WTW Standard (DIN/NIST) Buffer Solution PL 7, pH 6.865/6.87, PTB/NIST-traceable Calibration Solution in Accordance with DIN 19266-01:2000, ± 0.02 pH, 250 ml, 109120

An electrode that is incorrectly calibrated in the neutral range will introduce a bias into every subsequent measurement, particularly where pH 7, as the zero point of the scale, plays a key role. The WTW Standard (DIN/NIST) buffer solution PL 7 provides a traceable reference for this purpose: a working reference buffer solution in accordance with DIN 19266-01:2000, certified as traceable to PTB/NIST, with an accuracy of ± 0.02 pH at 25 °C. Together with PL 4 and PL 9, it forms the midpoint of a complete, documentable three-point calibration.

Standard (DIN/NIST) buffer solution PL 7: higher accuracy class than the technical range

The key difference from the ‘technical’ WTW buffer solution range lies in the accuracy class and traceability. The WTW Technical Buffer Solution pH 7 is designed for daily routine calibration and reliably covers everyday laboratory work. The Standard (DIN/NIST) Buffer Solution PL 7 goes one step further: As a working reference buffer solution in accordance with DIN 19266-01:2000, it is certified as PTB/NIST-traceable and specified to ± 0.02 pH at 25 °C. This makes it the ideal choice wherever the calibration itself must be documented and traceable, such as in ISO/IEC 17025-accredited testing laboratories, in pharmaceutical analysis or in metrologically demanding research.

Buffer system comprising two phosphate salts

PL 7 is based on a mixture of disodium hydrogen phosphate and potassium dihydrogen phosphate as a buffer system and achieves a pH value of 6.87 at 20 °C, stated on the label as pH 6.865 according to DIN or pH 6.87 according to NIST. The density is 1.01 g/cm³, the melting point is -0.3 °C and the boiling point is 100 °C. Under the CLP Regulation, the solution is not classified as a hazardous substance and is not a dangerous good, which simplifies storage, transport and handling in day-to-day laboratory work. Each bottle bears its own batch code with an expiry date, ensuring that the batch used can always be linked to the corresponding test certificate and remains clearly traceable within the documentation of a quality management system.

Traceability as part of quality assurance

Traceability means that the printed pH value can be traced back through an unbroken chain of reference measurements to the national metrology institutes PTB and NIST. For a testing laboratory accredited to ISO/IEC 17025, this is precisely the difference between a plausible and a robust measurement: not only the electrode itself, but also the reference used to calibrate it, must be documented and traceable. The zero point at pH 7, in particular, affects the entire calibration curve, which is why a traceable, certified buffer is especially important at this point. With PL 7, this requirement can be met directly via the batch identification and the associated test certificate, without the need for additional in-house verifications in the laboratory.

Storage and handling

Storage in the tightly sealed original container at 15 to 25 °C is recommended. According to TRGS 510, the solution is classified in storage class LGK 12 (non-flammable liquids) and is not considered to pose a risk to water. As PL 7 is neither classified as a hazardous substance nor declared as dangerous goods, no special protective measures are required when opening or dispensing it. Nevertheless, safety goggles with side protection are recommended for the safe handling of the reference solution, and the container should be resealed promptly after use to prevent the reference value from shifting due to evaporation or the absorption of CO2 from the air.

Typical applications

PL 7 is used wherever the calibration of a pH electrode must not only be ‘adequate’ but also verifiable through documentation:

• Two- or three-point calibration of pH electrodes in the neutral measuring range, in conjunction with the WTW buffer solutions PL 4 and PL 9
• Zero-point adjustment of the pH scale as the central reference point for every calibration routine
• Traceable calibration in accredited testing laboratories in accordance with ISO/IEC 17025
• Quality control in pharmaceutical and active ingredient analysis
• Calibration prior to the analysis of approximately neutral samples, such as drinking water or cell culture media
• Research institutions with stringent requirements regarding measurement uncertainty

Buffer solutions in accordance with DIN 19266-01:2000 are among the most widely used working reference buffer solutions, as their composition is specified directly in the standard. When PL 7 is used in conjunction with PL 4 and PL 9 for a three-point calibration, this trio covers the entire pH range relevant to practical applications, from acidic through neutral to alkaline, with the same accuracy class and the same traceability to PTB/NIST. This simplifies documentation, as all three calibration points can be included in the same test certificate format.

Technical details

Scope of delivery

• 1 bottle of WTW Standard (DIN/NIST) buffer solution PL 7, 250 ml, with batch number and expiry date

Individual requirements and procurement service

For a complete three-point calibration, we also stock the WTW Standard (DIN/NIST) buffer solutions PL 4 (pH 4.006/4.01) and PL 9 (pH 9.180/9.18). If you carry out daily routine calibration using the technical accuracy class, you will find the WTW Technical Buffer Solution pH 7 with the same pH value in our range. You can find suitable pH electrodes for calibration in our range of electrodes. Our procurement service is on hand to assist you with additional accessories for your pH measurement technology or with individual procurement enquiries.