# Water hardness in process fluids

Calcium and magnesium hardness, reporting bases, alkalinity distinctions, and limits of measurement in formulated process fluids.

Reviewed: 2026-10-30

Tags: chemicals, process-water, measurement

## Identity and scope

From calcium and magnesium in mg/L, hardness is **approximately 2.497 × calcium + 4.118 × magnesium** in mg/L as CaCO₃. At 40 mg/L calcium and 12 mg/L magnesium, the result is about 149 mg/L as CaCO₃. The 52 mg/L sum is ion mass concentration. With independently measured carbonate alkalinity of 100 mg/L as CaCO₃, the conventional partition is about 100 mg/L carbonate hardness and 49 mg/L noncarbonate hardness, on the same CaCO₃ basis. [USGS, *Hardness of water*, August 24, 2026](https://www.usgs.gov/water-science-school/science/hardness-water) [ASTM D1126-17(2025), public significance and use](https://store.astm.org/d1126-17r25.html)

Other polyvalent metals may contribute to a method’s hardness result. Incoming water, fresh fluid and used emulsion are separate sample bases.

## Hardness, alkalinity, and carbonate terminology

| Term | Meaning and boundary |
| --- | --- |
| Calcium hardness | Calcium's contribution expressed on the declared hardness basis. |
| Magnesium hardness | Magnesium's contribution on the same basis. |
| Calcium plus magnesium hardness | Sum of those two contributions. Often called total hardness in natural-water reporting. |
| Carbonate hardness | The portion of hardness chemically equivalent to bicarbonate and carbonate alkalinity. |
| Noncarbonate hardness | The excess of total hardness over that carbonate-associated portion. |

The calcium and magnesium contributions add on a common basis. [Hach Method 8213, hardness relationships, page 3](https://cdn.hach.com/7FYZVWYB/at/mp3b4pzqfmz4tt8wsjvmjn8/DOC3165301176.pdf). The carbonate distinction follows [USGS WRIR 02-4045, table 3, hardness and bicarbonate/carbonate rows](https://pubs.usgs.gov/wri/wri024045/htms/table3.htm). As chemical interpretation, these are equivalent-concentration relationships rather than an identification of discrete dissolved salt molecules.

Alkalinity measures acid-neutralizing capacity. It can include carbonate and noncarbonate species. In USGS's field-manual terminology, alkalinity concerns a filtered sample, while acid-neutralizing capacity includes solutes and particulates in an unfiltered sample. Its operational filtration definition uses a membrane of 0.45 µm or smaller, without clogging. Carbonate alkalinity concerns bicarbonate and carbonate specifically. [USGS National Field Manual, chapter A6.6, version 3.0, July 2006, page ALK-3 and footnote 1](https://pubs.usgs.gov/twri/twri9a6/twri9a66/twri9a_chapter6.6._v3.pdf).

For nonnegative total hardness H and carbonate alkalinity A, expressed in the same equivalent units, the conventional partition is:

`carbonate hardness = min(H, A)`

`noncarbonate hardness = H − min(H, A)`

This algebra is synthesis of the USGS definition. Substitution of total alkalinity for A requires carbonate and bicarbonate to account for that alkalinity. A formulated cleaner or coolant does not acquire that assumption merely because its alkalinity is reported "as CaCO₃." Hardness, alkalinity, and pH describe different properties. [USGS table 3, hardness, pH, and bicarbonate/carbonate rows](https://pubs.usgs.gov/wri/wri024045/htms/table3.htm); [USGS field manual, definitions on ALK-3](https://pubs.usgs.gov/twri/twri9a6/twri9a66/twri9a_chapter6.6._v3.pdf).

## Units and chemical basis

"mg/L as CaCO₃" expresses chemical equivalence to calcium carbonate. It does not mean that the sample contains that mass of dissolved CaCO₃. Ion mass, amount of substance, and charge equivalents are distinct bases. USGS relates equivalents to ionic charge and reports hardness by multiplying calcium-plus-magnesium meq/L by approximately 50. [Hem, 1985, pages 55–56 and 158](https://pubs.usgs.gov/wsp/wsp2254/pdf/wsp2254a.pdf).

The following are derived conversions. The stated molar masses are arithmetic assumptions, rounded to Ca = 40.078, Mg = 24.305, and CaCO₃ = 100.086 g/mol. Both metal ions have charge +2.

| Starting quantity | Equivalent expression |
| --- | --- |
| Summed Ca²⁺ and Mg²⁺ amount concentration of 1 mmol/L | 2 meq/L, or approximately 100.086 mg/L as CaCO₃ |
| 1 meq/L of hardness | Approximately 50.043 mg/L as CaCO₃ |
| Calcium, C mg/L as Ca | Approximately 2.497 × C mg/L as CaCO₃ |
| Magnesium, M mg/L as Mg | Approximately 4.118 × M mg/L as CaCO₃ |

On those assumptions, `H ≈ 2.497C + 4.118M`. This calcium-plus-magnesium calculation does not include other metals that a particular titration might detect. Hem's historical table 8 gives 4.116 for the magnesium factor. Factors therefore differ slightly between this calculation and that historical table. [Hem, table 8 and hardness definition](https://pubs.usgs.gov/wsp/wsp2254/pdf/wsp2254a.pdf).

Synthetic arithmetic example, not a process target: C = 40 mg/L and M = 12 mg/L gives H ≈ 149 mg/L as CaCO₃. The ion masses sum to 52 mg/L. If carbonate alkalinity is independently known to be 100 mg/L as CaCO₃, the conventional partition gives approximately 100 mg/L carbonate hardness and 49 mg/L noncarbonate hardness. The total alone cannot recover the separate calcium and magnesium concentrations.

For dilute water of density approximately 1 kg/L, mg/L and mass-based ppm are numerically close. They are not identical unit definitions. A formulated fluid's density and the stated ppm basis matter. Grains per gallon also requires the gallon convention. [Hem, pages 54–55 and table 8](https://pubs.usgs.gov/wsp/wsp2254/pdf/wsp2254a.pdf).

## Measurement and matrix limitations

ASTM D1126-17(2025) publicly describes titrimetric measurement for clear waters without chemicals that complex calcium or magnesium. Its reported lower detection limit is approximately 2–5 mg/L as CaCO₃. Testing used reagent water only. These are analytical scope limits, not permissible process-water levels. As synthesis, clarity alone does not demonstrate method validity for a formulated fluid containing complexants. [ASTM public scope, sections 1.1–1.2](https://store.astm.org/d1126-17r25.html).

Hach's Method 8213 illustrates why "total" is method-dependent. Barium and strontium can titrate with calcium. Other polyvalent metals can enter the result. Polyphosphates interfere, orthophosphate can slow the endpoint through calcium phosphate formation, and highly buffered samples can prevent the reagents from establishing the intended pH. Its interference treatment with CDTA can still include hardness from interfering metals in the result. [Hach DOC316.53.01176, Edition 8, April 2015, tables 4–6 and accompanying text, pages 4–5](https://cdn.hach.com/7FYZVWYB/at/mp3b4pzqfmz4tt8wsjvmjn8/DOC3165301176.pdf).

The following distinctions are explanatory synthesis of those method limits:

| Reported result | Limit of interpretation |
| --- | --- |
| Hardness calculated from calcium and magnesium assays | Depends on the assays' sample fraction and preparation. Does not identify free-ion concentrations or count other metals. |
| Titrimetric total hardness | Depends on the named method, endpoint behavior, complexation, and interfering species. |
| A result from a filtered or separated sample | Describes that fraction. It does not automatically represent the whole fluid. |
| Conductivity or total dissolved solids | Does not identify the calcium and magnesium contribution. |

Specific conductance varies with ionic composition, concentration, temperature, and ionization. Sodium and potassium can contribute dissolved mineral content without being calcium or magnesium. Consequently, there is no universal hardness conversion from conductivity or total dissolved solids. [USGS WRIR 02-4045, table 3, specific-conductance, hardness, sodium/potassium, and dissolved-solids rows](https://pubs.usgs.gov/wri/wri024045/htms/table3.htm).

## Process effects and product-specific evidence

Hardness salts can contribute to pipe or tube scale, with consequences for flow and heat transfer. A hardness result alone does not predict a deposit's mass or composition. [ASTM D1126-17(2025), public significance and use](https://store.astm.org/d1126-17r25.html).

Henkel's BONDERITE L-MR 21718 data sheet makes a product-specific claim about emulsion stability and low foam over a stated water-hardness interval. The February 2022 sheet uses °GH. Henkel's Croatia English product page uses °dH and gives different numerical bounds for foam and hard-water stability. The sources do not explain that discrepancy, so this reference does not combine them into an operating window. The sheet also distinguishes a pH result in deionized water from lubricating tests at a specified hardness. Those test contexts are not interchangeable. [Henkel TDS, February 2022, page 1](https://datasheets.tdx.henkel.com/BONDERITE-L-MR-21718-en_GL.pdf); [Henkel product description, accessed October 30, 2026](https://next.henkel-adhesives.com/hr/en/products/industrial-lubricants/central-pdp.html/bonderite-l-mr-21718/676717IB.html).

As synthesis, a product's hardness-tolerance claim cannot establish tolerance for another formulation, a used fluid, or a particular machine. General labels such as "soft" and "hard" classify water rather than qualify a manufacturing process. [USGS, measures of water hardness](https://www.usgs.gov/water-science-school/science/hardness-water).

## Source versions and coverage limits

The chemical reference is Hem's *Study and interpretation of the chemical characteristics of natural water*, third edition, 1985, supplemented by USGS's 2006 field-manual definitions and 2002 report table. Hach's reviewed method is Edition 8 of April 2015. ASTM coverage is limited to the public scope and significance text for D1126-17(2025), updated July 10, 2025. The paid standard's detailed procedure was not reviewed.

The Henkel PDF retains a February 2022 technical revision and a November 27, 2025 digital-generation timestamp. Its page 2 limits the data's applicability to conditions beyond Henkel's control. The generation date does not establish a technical revision. [Henkel TDS, pages 1–2](https://datasheets.tdx.henkel.com/BONDERITE-L-MR-21718-en_GL.pdf).

The reviewed evidence does not establish a universal hardness target, treatment method, emulsion operating window, corrosion prediction, surface-finish acceptance criterion, or customer qualification. Water treatment and fluid preparation procedures are outside this reference.

[Surface-finishing processes](../../processes/finishing/index.md).