Computer image of the spectrum of seawater. Image: Lisa Robbins

pHabulous pH

Post by Lisa Robbins from onboard the R/V Atlantis:

One of the instruments that we are running during the cruise is a portable spectrometer that measures discrete seawater samples for pH using 3 different wavelengths – 730, 434, and 578. This pH spectrometer system is very convenient because of its size and ease of set up.

Lisa Robbins unpacks and sets up the pH Spec in the laboratory on board the R/V Atlantis.
Lisa Robbins unpacks and sets up the pH Spec in the laboratory on board the R/V Atlantis.

The system has three main components: a light source (to left), a fiber optic cable leading to and from cuvette holder, and the spectrometer shown in separate picture.

pH Spectrometer with halogen light source, optical fiber, cuvette holder, and spectrometer (cant see in this photo because its in a little box for protection). Photo Credit: Lisa Robbins
pH Spectrometer with halogen light source, optical fiber, cuvette holder, and spectrometer (cant see in this photo because its in a little box for protection). Photo Credit: Lisa Robbins
pH Spectrometer that is used for measuring pH by the USGS Arctic Ocean Acidification team.
pH Spectrometer that is used for measuring pH by the USGS Arctic Ocean Acidification team.

After a quartz cuvette is filled with seawater, the cuvette is checked for bubbles and lint and finger prints that will cause inaccurate measurements.

Lisa Robbins wipes cuvette before running a water sample.
Lisa Robbins wipes cuvette before running a water sample.
Lisa Robbins carefully makes sure there are no bubbles in the cuvette.
Lisa Robbins carefully makes sure there are no bubbles in the cuvette.

Indicator dye is added and then the cuvette goes in the holder where light from the halogen lamp passes through it to the spectrometer.  We add 2 separate doses of mCP to a cuvette filled with seawater and measure the 3 wavelengths two times. Based on the wavelength characteristics, a pH can be calculated. We also use a seawater standard (Dickson Certified Reference Material (CRM)) daily to make sure our spec is giving correct numbers.

Lisa Robbins places the cuvette in the holder on the pH spectrometer before the sample is analyzed.
Lisa Robbins places the cuvette in the holder on the pH spectrometer before the sample is analyzed.
Computer image of the spectrum of seawater. Image: Lisa Robbins
Computer image of the spectrum of seawater. Image: Lisa Robbins

While you can see obvious changes in color of the seawater of the cuvettes which reflect pH differences – the spec can “see” and record subtle, very minute differences. These small differences in pH, are actually very important in understanding how the oceans vary over space and time.

The pH data, in conjunction with data on carbon parameters and oxygen and hydrogen isotopes (also see post “Measuring the Ocean – One Drop at a Time”), help us characterize the surface and subsurface waters for saturation state, carbon fluxes, and influences of different freshwater sources (like sea ice melt or glacial melt) on the chemistry.   Ultimately, these data will help us predict how the ocean and coastal environments will change over time.

Nearly five hundred pH measurements and several blistered fingers later, this is what the lab looks like below.

Lisa Robbins reacts to blister on her finger from running water samples.
Lisa Robbins reacts to blister on her finger from running water samples.

Share This: