This document provides a protocol for measuring the pH of dilute nanoparticle suspensions of composition similar to NIST Reference Materials (RMs) 8011, 8012, and 8013, which contain gold nanoparticles in a dilute electrolyte solution. The pH is measured by combination semi-micro glass electrode using bracketing (2-point calibration). Traceability is to the pH values of the respective standard buffers. The measurement as described may be applied to small volumes (4 mL to 5 mL) and is therefore applicable to the contents of a single ampoule of each of the above RMs.
To minimize consumption of the nanoparticle suspension, this protocol for pH measurement may be performed following the measurement of the electrolytic conductivity, κ, of the same sample, provided that κ is measured according to the corresponding protocol listed on this Web site (PCC-12). However, seepage of KCl from the reference junction of the pH electrode during the measurement of pH contaminates the sample with K+ and Cl− ions, thereby increasing the κ value obtained subsequently. For this reason, in a serial measurement of pH and κ on a single sample, the κ measurement must be performed first.
Nanoparticle suspensions of significantly different composition from NIST RMs 8011, 8012, and 8013 may require substitution of different standard pH buffers to account for pH values outside the range 6.86 to 9.19. If such a substitution is performed, the two buffers should bracket the value of the sample being measured.
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