Performance Verification Statement for the Idronaut 305 Plus CTD pH Sensor

Ref. No. [UMCES] CBL 2015-010

ACT VS15-03

followed by depth profiling (50%), then hand-held portable use (48%), then flow-through

systems (26%). Respondents used a variety of calibration procedures including commercial

buffers (68%), CO

chemistry (35%), seawater CRMs (23%), pH indicator dyes (18%), and

supplied by manufacturer (13%). The four areas where respondents expressed the greatest

concern over the use of in situ pH sensors were ruggedness (49%), calibration life (46%), level

of measurement uncertainty (43%), and reliability (41%). The complete needs and use

assessment reports can be found at:

INSTRUMENT TECHNOLOGY TESTED

The Idronaut Ocean Seven 305 Plus CTD pH sensor is comprised of a potentiometric

sensor that has a voltage output. A potentiometric sensor consists of two electrochemical cells or

electrodes: the measuring sensor and the Ag/AgCl reference sensor. The active part of a pH

measuring electrode is the sensor glass membrane. The glass membrane can be manufactured in

different shapes, depending on the application and is constructed from a special composition

glass which senses the hydrogen ion concentration. This glass is mostly amorphous silicon

dioxide (SiO2), with embedded oxides of alkali metal, mainly Na. It is made to be as thin as

possible, about 0.1 mm thick. To keep its electrical impedance as low as possible, the pH

measuring electrode, which is sensitive to the hydrogen ion, develops a potential (voltage)

directly related to the hydrogen ion concentration of the solution. The reference electrode

provides a stable potential against which the measuring electrode is referred. When immersed in

the solution, the reference electrode potential does not change with the changing hydrogen ion

concentration. A solution in the reference electrode makes the electrical contact with the sample

solution and the measuring electrode through a junction, completing the circuit. Output of the

pH measuring electrode changes with temperature (even though the process remains at a constant

pH), according to the Nerst equation, so a temperature sensor is necessary to correct it. The

measurement of pH in seawater demands high accuracy since seawater has a high ionic strength

and is weakly buffered. The pH range in the oceans is particularly restricted and, only in very

special cases, the observed values are outside the range of 7.8 and 8.4 pH and, in some seas, the

range extends from 6.5 to 9.0 pH. Some problems have always arisen from the use of traditional

reference sensors with porous diaphragms, when measuring the pH in seawater in particular at

pressures in excess of a few bars, due the high and variable junction potentials that are generated.

The Idronaut pH sensor has a glass body and a very low-impedance “blue” pH sensitive glass tip,

which can withstand pressures up to 700 bar. The sensor head is made of titanium. The

IDRONAUT reference sensor is in contact with the sample by means of a small hole in the glass

tip. This minimizes and stabilizes the junction potential between the inner gel electrolyte and the

liquid to be measured. The reference sensor is an Ag/AgCl cell in a saturated KCL solid gel.

The Idronaut Ocean Seven 305 Plus CTD pH sensor was calibrated by ACT staff using

commercial NBS buffers prior to the laboratory study and prior to each moored field

deployment. A two point calibration was done using pH buffers 7 and 10 from Fisher Scientific

following the standard operating procedures provided by the company in training videos.