ASTM D1126-17

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Designation: D1126 − 12 D1126 − 17
Standard Test Method for
Hardness in Water
This standard is issued under the fixed designation D1126; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope*
1.1 This test method covers the determination of hardness in water by titration. This test method is applicable to waters that are
clear in appearance and free of chemicals that will complex calcium or magnesium. The lower detection limit of this test method
is approximately 2 to 5 mg/L as CaCO ; the upper limit can be extended to all concentrations by sample dilution. It is possible
to differentiate between hardness due to calcium ions and that due to magnesium ions by this test method.
1.2 This test method was tested on reagent water only. It is the user’s responsibility to ensure the validity of the test method
for waters of untested matrices.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.5 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D1066 Practice for Sampling Steam
D1129 Terminology Relating to Water
D1193 Specification for Reagent Water
D2777 Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
D3370 Practices for Sampling Water from Closed Conduits
D5847 Practice for Writing Quality Control Specifications for Standard Test Methods for Water Analysis
3. Terminology
3.1 Definitions—For definitions of terms used in this test method, refer to Terminology D1129.
3.1 Definitions:
3.1.1 For definitions of terms used in this standard, refer to Terminology D1129.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 equivalent per million (epm), n—a unit chemical equivalent weight of solute per million unit weights of solution.
3.2.2 laboratory control sample (LCS), n—a solution with certified hardness.
4. Summary of Test Method
4.1 Calcium and magnesium ions in water are sequestered by the addition of disodium ethylenediamine tetraacetate. The end
point of the reaction is detected by means of Chrome Black T , which has a red color in the presence of calcium and magnesium
and a blue color when they are sequestered.
This test method is under the jurisdiction of ASTM Committee D19 on Water and is the direct responsibility of Subcommittee D19.05 on Inorganic Constituents in Water.
Current edition approved March 1, 2012Dec. 1, 2017. Published March 2012December 2017. Originally approved in 1950. Last previous edition approved in 20072012
ε1
as D1126 – 02 (2007)D1126 – 12. . DOI: 10.1520/D1126-12.10.1520/D1126-17.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
3–Hydroxy–4-(1–hydroxy-2–napththyl) azo-7–nitro–1 naphthalenesulfonic acid, sodium salt, Color Index 14645.
*A Summary of Changes section appears at the end of this standard
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D1126 − 17
TABLE 1 Freedom of Reaction from Interferences
Maximum Maximum
Concentration Concentration
Without Without
Substance Interference Interference
in the Total in the Calcium
Hardness Hardness
Test, mg/L Test, mg/L
+ + +
Aluminum, Al 20 5
+ A
Ammonium, NH 2 000
−
Bicarbonate, HCO . 500
Bromine, Br . 2
+ +
Cadmium, Cd 20 .
− −
Carbonate, CO 1 000 50
−
Chloride, Cl 10 000 .
Chlorine, Cl . 2
− −
Chromate, CrO 500 500
+ +
Cobalt, Co 0.3 .
+ +
Copper, Cu 20 2
+ ++ B
Iron, ferric, Fe 10 20
+ + B
Iron, ferrous, Fe 10 20
++
Lead, Pb 20 5
+ + C C
Manganese, Mn 1 10
+ + D
Nickel, Ni 0.5 .
−
Nitrate, NO 500 500
−
Nitrite, NO 500 500
− − −
Phosphate, PO 100 .
− −
Silicate, SiO 200 100
+ + E E
Strontium, Sr
− −
Sulfate, SO 10 000 10 000
− −
Sulfite, SO 500 500
Tannin, Quebracho 200 50
+ +++
Tin, stannic, Sn 10 5
+ +
Tin, stannous, Sn 10 5
++
Zinc, Zn 20 5
A
No data are available.
B
Iron will not interfere in concentrations up to 200 mg/L. However, the red color of
the end point may return in about 30 s.
C
Manganese will not interfere in concentrations up to 10 mg/L if a few crystals of
K Fe(CN) ·3H O are added to the buffer immediately before use.
4 6 2
D
Accurate results can be obtained in the presence of 1 mg/L nickel, but the end
point is slow under these conditions.
E
If strontium is present, it will be titrated with calcium and magnesium.
5. Significance and Use
5.1 Hardness salts in water, notably calcium and magnesium, are the primary cause of tube and pipe scaling, which frequently
causes failures and loss of process efficiency due to clogging or loss of heat transfer, or both.
5.2 Hardness is caused by any polyvalent cations, but those other than Ca and Mg are seldom present in more than trace
amounts. The term hardness was originally applied to water in which it was hard to wash; it referred to the soap-wasting properties
of water. With most normal alkaline water, these soap-wasting properties are directly related to the calcium and magnesium
content.
6. Interferences
6.1 The substances shown in Table 1 represent the highest concentrations that have been found not to interfere with this
determination.
6.2 The test method is not suitable for highly colored waters, which obscure the color change of the indicator.
7. Reagents
7.1 Purity of Reagents—Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all
reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society. Other
grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening
the accuracy of the determination.
Reagent Chemicals, American Chemical Society Specifications, American Chemical Society, Washington, DC. For Suggestions on the testing of reagents not listed by
the American Chemical Society, see Annual Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia and National
Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville, MD.
D1126 − 17
7.2 Purity of Water—Unless otherwise indicated, reference to water shall be understood to mean reagent water conforming to
Specification D1193, Type I. Other reagent water types may be used provided it is first ascertained that the water is of sufficiently
high purity to permit its use without adversely affecting the precision and bias of the test method. Type II water was specified at
the time of round robin testing of this test method.
7.3 Ammonium Hydroxide Solution (1 + 4)—Mix 1 volume of NH OH (sp gr 0.90) with 4 volumes of water.
7.4 Buffer Solution—Prepare the buffer solution in three steps as follows:
7.4.1 Dissolve 40 g of sodium tetraborate (Na B O ·10H O) in 800 mL of water.
2 4 7 2
7.4.2 Dissolve 10 g of sodium hydroxide (NaOH), 10 g of sodium sulfide (Na S·9H O), and 10 g of potassium sodium tartrate
2 2
(KNaC O ·4H O) in 100 mL of water.
4 6 2
7.4.3 When cool mix the two solutions and add 1 g of magnesium disodium ethylenediamine tetraacetate, having a
magnesium-to-EDTA mole ratio of 1 to 1. Make up to 1 L with water. Keep the solution bottle stoppered when not in use. The
reagent will be effective for at least 1 month.
7.5 Calcium Solution, Standard (1 mL = 0.20 mg CaCO )—Dissolve 0.2000 g of CaCO in 3 to 5 mL of HCl (1 + 4). Dilute
3 3
to 1 L with water.
7.6 Calcium Indicator—Use powdered hydroxynaphthol blue, or grind solid hydroxynaphthol blue to 40 to 50 mesh size.
7.7 Hardness Indicator—The hardness indicator can be prepared, stored, and used in liquid or powder form.
7.7.1 Hardness Indicator Solution—Dissolve 0.5 g of Chrome Black T in 50 mL of diethanolamine or triethanolamine. Store
the solution in a dark-colored bottle. This solution has a storage life of several mont
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