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Chemistry

DIY Science: Scientists create a LED photometer that can measure the pH of seawater with an accuracy of 0.01, for under $50

Bo Yang, Mark C. Patsavas, Robert H. Byrne, Jian Ma, Seawater pH measurements in the field: A DIY photometer with 0.01unit pH accuracy, Marine Chemistry, Volume 160, 20 March 2014, Pages 75-81, ISSN 0304-4203, http://dx.doi.org/10.1016/j.marchem.2014.01.005.

In elementary school you may have measured pH with a strip, but that only works for freshwater. You can’t measure the pH of seawater with strips because the salt interferes with the indicator. You can measure the pH of natural waters with a salinity greater than ~5 (water found in estuaries, mangrove swamps, and some lakes) using a standard hydrogen/silver-silver chloride electrode after you calibrate the electrode using seawater buffer solutions. The cost for simple hand held devices range from $100 to $200. However, the accuracy and precision for these devices is limited.

As ocean and estuarine acidification has gained attention from scientists and policy makers over the last several years, the need to develop cost-effective and accurate methods to monitor the chemistry of our coasts and oceans has become increasingly important. The X-PRIZE Foundation is offering a $1 million reward to inventors who can create an instrument that is cost-effective and can accurately measure pH, an indicator of ocean acidity. This prize is for good reason: the cost for a pH sensor that can be used in the field for scientific research ranges from $8,500 to $6,000. Bench top pH meters that are used in the lab range from $200 to $1000.

As the cost of hardware has dropped and the open source movement has gained steam there has been a growth in DIY projects using open-source hardware and software. It appears that the open-source movement has already grabbed the interests of scientists in materials science. This application of hardware would be particularly useful for labs or environmental monitoring organizations with limited funding.

Yang and colleagues from the University of South Florida devised a portable LED photometer that produces pH measurements within .01 units of state-of-the-art spectrophotometric measurements (the state-of-the-art model used in this study costs about $6,000) and has a precision of +/- .002. All of the parts used to build the instrument can be found online and their total cost amounts to ~$50, including shipping. To test the validity of the measurements, Yang compared the measurements of the LED photometer and bench top spectrophotometer of laboratory and field samples.

How does this method work to measure pH?

  • Build the LED photometer (steps are outlined in open-source publication)
  • Use the device to determine “how much” light is absorbed in your sample compared to a reference.
  • Use the values from the  measurement to calculate pH

By using a spectrophotometric method, you determine the pH of your seawater sample by measuring the intensity of a light beam through your sample after you add an indicator dye. You use the wavelength to define the intensity of light. This approach works because each form of the indicator (dye) absorbs energy (light) at a distinct wavelength, and the variation of it called the absorption spectrum. Your goal is to determine the wavelength at which absorbance is the greatest, known as the wavelength of maximum absorption.

In this study the scientists used a sulfonephthalein indicator called meta-cresol purple. In the equation below, the indicator is represented by “I” and hydrogen ion concentration (which is used to determine pH) is shown as H+.

HI(aq) = H + (aq) + I2-

When you mix the indicator and your sample, the hydrogen ions in your sample will combine with the indicator to form HI. First you measure the absorbance of I2-, your indicator. Then you add a known volume of the dye to your seawater sample and measure its absorbance. Using the ratio of the indicator and seawater measurements, you get “Rb”. “Rb” can be used in an equation to determine pH.

Rb = (wavelength of maximum absorbance of the  indicator x absorbance of the  indicator)/(wavelength of maximum absorbance of the sample mixed with the indicator x absorbance of the sample mixed with the indicator).

The values you get from the LED photometer are the values used to calculate Rb. If you are doing measurements for the first time, you must do a calibration. According to Yang, it only has to be done once. Measure the salinity and temperature of your samples (in this study Yang used a digital thermometer and salinity pen). You make a linear fit of your Rb values, this is the calibration curve. Afterwards you correct your Rb values from your calibration curve.

Figure 1: Left- This diagram shows the maximum wavelengths of the LED lights compared to the absorbance of the acidic and basic forms of indicator (meta-cresol purple) from parts used in this study. The maximum of the peaks should line up. Right- The calibration curve obtained from the LED photometer. Rb is the value obtained from the device, the Rn is obtained by a correction used in Lui et al., 2011 (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3103190/#eq18).

Figure 1: Left- This diagram shows the maximum wavelengths of the LED lights compared to the absorbance of the acidic and basic forms of indicator (meta-cresol purple) from parts used in this study. The maximum of the peaks should line up. Right- The calibration curve obtained from the LED photometer. Rb is the value obtained from the device, the Rn is obtained by a correction used in Lui et al., 2011 (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3103190/#eq18).

Use your Rn, salinity, and temperature value into a simple equation shown in Yang’s paper to determine your pH.

Figure 2: The setup of the LED photometer. The rig is 90x90x100 mm, small enough to fit in a shoe box.

Figure 2: The setup of the LED photometer. The rig is 90x90x100 millimeters, small enough to fit in a shoe box.

Yang and his colleagues provide the source code to show the values of wavelength and absorbance on the LED monitor of the device in his supplementary materials section (see below). The Arduino language is based on C/C++.

Results: Accuracy of the Method

The accuracy of the pH measurements will depend on 1) the quality of the buffers used to make the calibration curve, 2) quality and age of the indicator dye, 3) the quality of the hardware, and 4) the accuracy of your temperature and salinity measurements. Most of these requirements are standard when measuring pH with other devices such as an electrode. In this study, there is the added requirement that the LED lights used in the photometer must emit light at an intensity that the sample can absorb. Yang and colleagues determined the instrument’s accuracy by comparing its pH measurements to those made by the Agilent 8453, a narrowband bench top spectrophotometer (shown as pH t(N)).

Figure 3: Left-The difference between the pH of the LED photometer and Agilent spectrophotometer is on the y-axis. On the x-axis is 2) the pH value of the spectrophotometer, b) salinity, c) temperature. Right-The depth profile of seawater pH measured by both devices during a research cruise, in northeaster Gulf of Mexico.

Figure 3: Left-The difference between the pH of the LED photometer and Agilent spectrophotometer is on the y-axis. On the x-axis is 2) the pH value of the spectrophotometer, b) salinity, c) temperature. Right-The depth profile of seawater pH measured by both devices during a research cruise, in northeaster Gulf of Mexico.

The samples used to make the measurements in Figure 3 on the left side were from two locations in the Gulf of Mexico. The original salinity of the samples was between 36.1 and 36.2 parts per thousand. To test the device at different pH and salinity, the samples were diluted with deionized water. To evaluate the accuracy of the device at different temperatures, the samples were either warmed or cooled with a water bath. Figure 3 on the right  side shows pH measurements over depth (0 to 1,400 meters) taken during a cruise in the Gulf of Mexico. It was important to Yang and colleagues that the device be easy to use on a ship.

The pH in a reef aquarium tank can increase due to photosynthesis or decrease due to respiration. Figure 4 shows the constancy of pH measurements over a 16 hour period. Note that the top two lines are pH measurements made by probes, while the bottom two are from the newly developed LED photometer and the state-of-the art spectrophotometer.

Figure 4: How pH changes over time in an aquarium tank, using different instruments. The probes were calibrated with different buffer called “NBS” and are on a different pH scale than the measurements made by the LED photometer and Agilent 8453. The offset of the two measurements may be partly due to the differences in the pH scales.

Figure 4: How pH changes over time in an aquarium tank, using different instruments. The probes were calibrated with different buffer called “NBS” and are on a different pH scale than the measurements made by the LED photometer and Agilent 8453. The offset of the two measurements may be partly due to the differences in the pH scales.

 

Based on a single calibration, the D.I.Y LED photometer provided pHT measurements within 0.01 units of spectrophotometric measurements (7.6 ≤ pH ≤ 8.2, 30 ≤ S ≤ 36.2, and 15 °C ≤ t ≤ 30 °C) and has a pH precision of ± 0.002. At this salinity range, the D.I.Y LED photometer can be used to make pH measurements in the open ocean. pH measurements in the open ocean is important as it helps understand how humans are changing the chemistry of our world’s oceans from our CO2 emissions. pH is considered a “master” variable because it can affect how other chemical reactions in our ocean proceed and can help us understand what reactions have taken place in the recent past. This is because marine scientists use three interconnected values (pH, total dissolved inorganic carbon, and total alkalinity) to characterize a water parcel, much like a fingerprint. For instance, water from the deep ocean from the Pacific Ocean tends to have a pH, dissolved inorganic carbon, and alkalinity concentration that is different from surface water found in Narragansett Bay, Rhode Island.

Significance

While the achievement of creating an LED photometer with this kind of accuracy at such a low price is tremendous, it is important to keep in mind that accuracy may be affected by extreme changes in temperature and salinity that could be encountered in the field. The deep ocean can be cold, around 0 to 3 °C. However, Figure 3 shows that the measurement is accurate at deep depths. More work should be done to develop a similar photometer that can be used in environments that have a large salinity range, such as an estuary. As the open-source movement continues to grow, it will be interesting to see more innovative technologies developed for use in oceanography.

Where do I start?

I would say that the rig would amount to ~$100  if you are starting from square one, which is around the price of a simple electrode. The benefit of this device that Yang et al. claims is the device only has to be calibrated once and is more precise and accurate than simple electrodes. In my opinion, I would perform a calibration once in a while because I am not sure if the accuracy of the measurements might change with the device’s age.

  1. Download the source code and make and model of the parts can be found in the supplementary material
  2. Check out Arduino’s set-up guide  and copy the functions that are called in the Yang’s source code, such as liquidcrystal. If you become stuck, there are many online communities where you can post questions and people will readily answer (e.x. forums).
  3. Get the tools and parts you need. I was able to find nearly all of the parts in this website. I believe one resistor was discontinued but you may find an alternative or get it from another website. After adding all of the items in the cart the total came shy of $50, after including an estimate of shipping. However, the cost does not include a device to measure temperature and salinity. Additionally, you need a 100 mL glass bottle, the indicator, and buffers. You will also need basic things for your Arduino such as a Wall Adapter Power Supply (9V DC 650mA). You can buy starter kits with instructions and parts on Amazon for ~$100. Keep in mind that you can get the same information from Arduino’s website…for free.

 

Cathleen Turner
Cat Turner is a Masters Candidate at the University of Rhode Island. Her research topic is on pH and dissolved inorganic carbon (DIC) fluctuations of Narragansett Bay, R.I. In her spare time she draws cartoons, reads horror stories, and collects wine corks. She likes to sail in fair weather.

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