Open Sensor Library - User contributions [en]

PH Sensor

2019-11-06T20:32:07Z

Kristi:

===== '''What is it, and why study it?''' =====
By measuring pH, you are checking the concentration of hydrogen ions (H+) in a liquid. An acidic solution has a high concentration of hydrogen ions (H+), greater than that of pure water. A basic solution has a low H+ concentration, less than that of pure water. The pH of an aqueous solution can be measured on a scale from 0-14. The lower the number, the more acidic the solution. The higher the number, the more basic the solution.

///<insert PH_Scale.png\> [tag Wikimedia Commons as source]

The pH scale is used to rank solutions in terms of acidity or basicity (alkalinity). Since the scale is based on pH values, it is logarithmic, meaning that a change of 1 pH unit corresponds to a ten- fold change in H+ ion concentration. The pH scale is often said to range from 0 to 14, and most solutions do fall within this range, although it’s possible to get a pH below 0 or above 14. Anything below 7.0 is acidic, and anything above 7.0 is alkaline, or basic.

///<insert PH_Scale-_Acidic_vs._Basic_(Alkaline).png\> [tag Wikimedia Commons as source]

For reference, most drinking water has a pH range of 6.5 to 9.5. Many animals cannot live in a pH level below 5 or above 9. Once the normal pH range for water has been established, a rise or fall in pH can indicate chemical pollution, acid rain, treatment plant issues, or problems with the supply pipe network. Low pH levels can precipitate a negative feedback loop in the environment, facilitating the solubility of heavy metals. As the level of hydrogen ions increases, metals such as aluminum, lead, copper, and cadmium are released into the water instead of being absorbed into the sediment.

While important in environmental systems health and water treatment, measurements of pH are also important in chemistry, soil management for crops, and medicine.

=== Scientific Uses1 ===
From waterways to soils to even our drinking water, scientists measure pH to monitor safe levels for human, plant, and animal health alike. It is important to establish a baseline for pH levels to determine if external factors are affecting the pH of an environment.

==== FRESHWATER ====
Measuring pH can help in monitoring freshwater lakes and rivers. As pH fluctuates out of its natural ranges, it can put stress on animals, leading to reduced hatching and survival rates. There are increased dangers when pH reaches extreme levels. These conditions can facilitate the solubility of toxic elements and compounds, thus increasing the risk of absorption by aquatic life.

Not only does pH serve as a metric to test for safe living conditions for organisms, but changing levels can also serve as an indicator of increased pollution, acid rain, or wastewater and mining discharges. Some states, such as Alaska, are attempting to maintain a pH standard for water quality. [https://dec.alaska.gov/water/water-quality/standards/ The Alaska Water Quality Standard] requires pH levels between 6.5 and 8.5 to protect the many salmon populations.

[[File:Ocean Acidification|thumb|Image: NOAA]]

===== Oceans2 =====

Over the past 300 million years, ocean pH has been slightly basic, averaging about 8.2 pH. For millions of years, the oceans maintained a relatively stable level of acidity, creating ideal conditions for complex webs of ocean life to flourish. The industrial revolution arose some 200 years ago, precipitating the release of billions of tons of carbon dioxide into the atmosphere. Since the ocean absorbs about a third of those human-generated CO2 emissions, the pH of the ocean has dropped 0.1 pH units, putting it currently around 8.1 pH, representing a 25-percent increase in acidity over the past two centuries. Projections based on these numbers show that by the end of this century, continued emissions could reduce ocean pH by another 0.5 units. Shell- forming animals including corals, oysters, shrimp, lobster, many planktonic organisms, and some fish species are in danger from these changes, in effect impacting the entire food chain and ecosystem.

<insert WOA05_GLODAP_del_pH_AYool.png\> [caption: Estimated change in sea water pH caused by human created C02 between the 1700s and the 1990s, from the Global Ocean Data Analysis Project (GLODAP) and the World Ocean Atlas [Creative Commons]]

=== Soil ===

[https://www.scientificamerican.com/article/precision-farming/ Precision agriculture] is a farm management strategy based on information technology which can help optimize soil and crop quality, production levels, sustainability, and food safety. The nutrition growth and yields of most crops decrease where pH is low and increase as pH rises to an optimum level.

Specifically, pH levels influence the solubility of nutrients while also affecting the ability of micro-organisms to break down organic matter and transform the chemistry of the soil. Additionally, most pesticides are labeled for specific soil conditions. If soils have a pH outside the allowed range, the pesticides may become ineffective, changed to an undesirable form, or may not degrade as expected, which results in problems for the next crop period.

[[File:How Soil pH affects availability of plant nutrients.jpg|thumb|pH and plant nutrients]]

=== Hydroponics3 ===

Managing pH levels in a soil-less hydroponic system are essential to maintaining plant health. In these systems, plants absorb nutrient solutions through their roots. In this scenario, the absorption capacity is affected by the pH of the solution. For example, acidic solutions facilitate the absorption of aluminum, hydrogen, and manganese which can be toxic to the plant. Conversely, at low pH, calcium and magnesium are less readily absorbed. This can lead to deficiencies of these nutrients. The same is true in alkaline environments, which increase the availability of molybdenum and macronutrients and decrease availability of phosphorus, iron, zinc, copper, and cobalt.

==== Acid Rain4 ====

[[File:450x375 acidrainpathway 3-28.png|thumb|Image: EPA]]

Acid rain, defined as precipitation with a pH level less than 5.0. is one of the best known examples of human influence on the pH of water. Mining and smelting operations produce nitrogen oxides and sulfur oxides. These are then transported by wind and air in the atmosphere and react to form sulfuric and nitric acids before combining with water and falling back to earth as wet depositions. High C02 levels associated with fossil fuel combustion also contribute to the acidity of rain.

Acidic particles and gases can also deposit from the atmosphere in the absence of moisture as dry deposition. The acidic particles and gases may deposit to water bodies, vegetation, and even buildings. When the accumulated acids are washed off a surface by the next rain, this acidic water flows over and through the ground, and can harm plants and wildlife.

[[File:Hydrogen ion concentration.png|thumb|Central Analytical Library]]

=== Drinking Water5 ===

You may have noticed Alkaline water sold in your local store. Some say it can help slow the
aging process, regulate your body’s pH level, and prevent chronic diseases like cancer. Water that
is naturally alkaline occurs when water passes over rocks and picks up minerals, which increase
its alkaline level. However, alkaline water available for purchase generally has achieved its status
through a chemical process called electrolysis. This technique uses a product called an ionizer to
raise the pH of regular water.

According to the [https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/expert-answers/alkaline-water/faq-20058029 Mayo Clinic], regular water is best for most people. They state that there is no
scientific evidence that fully verifies the claims made by supporters of alkaline water. However,
there are a few studies that suggest alkaline water might be helpful for certain conditions.

==== Social Uses ====
pH measurements can offer clear indications of the overall health of a water ecosystem. In its
specificity, pH can be used to introduce people to environmental monitoring and basic chemistry
principles. By establishing baselines and observing changes in pH levels, activists and educators
alike can link changes in ecosystems help to external influences.

=== Citizen science ===
Citizen science initiatives are increasingly using new devices, smartphone apps, and interactive
websites to improve data quality and collection procedures, better visualize trends and changes,
and increase volunteer engagement and retention.

Water quality projects such as, [http://testingwaters.org/dashboard.htm “Testing the Waters: A Paddle and Probe Adventure”], offer multistage citizen science testing and data collection occurring down the length of the Rock River in
Wisconsin. The event was successful in collecting 30,000 GPS-tagged water samples across 324
km, providing a unique profile of the river’s water (including pH), identifying potential water
quality issues, and establishing a baseline for comparison.

The [http://www.worldwatermonitoringday.org/ EarthEcho Water Challenge] (formerly World Water Monitoring Challenge) is an
international program that runs annually from March 22 (the United Nations World Water Day)
through December and equips anyone to protect the water resources we depend on every day.
The EarthEcho Water Challenge builds public awareness and involvement in protecting water
resources around the world, engaging local citizens to both test water quality (including pH) and
share their data online.

=== Activism ===
pH is a relatively straightforward metric that can be used by activists to monitor the changes in
water quality. Pollution can change a water's pH, which in turn can harm animals and plants
living in the water. For instance, water coming out of an abandoned coal mine can have a pH of 2
—an extreme level of acidity that would affect nearly any plant or animal living in its vicinity.

Point source pollution is a common cause that can increase or decrease pH depending on the
chemicals involved. These chemicals can come from agricultural runoff, wastewater discharge or
industrial runoff. Mining operations (particularly coal) produce [http://www.sosbluewaters.org/epa-what-is-acid-mine-drainage%5B1%5D.pdf acid runoff] and acidic
groundwater seepage, especially if the surrounding soil is poorly buffered. Wastewater discharge
that contains detergents and soap-based products can cause a water source to become too basic.

Communities that live near or downstream of toxic mining facilities must receive attention for
the potential risks they are disproportionately bearing. Often times, communities that are at the
highest risk of environmental contamination are marginalized and do not have a say among
decision makers. [https://link.springer.com/article/10.1007/s10668-018-0149-4 Environmental justice struggles] continue for people affected by acid mine
drainage and extreme pH levels.

=== Education ===
Through teaching students about pH and introducing testing techniques, educators can introduce
their students to both computational and laboratory skills. In measuring pH, students can connect
to the variability in everyday substances like orange juice, soda, or laundry detergent to
demonstrate the difference in pH levels.

Affordable options ranging from litmus paper to pH paper to affordable probes can offer students
hands-on laboratory experience where they can make and test their hypotheses. One common
resource—red cabbage—changes color in response to pH from red (pH = 2) through blue at
neutral pH to greenish-yellow at pH = 12. Testing pH in this way can give students hands-on
experimentation to connect them with experimentation principles.

== Technologies of Sensing pH ==

=== Potentiometric sensor6 ===
Potentiometric sensors are a type of electrochemical
sensor and are the most most commonly used sensors
in the scientific community. When the probe is
submersed in water, it measures the pH of the water
relative to the pH of a solution within the probe.
[[File:PH-electrode-anatomy-YSI.jpg|thumb|Image: https://www.ysi.com/parameters/ph]]
The pH electrode has a glass bulb filled with a pH 7
solution. The difference between the pH in the water
and the pH 7 solution within the glass bulb generates
an electrical potential, which is measured relative to a
stable electrical potential generated by a reference
electrode located within the pH probe. The pH of the
solution is then determined by the difference between
these two electrical potentials.

==== Epoxy vs Glass Body ====

Epoxy-body electrodes are impact resistant and ideal for rough handling, but should not
be used at higher temperatures. Glass-body electrodes withstand high temperatures and
highly corrosive materials or solvents.

=== Ion-Sensitive Field-Effect Transistors (ISFETs) ===
This technology measures ion concentrations in a sample solution that sit in direct contact with a
field-effect transistor (FET). The FET gate-electrode material is typically covered by an ionsensitive membrane, and in this case, sensitive to hydrogen ions for pH measurements.

Key advantages of ISFETs are their small size which allows them to be used with small volumes,
low cost, good stability, and their ability to work in wide temperature ranges. Solid-state ISFET
electrodes with a non-glass measuring surface won’t break and wipe clean for dry storage. This
is a good choice for use in the food industry to avoid any chance of glass breaking and
contaminating samples. Another benefit of ISFET electrodes is their ability to measure smaller
volumes of samples, in most cases as little as 30 microliters.

The key disadvantages of ISFETs include [https://www.researchgate.net/figure/The-stability-of-ISFET-in-drift-test-Like-the-time-drift-effect-the-hysteresis-also_fig5_221906746 long-term drift], [https://www.merriam-webster.com/dictionary/hysteresis hysteresis], and a relatively short lifespan.

=== Colorimeters & Spectrophotometers7 ===
Both of these technologies measure a sample’s absorbance to determine concentrations of pH,
often placing a pH-sensitive indicator dye to a water sample. One benefit of these technologies is
their capacity to measure pH using a very small sample size. This technology, while very
accurate, has a limited range of pH it can measure.

==== pH paper ====

The use of pH paper is a simple and cheap method of measuring solutions in an educational
setting. Particular colors indicate certain values, and each measurement is only accurate within a
unit or two. While pH paper is great for quick qualitative work, it fails at highly accurate
quantitative work. If the accuracy you desire is within one pH value or two, pH paper is the way
to go. An even less specific pH test is with [https://www.thoughtco.com/what-is-litmus-paper-3976018 Litmus paper], which can give you a quick insight to
see if your solution is acidic, neutral or basic.

== Best practices for Deployment & Calibration8 ==

Any style of electrochemical sensor requires calibration to provide accurate measurements. The
probe is immersed in a solution that has a known pH, such as pure water with a neutral pH of
7.0. When performing a pH calibration it is recommended to calibrate to at least two points,
considering the pH range of the samples being measured. pH calibration is most effective when
calibration points bracket the sample pH. For example, one solution standard with pH value
slightly lower than the minimum expected measurement and another with pH value slightly
higher than the maximum expected measurement.

Ensure the glass bulb of the pH electrode is hydrated before performing a calibration. Calibrating
a dry pH electrode will result in the need for frequent calibration. Use an adequate volume of pH
buffer to fully submerge the pH bulb and the reference junction. Open or loosen the fill hole on
the electrode to ensure the flow or reference electrolyte into the sample. This is crucial for fast
electrode response and stable electrode readings. During pH calibration and measurement, the
solution should always be stirring.

Offset and slope have a significant impact on the condition of a pH meter’s electrode. The offset
is the mV reading of the electrode when it’s submerged in pH 7 buffer. The slope of the electrode
is calculated by determining the mV change between two different pH buffers. By dividing the
mV difference by the change in pH units, users can get the actual slope of the electrode.

pH calibration and measurements are only as good as the condition of pH electrode. pH
electrodes should never be stored in distilled or deionized water, as this can dehydrate the
sensing glass of the pH electrode and deplete reference electrolyte. Instead, storing the electrode
in storage solution will provide stable and highly responsive readings due to a properly hydrated and responsive bulb.

== The Importance of pH Sensor Calibration9 ==

'''(***This through the sections on temperature are examples provided by Sensorex for their
sensor. It could be an asset to create to reflect Field Kit’s particular sensor.)''' A combination
pH sensor outputs a mV signal, which corresponds to a pH value. An output of 0mV is equal to
pH 7. Moving one pH unit along the scale in any direction will correspond to a change in voltage
unique to each particular sensor. This is called the “slope” of the sensor. These are theoretical
values though, and in reality, there is no such thing as a sensor with perfect output. This is where
pH sensor calibration comes in.

[[File:Acids bases.png|thumb|acids and bases]]

== Sources of pH Sensor Variability ==

There are a few main characteristics to consider related to calibration:

• Offset: Output at pH 7 will be slightly above or below 0mV, within a certain tolerance.

• Slope: May not be exact.

• Span: The slope may not be constant across the entire pH scale. For example, the sensor
may be close to theoretical output at pH 7 but have a larger deviation at the extreme ends
of the pH scale.

The important thing to note is that these variances do not mean the sensor is faulty.

== Adjusting for Temperature10 ==

Temperature must also be considered. See the figure below for the effects of temperature on pH
readings. It is important to calibrate the sensor in solution close to 25°C, at the same temperature
as the sample or process, or to use a system with automatic temperature compensation.

[[File:Ph vs temperature error chart.png|thumb|ph vs temperature]]

Note: Values in light blue are less than 0.1 error and may not require temperature compensation.
Values in dark blue are temperature and pH in which there is no error in pH from temperature.

'''== SUPPLIERS =='''

'underline'Atlas Scientific Lab Grade Probe'underline'
Range: 0-14
Accuracy: +/– 0.002
Resolution: +/- 0.0001
Temperature Range: 1-99 degrees Celsius

'underline'Atlas Consumer Grade Probe'underline'
Range: 0-14
Accuracy: +/– 0.002
Resolution: +/- 0.0001
Temperature Range: 1-60 degrees Celsius

'underline'Libelium: Wasp Mote Plug & Sense'underline'
0-14 pH
Accuracy +/– 0.01

'underline'YSI 6561 pH Sensor'underline'
Range: 0-14
Accuracy: +/–: 0.2
Resolution: 0.01

'underline'YSI pHotoFlex'underline'
The pHotoFlex pH colorimeter allows for the connection of an external pH sensor.

'underline'YSI MultiLab pH and Temperature Sensor'underline'
Range: 0-14
Accuracy: ± 0.004
Vernier Go Direct Glass Body Sensor
Range: 0-14
Accuracy: ±0.05

'underline'Horiba LAQUAtwin pH-33'underline'
Range 0-14
Accuracy ±0.01 pH

'underline'Oakton EcoTestr™ pH 2+ Pocket pH Meter'underline'
Range 0-14
Accuracy ±0.1 pH
Resolution 0.1 pH

'underline'SensorEx pH3000 Research-Grade pH Sensor'underline'
Range 0-14
Accuracy ±0.04 pH

'underline'Edaphic Portable pH Soil Meter'underline'
Range: (-2)- 16
Accuracy ±0.02 pH
Resolution 0.01 pH

'underline'Oakton pH 150 Waterproof Portable Meter with Single-Junction All-in-One Electrode'underline'
Range (-2)-16
Accuracy ±0.01 pH
Resolution 0.01 pH

== Field Kit (What we’re doing) ==
===== TBD =====

''Potential assets: Rather than relying on this calibration table and breakdown specific to Sensorex, it
would be useful to have visual aids to assist in the calibration of the Field Kit specific sensor.''

1 https://www.fondriest.com/environmental-measurements/parameters/water-quality/ph/
https://www.usgs.gov/special-topic/water-science-school/science/ph-and-water?qt-science_center_objects=0#qt-science_center_objects

2 https://www.noaa.gov/education/resource-collections/ocean-coasts-education-resources/ocean-acidification
https://www.sciencedirect.com/science/article/abs/pii/S0165993612002427
https://www.nationalgeographic.com.au/nature/ocean-acidification.aspx

3 https://sensorex.com/hydroponics/

4 https://www.epa.gov/acidrain/what-acid-rain

5 https://www.healthline.com/health/food-nutrition/alkaline-water-benefits-risks#risk-factors

6 https://www.coleparmer.com/tech-article/ph-electrode-selection-guide
https://www.sciencedirect.com/topics/chemistry/glass-electrode

7 https://oceanoptics.com/wp-content/uploads/Introduction-to-pH-and-Optical-pH-Sensing.pdf
https://www.io-warnemuende.de/spectrophotometric-ph-measurements.html

8 https://www.rscal.com/6-important-tips-calibrating-ph-meter/
https://www.rdmag.com/article/2015/02/six-tips-%C2%A0use-when-calibrating-your-ph-meter

9 https://sensorex.com/blog/2016/05/09/ph-sensor-calibration/

10 https://sensorex.com/blog/2016/05/09/ph-sensor-calibration/

File:Ph vs temperature error chart.png

2019-11-06T20:23:40Z

Kristi:

ph vs temperature chart

File:Acids bases.png

2019-11-06T20:20:48Z

Kristi:

acids and bases scale

File:PH-electrode-anatomy-YSI.jpg

2019-11-06T20:10:44Z

Kristi:

structure of a typical combination pH electrode with glass body

File:Hydrogen ion concentration.png

2019-11-06T19:57:19Z

Kristi:

Hydrogen ion concentration as pH from measurements made at the Central Analytical Laboratory, 2017

PH Sensor

2019-10-31T20:01:28Z

Kristi: Created page with "===== '''What is it, and why study it?''' ===== By measuring pH, you are checking the concentration of hydrogen ions (H+) in a liquid. An acidic solution has a high concentrat..."

File:450x375 acidrainpathway 3-28.png

2019-10-31T19:59:33Z

Kristi:

pathway of acid rain

File:How Soil pH affects availability of plant nutrients.jpg

2019-10-31T19:55:20Z

Kristi:

pH and availability of plant nutrients

File:Pmel-oa-imageee landscape.jpg

2019-10-31T19:12:49Z

Kristi:

describes ocean acidification