14 In Aargau this includes total hardness in French degrees, classed as soft (0–15 °fH), medium hard (15–25 °fH) and hard (above 25 °fH).14 Water for coffee lists finer Swiss classes.

None of these rules names alkalinity as a reporting item. For coffee it usually has to be found in the full water analysis or measured.1,13,14

Reference values#

Parameter Value Source
pK₁ of carbonic acid (CO₂ + H₂CO₃) About 6.3 USGS7
pK₂ of carbonic acid, 25 °C 10.33 (our calculation from the published equation) Plummer & Busenberg8
pK₂, 20 °C and 90 °C 10.38 and 10.14 (our calculation) Plummer & Busenberg8
Bicarbonate the main form About pH 6.3–10.3 (our calculation) Calculated from7,8
Bicarbonate share of dissolved carbonate at pH 6, 7, 8 ≈33%, ≈83%, ≈98% (our calculation, 25 °C) Calculated from7,8
Total alkalinity end point pH 4.3 (SCA, DIN 38409-7); about pH 4.5 (USGS) SCA1, DIN10, USGS6
Carbonate end point pH 8.2 (DIN 38409-7); about pH 8.3 (USGS) DIN10, USGS6
1 mmol/L bicarbonate 50.04 ppm as CaCO₃ SCA1
ISO 9963-1 range 0.4–20 mmol/L (≈20–1,000 ppm as CaCO₃, our calculation) ISO9
ISO 6059 lowest determinable 0.05 mmol/L ISO11
Acid neutralised by 50 ppm alkalinity 1.0 mmol H⁺ per litre (our calculation) From the SCA factor1
WHO health-based value for hardness None proposed WHO12
EU parametric value for hardness None; pH 6.5–9.5 Directive 2020/218413
Aargau reporting classes Soft 0–15; medium hard 15–25; hard > 25 °fH Kanton Aargau14

Open questions#

  • Which end point for coffee water. The SCA uses pH 4.3, the USGS about 4.5.1,6 No study opened for this entry compared the two on low-alkalinity brewing waters.
  • Temperature. Alkalinity is defined and measured at room temperature, while coffee is brewed near boiling. Only pK₂ has a cited temperature equation here;8 no study opened for this entry measured bicarbonate buffering at brewing temperature.
  • Bicarbonate versus hardness in the cup. The cold brew study’s abstract does not separate the three ions, so their separate effects remain open.15
  • Other buffers. The acid capacity to pH 4.3 also includes buffers other than bicarbonate.10 No study opened for this entry measured how much they matter in drinking water used for coffee.

Key studies#

Plummer & Busenberg, 1982 – Calcite solubility and carbonic acid. Measured the solubility of calcite, aragonite and vaterite in CO₂–water solutions from 0 to 90 °C and fitted temperature equations for the equilibrium constants of the CaCO₃–CO₂–H₂O system. Their equation for the second dissociation constant of carbonic acid is the basis for the pK₂ values given here.

Geochimica et Cosmochimica Acta 46(6), 1011–1040 · doi:10.1016/0016-7037(82)90056-4

Hendon et al., 2014 – Cations and extraction. Used density functional theory to calculate how strongly sodium, magnesium and calcium ions bind to five coffee acids, caffeine and eugenol. It is a model of the hardness ions, not a brewing trial; its abstract does not address alkalinity.

Journal of Agricultural and Food Chemistry 62(21), 4947–4950 · doi:10.1021/jf501687c

Chen et al., 2024 – Water and cold brew. Made cold brew with six commercial waters and related their ions to the brews’ chemistry and flavour. Calcium, magnesium and bicarbonate correlated negatively with titratable acid, and higher levels reduced fruity and floral flavours.

Science and Technology of Food Industry 45(5), 70–80 · doi:10.13386/j.issn1002-0306.2023050076

Contested
  • Titration end points: the SCA handbook and the German standard DIN 38409-7 titrate alkalinity to pH 4.3,1,10 while the USGS places the bicarbonate equivalence point of most environmental waters near pH 4.5.6 The carbonate end point is pH 8.2 in DIN 38409-7 and about 8.3 for the USGS.6,10 By our calculation, the free hydrogen ions between pH 4.5 and 4.3 alone add about 1 ppm as CaCO₃ to the result; the difference matters only for very soft waters.
  • Carbonate hardness as a name for alkalinity: the SCAA handbook links carbonate and bicarbonate with temporary hardness and sulfate with permanent hardness,3 and many test kits sold for “carbonate hardness” measure alkalinity.1,2 The SCA handbook calls this use misleading, because carbonate hardness equals alkalinity only when total hardness is the larger of the two.1
  • English units in the SCA handbook: its glossary defines the Clark degree as one grain of calcium carbonate per imperial gallon (4.55 L), but its entry “grains per imperial gallon” uses the US gallon (3.78 L), gives 17.1 ppm and calls this unit a Clark degree as well; it also gives the grain once as 64.8 g and once as 64.8 milligrams. Its conversion table keeps the two units apart (17.12 and 14.25 ppm as CaCO₃).1 A grain is exactly 64.79891 mg, a US gallon 3.785 L and an imperial gallon 4.546 L.17 By our calculation, one grain per imperial gallon is 14.25 ppm and one grain per US gallon 17.12 ppm, so the table is right and the glossary is not.
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Change log

  • – Hydroxide at pH 8.3 corrected from about 0.0001 to about 0.1 ppm as CaCO₃. Reason: Arithmetic error; 10⁻⁵·⁷ mol/L × 50,040 mg/mol ≈ 0.1 mg/L.
  • – Removed the test-kit end point of pH 4.5 (Contested, Open questions); reworded the English-units item and added NIST Handbook 44 for the grain and gallons. Reason: The test-kit value rested only on a textbook chapter; the SCA glossary inconsistency was stated incompletely.

Sources

  1. A/BWellinger, M.; Smrke, S.; Yeretzian, C. (2018). The SCA Water Quality Handbook, edition 2 (ISBN 978-0-9995807-3-8). Specialty Coffee Association.
  2. BWellinger, M.; Smrke, S.; Yeretzian, C. (2017). Water for Extraction – Composition, Recommendations, and Treatment. In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 16, pp. 381–398. doi:10.1016/B978-0-12-803520-7.00016-5 · other copies
  3. A/BBeeman, D.; Songer, P.; Lingle, T. (ed. Everage, L.) (2011). The Water Quality Handbook (ISBN 978-1-882552-08-5). Specialty Coffee Association of America.
  4. AU.S. Geological Survey, Water Science School Hardness of Water. U.S. Geological Survey.
  5. AU.S. Geological Survey, Water Science School Alkalinity and Water. U.S. Geological Survey.
  6. AU.S. Geological Survey, Oregon Water Science Center Alkalinity Calculator – Frequently Asked Questions. U.S. Geological Survey.
  7. AU.S. Geological Survey, Oregon Water Science Center Alkalinity Calculation Methods. U.S. Geological Survey.
  8. APlummer, L. N.; Busenberg, E. (1982). The solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90 °C, and an evaluation of the aqueous model for the system CaCO3-CO2-H2O. Geochimica et Cosmochimica Acta 46(6), 1011–1040. doi:10.1016/0016-7037(82)90056-4 · other copies
  9. AInternational Organization for Standardization (1994). ISO 9963-1:1994 Water quality – Determination of alkalinity – Part 1: Determination of total and composite alkalinity (abstract). ISO (abstract as listed by the Association Sénégalaise de Normalisation).
  10. ADIN Arbeitskreis 1.16 „Säure- Basekapazität“ (2005). Basisvalidierung zur Überarbeitung der Norm DIN 38409-7 (H7) vom Mai 1979, Bestimmung der Säure- und Basekapazität. Wasserchemische Gesellschaft in der GDCh / DIN (validation document).
  11. AInternational Organization for Standardization (1984). ISO 6059:1984 Water quality – Determination of the sum of calcium and magnesium – EDTA titrimetric method (abstract). ISO (abstract as listed by the Association Sénégalaise de Normalisation).
  12. AWorld Health Organization (2009). Hardness in Drinking-water: Background document for development of WHO Guidelines for Drinking-water Quality (WHO/HSE/WSH/09.01/10). World Health Organization.
  13. AEuropean Parliament; Council of the European Union (2020). Directive (EU) 2020/2184 of the European Parliament and of the Council of 16 December 2020 on the quality of water intended for human consumption, Annexes I and IV. Official Journal of the European Union (text as published by legislation.gov.uk).
  14. AKanton Aargau, Departement Gesundheit und Soziales, Amt für Verbraucherschutz (2020). Informationspflicht Trinkwasser (Merkblatt). Kanton Aargau.
  15. AChen, S.; Xiao, Y.; Jiang, F.; Jiang, T.; He, X.; Zhu, J.; Tang, W.; Zhou, Y. (2024). Effects of Water Quality Factors on the Main Components and Flavor of Cold Brew Coffee. Science and Technology of Food Industry (Shipin Gongye Keji) 45(5), 70–80. doi:10.13386/j.issn1002-0306.2023050076 · other copies
  16. AHendon, C. H.; Colonna-Dashwood, L.; Colonna-Dashwood, M. (2014). The Role of Dissolved Cations in Coffee Extraction. Journal of Agricultural and Food Chemistry 62(21), 4947–4950. doi:10.1021/jf501687c · other copies
  17. ANational Institute of Standards and Technology (2026). Handbook 44–2026, Appendix C: General Tables of Units of Measurement. NIST, U.S. Department of Commerce.

Tier A · authoritative · B · trade and expert · C · general. Supporting quotes for every source are in the data version of this entry. If a publisher’s site refuses access, try the doi link or “other copies”.

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