BAM Certified Reference Materials for Titrimetry and Ion Chromatography
By: Jürg Wüthrich, Nicole Amann, AnalytiX Volume 8 Article 3
It is the responsibility of certain federal agencies and metrological institutes to assign traceable values, providing the ultimate link to the SI unit by performing a primary measurement. However, not every agency may be adequately equipped for the efficient production of standards and calibration solutions. Often commercial producers are better equipped for this task. An ideal situation is to combine these complementary skills; like the decadelong cooperation between the Sigma-Aldrich production facility in Buchs, Switzerland, and the Swiss Federal Institute for Materials Science and Technology (EMPA) and Federal Institute for Materials Research and Testing (BAM) for the certification of titrimetric standards and calibration solutions. In this partnership, Sigma-Aldrich was responsible for the supply and production of different reference materials. When EMPA terminated its activities in chemical metrology in 2004 and no longer acted as a national metrological institute, BAM and Sigma-Aldrich intensified their close collaboration and launched the current series of certified anion standards.
Although cutting-edge analytical instruments like spectrometers, AAS, ICP-OES, etc. are now commonplace in analytical laboratories, volumetric titration is still widely used because it is fast, inexpensive and very precise. The most important titrimetric CRMs offered by Sigma-Aldrich and their typical use are summarised in Table 1.
For the standardisation of titrants, a primary standard should be used to assure traceability to a well-defined reference. A primary standard should have:
- Well-defined content and high purity (>99.8%) with a stated measurement uncertainty
- Traceability to SI unit
- Good stability for handling under ambient conditions and during storage (shelf life)
- High equivalent weight for optimal weighing
- Good solubility, especially important for titrimetry
Industries like pharmaceutical or semiconductor may need to work with certified standards with higher qualification to comply with GMP, GLP or ISO guidelines. Labs working in a regulated environment usually have to prove traceability of their measurements, evaluate measurement uncertainty and validate their analytical methods.
The increasing trend of using CRMs for regular use is reinforced by official regulations of the European Community within the guideline 96/23/EG, paragraph 220.127.116.11. which states:
“The determination of trueness (one component of accuracy) is described: trueness can only be established by means of CRMs. A CRM should be used whenever available.”
The U.S. Food and Drug Administration (FDA) demands in their Inspection Guide the use of reference standards in analytical methods.
Even when a laboratory performs measurements under no special regulations, it is often advantageous to use a CRM when methods need to be verified or instruments qualified. This explains the increasing demand for CRMs certified by an independent metrological institute.
Table 1 BAM-certified reference materials for titrimetry including the new CRM potassium dichromate (no. 42019, available in 50 g package size) with their typical use
|Titration method||Standard for volumetric titration||Cat. no.||Examples of use|
|Acid-base titration||Benzoic acid||12353||Acidimetric CRM for standardisation of bases in non-aqueous solution|
|Potassium phthalate monobasic||60357||Acidimetric CRM for standardisation of bases in aqueous solution|
|Sodium carbonate||71363||Basimetric CRM for standardisation of acids in aqueous solution|
|Tris(hydroxymethyl)aminomethane (Tris base, Trizma® base)||93440|
|Redox titration||Arsenic trioxide||17971||Reductometric CRM for standardisation of iodine solution|
|Sodium oxalate||71804||Reductometric CRM for standardisation of permanganate solution|
|Potassium dichromate||42019||Reductometric CRM for standardisation of thiosulfate solution|
|Complexometric titration||Calcium carbonate||21067||Complexometric standard for standardisation of EDTA solution|
|Precipitation titration||Sodium chloride||71387||Argentometric standard for standardisation of silver solution|
Each BAM-certified standard for titrimetry is certified according to ISO/IEC 17025 and accompanied by a printed certificate fulfilling ISO Guide 31. The following parameters are provided in each certificate:
- Certified property value and its uncertainty
- Detailed analysis report of BAM, including traceability statement and measurement procedure
- Intended use, storing and handling instructions
- Expiry date based on stability studies
- Homogeneity and the recommended minimal amount to use
The label on each bottle contains the following information:
- Certified property value with uncertainty
- Sigma-Aldrich catalogue number and product name
- Batch number
- Expiry date
Please visit www.sigma-aldrich.com/titrimetry for more information.
After the successful introduction of the BAM-certified titrimetry CRMs, Sigma-Aldrich and BAM launched a series of CRM for ion chromatography. With PRIMUS two ready-to-use Primary Multi-ion Solutions were introduced and the outstanding characteristics of these CRM were described in an article of “Metrohm Information”. In addition, a series of mono-anion CRM were launched which are described in more detail here (Table 2): To guarantee the highest reliability of the 1000 mg/L anion standards, they are certified by two independent certification bodies. The batch solution of the reference material is prepared gravimetrically, using well-characterised high-purity starting materials (SM) in a double-accredited laboratory under ISO/IEC 17025 and ISO Guide 34. The SM content is calculated by 100% minus all impurities. The calculated SM content is verified by a comparison with an appropriate standard reference material from the National Institute of Standard & Technology (NIST). If contamination and loss of material during the whole production procedure is strictly prevented, this approach allows high accuracy and small uncertainties. The first certified value of the bottled CRM is based on this approach and is therefore directly traceable to the SI unit kilogram.
After bottling the batch solution under clean-room conditions and random sampling, a second certified value is produced by BAM using volumetric titration or precipitation analysis. The measurements are traced directly to SI unit or to a Standard Reference Material from NIST. Both property values, the certified value from BAM and from Sigma-Aldrich, are combined to the certified value of the anion standard solution.
The equipment and materials used to package the standard solution must maintain the quality instilled by the certification procedure. Easy handling, low impurities and good storage stability are essential. The use of HDPE bottles with aluminium-coated bags extends the shelf life of the unopened CRM up to four years and in addition this special packaging leads to smaller uncertainties. Only this combination of high-precision analytical measurement and packaging know-how leads to uncertainties of the certified values down to 0.2% (expanded uncertainty). Figure 1 shows a comparison between storage with and without additional packaging. Detailed data on trace impurities and data from transpiration studies are given in the printed certificate.
Please visit www.sigma-aldrich.com/ic for more information.
Figure 1. Aqueous standards in tightly closed plastic bottles (HDPE, LDPE, etc.) loose a significant percentage of solvent at room temperature within a storage time of several years. Aluminized bags and also storing at low temperature (i.e. refrigerator) will reduce this transpiration rate to an acceptable level.
Table 2 BAM and Sigma-Aldrich certified reference materials for ion chromatography
|Cat. no.||Constituents||Starting material||Quality||Package size|
|87969||Bromide||NaBr||Certified reference materials for ion chromatography||100 mL HDPE bottle sealed in aluminised bag|
|87603||Chloride||NaCl||Certified reference materials for ion chromatography||100 mL HDPE bottle sealed in aluminised bag|
|80373||Fluoride||NaF||Certified by BAM and gravimetric assay||100 mL HDPE bottle sealed in aluminised bag|
|86576||Nitrate||NaNO3||1000 mg/L in water, exact content and uncertainty on label and enclosed certificate. Shelf life: four years.||100 mL HDPE bottle sealed in aluminised bag|
|81193||Phosphate||Na2HPO4||1000 mg/L in water, exact content and uncertainty on label and enclosed certificate. Shelf life: four years.||100 mL HDPE bottle sealed in aluminised bag|
|80218||Sulphate||Na2SO4||1000 mg/L in water, exact content and uncertainty on label and enclosed certificate. Shelf life: four years.||100 mL HDPE bottle sealed in aluminised bag|
- ISO Guide 31, Reference materials – Contents of certificates and labels, 2000.
- PRIMUS – Certified standard solutions for ion chromatography, Metrohm Information, Vol. 33 (2004) No. 1 (available at www.metrohm.com/company/metrohminfo/2004/mi2004_1e.html).
- TraceCERT® – Traceable Certified Reference Materials. Part 3: Challenges in the characterisation of high purity starting materials, Analytix, Vol. 2, 2007 (available at www2.sigmaaldrich.com/ analytix).
- ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories, 2005. ISO Guide 34, General requirements for the competence of reference material producers, 2000.