pH Measurement of Pure Water using 9600-10D and 9615S-10D pH Electrodes

The 9600-10D PUREIL pH electrode equipped with gelled ionic liquid salt bridge provided stable pH reading of pure water in 5 minutes without affecting its conductivity value. It showed faster response and more accurate reading than 9615S-10D ToupH Standard pH electrode equipped with KCl salt bridge.

Introduction

Measuring the pH of pure water and other low conductivity water is difficult due to their lack of buffering capacity, susceptibility to trace contaminants from sample containers, and exposure to pH electrode’s KCl filling solution during measurement. pH electrodes are typically designed to operate optimally in solutions with certain ionic strength and buffering capacity. In pure water, the lack of ions and buffering capacity can affect the sensitivity of the pH electrode leading to slow response or unstable pH readings. The pH electrode may also exhibit drift resulting in inaccurate and inconsistent pH measurements. Excessive KCl introduction from pH electrode into such samples will increase solution conductivity and alter solution pH.

HORIBA recommends 9600-10D PUREIL pH electrode to eliminate the issues encountered with low conductivity water and ensure accurate and reliable results. It is ideal not only for low conductivity water (< 100 μS/cm) such as distilled water, deionized water, rainwater, reverse osmosis water, and boiler water, but also for solutions that react with KCl. Unlike conventional pH electrodes with KCl salt bridge, the 9600-10D PUREIL pH electrode keeps KCl from leaking into the sample which results in better accuracy, reproducibility, and stability in pH measurement of low conductivity samples.

HORIBA recommends 9600-10D PUREIL pH electrode to eliminate the issues encountered with low conductivity water and ensure accurate and reliable results.

It is ideal not only for low conductivity water (< 100 μS/cm) such as distilled water, deionized water, rainwater, reverse osmosis water, and boiler water, but also for solutions that react with KCl. Unlike conventional pH electrodes with KCl salt bridge, the 9600-10D PUREIL pH electrode keeps KCl from leaking into the sample which results in better accuracy, reproducibility, and stability in pH measurement of low conductivity samples.

Method

Two pH electrodes with different reference electrodes were used to measure the pH of pure water – the 9615S-10D ToupH Standard pH electrode and 9600-10D PUREIL pH electrode.

The 9615S-10D ToupH Standard pH electrode is a refillable pH electrode constructed with responsive glass that is 10X stronger than JIS standards and KCl salt bridge whereas the 9600-10D PUREIL pH electrode is a non-refillable pH electrode constructed with gelled ionic liquid salt bridge as liquid junction. The 3551-10D conductivity electrode (k=0.1) was also used to
determine the conductivity of pure water and monitor the effect of KCl leaking into the sample to the pH value.

Calibration

Prior to measurement of pure water, the 9615S-10D and 9600-10D pH electrodes were calibrated using pH 4.01 and 6.86 buffers and the cell constant of the 3551-10D conductivity electrode was set into the meter.

Measurement

Two separate measurements were carried out:

To measure pH, both 9615S-10D and 9600-10D pH electrodes were immersed in pure water for 10 minutes. Aside from pH, the conductivity of the sample was also measured using 3551-10D conductivity electrode to confirm the effect of KCl leaking into the sample from the 9615S-10D pH electrode. See results in Figure 1.

Figure 1. Data of pH and conductivity measurements in pure water using 9600-10D PUREIL pH electrode, 9615S-10D ToupH Standard pH electrode, and 3551-10D conductivity electrode.

For comparison, only the 9600-10D pH electrode and 3551-10D conductivity electrode were immersed in a pure water sample
for 10 minutes. See results in Figure 2.

Figure 2. Data of pH and conductivity measurements in pure water using 9600-10D PUREIL pH electrode and 3551-10D conductivity electrode.

Electrode Specifications

Model	9600-10D PUREIL Electrode	9615S-10D ToupH Standard Electrode
pH Measurement Range	0 – 14
Operating Temperature Range	0 – 40 °C	0 – 100 °C
Storage Temperature Range	0 – 50 °C
Internal Electrode	Ag/AgCl
Thermistor	Yes
Liquid Junction	PVDF-HFP, ionic liquid	Porous ceramic
Filling Solution	–	3.33M KCl
Wetted Materials	Glass, PBT, PVDF-HFP, ionic liquid	Glass, ceramic
Electrode Dimensions	210 mm (L) x 12 mm (Ø)	151 mm (L) x 12 mm (Ø)
Cable	1 m

Revision 0, 04 July 2025