Final answer:
The pH values of 6.86 and 9.18 for calibration solutions correspond to the pH of standardized buffers at a certain temperature, providing reliable calibration for pH meters, especially those used in high-accuracy work such as in biochemistry or pharmaceutical development.
Step-by-step explanation:
The specific numbers you mentioned, 6.86 and 9.18, for calibrating pH sensors likely correspond to the pH values of standardized buffer solutions at a certain temperature, commonly referenced in precise pH measurements. These numbers are not arbitrary;
they represent the pH values of particular buffer solutions (phthalate for 4.00, phosphate for 6.86, carbonate for 10.01 at 25°C), and are selected to provide a reliable calibration over the range of pH values you are likely to measure.
When calibrating a pH meter, at least two buffer solutions with known pH values are used to cover the range of expected pH readings. A common buffer is pH 7.00 (neutral), and typically, either a pH 4.00 or pH 10.01 buffer is used depending on whether the expected sample pH is acid or basic, respectively.
The choice of 6.86 and 9.18 is probably linked to the expected pH range of the samples to be measured and the specifications of the pH sensor or meter, which could be intended for high-accuracy work where these non-standard buffer solutions provide the appropriate calibration points.
Researchers in fields requiring precise pH measurements, such as environmental science, biochemistry, or pharmaceutical development might use these specific calibration points.
High-accuracy and research-grade meters, such as the ones with the ability to have a resolution of 0.001 pH units and an accuracy of ±0.002 pH units might necessitate these specific calibration standards which are slightly unusual.