Deionized water
Ion removal, conductivity and resistivity, purity grades, and the limits of deionized-water claims.
Tags: chemicals, water, ion-exchange
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Identity and terminology
The theoretical pure-water reference at 25 °C is about 0.055 µS/cm or 18.2 MΩ·cm. Water self-dissociation accounts for finite conductivity. MilliporeSigma water quality monitoring Deionization identifies ion removal, while grade and lot records describe measured quality. Supelco EM3234 is marketed as both deionized and distilled, ASTM Type II. Supelco EM3234 product page
Cation and anion resins exchange ions for H⁺ and OH⁻, which form water. Separate beds split the resin types. Mixed beds combine them. Water remains H₂O, CAS 7732-18-5. Thermo Scientific waterbook, p. 12
Related treatments and purity descriptions
The following distinctions describe mechanisms, not equivalent grades. MilliporeSigma, laboratory water purification techniques.
| Term | Meaning and boundary |
|---|---|
| Deionization | Ion exchange removes dissolved ions. It does not effectively remove most organics, particles, or bacteria. |
| Softening | Common sodium-cycle softening exchanges calcium and magnesium for sodium. Removing hardness is different from removing dissolved ions generally. |
| Reverse osmosis, RO | Applied pressure drives water through a semipermeable membrane. Rejection of contaminants is selective and incomplete. |
| Distillation | Water becomes vapor and then condenses. Some contaminants can reach the condensate, so distilled water is not automatically free of every impurity. |
| Electrodeionization, EDI | Ion exchange combines with electrodialysis and ion-selective membranes. |
"Ultrapure" describes a broader quality claim than ion removal alone. Thermo Fisher states that multiple technologies are needed for its Type 1 water. Synthesis: DI, RO, distilled, and ultrapure are therefore insufficient as interchangeable descriptions of a supplied material. Thermo Scientific waterbook, p. 17.
Conductivity, resistivity, and units
Conductivity measures electrical conduction through the water. Resistivity is its reciprocal. Conductivity depends on the ions' concentrations, charges, and mobilities, so it does not identify individual contaminants. NIST defines conductivity in S/cm and resistivity in Ω·cm. Resistance in Ω also depends on the measurement cell geometry. NIST SP 260-142, introduction and section 1.1.
Calculated unit relationship: for the same sample, temperature, and reporting basis,
resistivity [MΩ·cm] = 1 / conductivity [µS/cm].
The numerical simplification follows because megaohm is 10⁶ Ω and microsiemens is 10⁻⁶ S. Thus, a hypothetical reading of 0.20 µS/cm corresponds to 5.0 MΩ·cm. This is arithmetic, not a grade or an acceptance limit. NIST reciprocal definition, section 1.1.
MilliporeSigma gives approximately 0.055 µS/cm and 18.2 MΩ·cm for theoretically pure water at 25 °C. Water's own dissociation supplies hydrogen and hydroxide ions, so pure water has finite conductivity. These rounded values describe an ionic-purity reference, not every DI product. Conductivity changes with temperature. An actual-temperature result and a result compensated to 25 °C have different reporting bases. The manufacturer's meters can display either. MilliporeSigma, water quality monitoring.
Carbon dioxide and pH limitations
Atmospheric CO₂ dissolves in water and produces hydrogen and bicarbonate ions, increasing conductivity. NIST identifies atmospheric conditions as a source of variation. Synthesis: a change after exposure to air does not by itself identify salt contamination or quantify a deionizer's performance. NIST SP 260-142, sections 1.3 and 2.2.
Low-conductivity water can produce slow, noisy, drifting, or inaccurate conventional pH-electrode readings. Thermo Fisher identifies low ionic strength, differences from calibration buffers, liquid-junction potentials, and CO₂ absorption as causes. Low buffering capacity also lets small contamination change the sample's pH. Synthesis: a pH value alone neither establishes ionic purity nor proves a particular impurity is present. A result's meaning depends on the measurement method and sample condition. Thermo Fisher Application Note 006, AN-PHLOWION-E 0914 RevA, 2014, pp. 1–2.
Purity properties remain separate
Thermo Fisher lists organics, particles, pyrogens, and bacteria among conventional DI's limitations. Its contaminant discussion also distinguishes dissolved gases. An ionic measurement is therefore only one part of water characterization. Thermo Scientific waterbook, pp. 6–7 and 12.
| Property | What remains distinct from conductivity |
|---|---|
| Total organic carbon, TOC | Organic contamination has a separate measurement basis. MilliporeSigma's TOC method oxidizes organics before detecting the resulting conductivity change. An ordinary conductivity reading is not a TOC result. |
| Particles | A particle-size or particle-count statement is separate from dissolved-ion content. |
| Microorganisms | A microbial count is separate from ionic purity. DI does not establish sterility. |
| Endotoxins | An endotoxin result is separate from a bacterial count and from conductivity. |
| Dissolved gases | A conductivity result does not quantify all dissolved gases. CO₂ can affect conductivity through its ionic species. |
Synthesis: these are separate evidence categories, not alternative names for one purity score. The table combines the waterbook's contaminant distinctions with MilliporeSigma's separate resistivity and TOC measurements and NIST's CO₂ discussion.
Named grades and evidence limits
ASTM's public D1193-24 summary describes four reagent-water types and three additional grades addressing microbiological contaminants. The type numbers do not represent a simple progression in purity. The public scope permits alternate production technologies subject to the applicable constituent specifications and application basis. This article uses the public summary, not the paid standard's full requirements. ASTM D1193-24 publisher summary.
ISO 3696:1987 addresses three grades for laboratory analysis of inorganic chemicals. Its public abstract excludes organic trace analysis, surface-active-agent analysis, and biological or medical analysis. ISO lists edition 1, published in April 1987 and confirmed in 2023. These scopes do not establish equivalence between ISO grades, ASTM types, or a supplier's "ultrapure" label. ISO 3696:1987 publisher abstract and status.
Synthesis: a generic reference, a supplier specification, a lot certificate of analysis, and a measurement at a particular point and time answer different questions. No current lot certificate or point-of-use result was examined here. The reviewed EM3234 page contains inconsistent rendered conductivity and resistivity units, so its numeric purity limits are not reproduced. Its supplier grade claim is not independent proof of conformity. EM3234 specifications, analysis note, and certificate section.
The Thermo Scientific waterbook is COL34480 0722, © 2022. The cited manufacturer web articles have no visible revision date. NIST SP 260-142 was issued in February 2000. Coverage is limited to definitions and measurement relationships, not full standards verification, process suitability, or medical use.
Related reference
Surface-finishing processes provides broader manufacturing context. This article supplies no rinse specification, formulation recipe, or operating settings.