1. 1 NIST Electrolytic Conductivity Standard Reference Materials ® Traceability and Stability Issues Kenneth W. Pratt National Institute of Standards and Technology (NIST) 100 Bureau Dr., Stop 8391 Gaithersburg, MD 20899-8391 USA kenneth.pratt@nist.gov NCSLI conference, San Antonio, TX USA 29 July 2009, Session 6D

2. 2 Charge carriers: electrons Units: Siemens (S) per meter (S·m -1 ) or (more often) µS·cm -1 1 S = 1 ohm -1 ( -1  = 1 mho (obsolete) Approximate κ values (µS∙cm -1 ): t = 25 °C except as noted Pure water 0.055 [resistivity = 18.2 MΩ·cm] Water equil. w/ atm. CO 2 1.0 [resistivity = 1.0 MΩ·cm] Tap water ~300 0.01 mol L -1 KCl 1 400 Sea water 53 000 (35 ‰ salinity) 1.0 mol L -1 KCl 110 000 30 % H 2 SO 4 740 000 (18 °C) Hg (0 °C) 10 629 500 000 ≈ 1.06 x 10 10 Ag (metal) 618 000 000 000 = 6.18 x 10 11 Electrolytic Conductivity ( κ ) Werner von Siemens 1816-1892

3. 3 In essence: a resistance ( R ) measurement Resistivity, ρ Conductivity Measurement of κ A l R = 10 kΩ Cell constant, K cell l = length, A = area Traceability of κ requires both K cell and R to be traceable to the International System of Units (SI)

4. 4 K cell and Traceability Calibration of a cell = determination of K cell Traceability –Via area, A, and length, l (absolute measurement) Only possible in specially-designed cells Accurately-known geometry of electric field A and l known from physical measurement –Calibration with a standard solution CRMs Other solutions with measured κ κ is indirectly traceable to an absolute measurement –must include uncertainty of the absolute measurement in the uncertainty of the cell calibration!

5. 5 NIST Absolute Measurement of κ Measurement #1: Insert Center Tube (b) between End Sections (a and a′) Measurement #2: Connect End Sections (a and a′) directly to each other. R with center tube − R no center tube = ΔR = l/A (of center tube) ′ A l

6. 6 Absolute Measurement of κ of Pure H 2 O: An Electrical Approach IF you can build a calculable capacitor ( C ) –Coaxial –Quadrupole (Thompson and Lampard) AND you can fill it with pure water –No CO 2 or other impurity ions AND you can show that the R and C lines of force have the same geometry/symmetry AND you can factor out electrode effects THEN you have an absolute measurement of κ of pure H 2 O !

7. 7 Contributors to κ – Anything Ionic Dissolved salts, acids and bases (including fumes) –KCl → K + + Cl - –HNO 3 → H + + NO 3 - –NH 3 (g) + H 2 O ↔ NH 4 + + OH - Atmospheric CO 2 –CO 2 (g) ↔ CO 2 (aq) ↔ H 2 CO 3 ↔ H + + HCO 3 - –~ in unbuffered solutions, proportional to square root of air pressure CO 2 mole fraction Leached ions from containers –glass → Na + + OH - Oxidation products from neutral organics –HDPE, alcohols + O 2 → weak acids ↔ H + + anion − Water –H 2 O ↔ H + + OH - (very minor contribution to SRMs)

8. 8 κ ~ Proportional to Concentration

9. 9 Why not obtain κ from the concentration? Relationship is not perfectly linear –Ionic interactions Dissolved CO 2 –Raises κ from 0.055 µS·cm -1 to ~1.0 µS·cm -1 –Lowers ρ from 18.2 MΩ·cm to ~1.0 MΩ·cm Impurities –Water in salts

10. 10 NIST Electrolytic Conductivity Standard Reference Materials (SRMs) w = mass fraction ν = amount content 1 HDPE = high-density polyethylene PET = polyethylene terephthalate Certified by κ measurement – not by gravimetric preparation Traceable to NIST absolute κ measurements 1

11. 11 NIST Spectrometric SRMs SRM Standard Solutions for 67 elements (1 SRM per element) Top-tier traceability link to the SI for elemental analysis Used as primary calibration standards and validation tools by commercial producers of Certified Reference Materials (CRMs) Elemental Analysis Lab a Elemental Analysis Lab b Standards Producer A Elemental Analysis Lab c Standards Producer B Elemental Analysis Lab d Elemental Analysis Lab e Standards Producer C NIST SRM 3100 Series NIST Primary Materials Elemental Analysis Lab f Standards Producer D Other NMI Calibration CRMs Other NMI Primary Materials SI Certified for the mass fraction of the major element only Produced from well-characterized, solid materials known as NIST Primary Materials Certified using 2 independent methods (usually gravimetric preparation and high-performance inductively-coupled plasma- optical emission spectrometry)

12. 12 Compare Spectrometric and Conductivity SRMs? It’s a bit like comparingand. But – you can compareand if...

13. 13... you call them both.

14. 14 Conductivity SRMs vs. Spectrometric SRMs Certified for κ (conductivity) –Value varies with SRM All ions contribute to cert. value In an ideal (stable) standard: –Solute (KCl, HCl) –CO 2 → H + + HCO 3 − –Water ↔ H + + OH - (minor) In a practical standard, also: –Acid fumes → H + (+ anion) –Leaching → alkali + OH − (or other) –Air oxidation: O 2 + 2H 2 O → 4OH − + 4e − 1-propanol → organic acids HDPE → organic acids Transpiration/Evaporation –Absent in sealed ampoules –Cap sealing on glass bottles –Al-PET bags used for SRMs in plastic bottles Certified for w M (mass fract. of metal, M) –Nominal w M = 10 mg·g -1 (1 %) –M is a minor component Only M contributes to cert. value –Acid matrix − no influence on w M Only a problem if M in source acid –Leaching Only a problem if M leaches Minor problem in ampoules Absent in plastic bottles –Precipitation Adjust matrix to keep M in solution Transpiration/Evaporation –A bsent in sealed ampoules –Al-PET bags used for SRMs in plastic bottles –Main contributor to change in w M

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17. 17 Matrix: 49.5 µg kg -1 KCl in water..

18. 18 Matrix: 5.3 µg kg -1 KCl in 30 % n- PrOH + 70 % water.

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