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Nitrate (NO3) comprised of Nitrogen and Oxygen, is a naturally occurring compound found in soils and is essential to plants and other living organisms as a source of sustenance. Although naturally occurring, high concentrations of nitrate in drinking water can be attributed to sewage treatment, septic systems, industrial waste, atmospheric pollutants, and fertilizers from farmland runoff. Because of its high solubility and inability to bind to soils, nitrates can infiltrate ground water and surface water as well. Bacteria using dissolved oxygen to oxidize ammonium can also contribute to nitrate concentrations in bodies of water.
The concentration of nitrate is a closely monitored parameter in drinking water and is a concern to public water systems nationwide because of its serious health risks in high quantities. High concentrations of nitrate in drinking water can cause a condition called methemoglobinemia (also referred to as "blue baby" syndrome) in infants where the blood's ability to carry oxygen is reduced causing the skin to take on a blue color and in severe cases death if not treated promptly. Pregnant women are also susceptible to having babies with birth defects due to the lower oxygen intake. Although no conclusive evidence of health risks have been found in adults, researchers suspect that high nitrate intake can increase the risk of diabetes, thyroid disease, and certain kinds of cancers. As a result of these risks, the Environmental Protection Agency has set a maximum contaminant goal level for nitrate at 10 ppm (parts per million).
Method of Detection
In the past, the method of measuring nitrates in water required the use of time consuming procedures where samples of water had to be collected, filtered, and have chemicals added to cause the samples to change colors with color intensity determined by the concentration of nitrates in the sample. This method was often laborious since every sample had to be individually tested and this was usually done by hand.
The In-Situ Ultraviolet Spectrophotometer (ISUS) was developed by Dr. Kenneth Johnson and Mr. Luke Coletti of the Monterey Bay Aquarium Research Institute as an alternative means of measuring concentrations of Nitrate in ocean bodies. The standard spectrophotometer is comprised of two major components: the spectrometer which produces any selected color of light and the photometer which measures the intensity of light. The amount of a certain wavelength of light that is absorbed by an aqueous solution and the amount of light absorbed depends on both the material and its quantity in the solution. Spectrophotometers work by placing a sample of solution between the spectrometer and photometer, a beam of light is then passed through the sample to the photometer on the other side of the sample, and the resulting intensity is captured by the photometer which in turn sends a voltage signal to a galvanometer which measures current.
Signal changes depend on the intensities measured by the photometer. The photometer contains a photodetector which is responsible for converting the measured light into a useable electrical signal which is made possible because of its photovoltaic/ photoresistive properties. The photodetector in the ISUS utilizes a photodiode which possesses the capability of changing radiant light energy into a current or voltage. Photodiodes are the opposite of light emitting diodes in that instead of radiating light they detect it and in turn create an electrical current.
The amount of absorption can be calculated from the ratio of recorded light intensities and written as:
Beer's law states that the relationship between absorbance of a particular wavelength, the travel path length, and the concentration of a solution are proportional.
The ISUS works on the same principle that concentrations of nitrate dissolved in water can absorb ultraviolet light at specific wavelengths, and in larger concentrations even more light. The ISUS unit is encased in an enclosure that is waterproof to protect the instrument from corrosion and the effects of pressure. The optical probe is a small cylinder with a notch to let water flow through the instrument so that it can be measured by the spectrophotometer. Nitrates in the water will absorb some of the ultraviolet light and a mirror at the end of the optical probe shoots the passing light back into the enclosure through the optical fibers and into the spectrometer which breaks the light into separate wavelengths. A computer inside determines how much nitrate is in the water based on the measured wavelengths.
ISUS Instrument (Source: http://www.mbari.org/twenty/isus.htm)
Potential Hydrogen or pH is a measure of the concentration of Hydrogen ions [H+] in a solution. The amount of Hydrogen ions in a solution determines how basic or acidic it is. The pH of a solution is measured on a logarithmic scale of 0 to 14, with 7 being considered the neutral point, values measured below 7 are considered to be acidic and values measured above 7 are considered to be basic. The pH of a body of water can be affected by a number of natural and man-made causes. The type of underlying rock (limestone bedrock), the amount of water usage and waste water discharge tend to increase the pH of water.
While not life threatening to humans, the EPA considers pH a part of its secondary drinking water regulations which place more of an emphasis on the aesthetic aspects of water such as taste, color and odor. As the pH increases, water begins to taste chalkier and it becomes difficult to lather soap (hard water), and a low pH causes the water to take on a bitter metallic taste. In addition to the aesthetic concerns, an imbalance in pH can cause problems for distribution systems which can lead to other complications. A high pH can cause deposits to collect along the pipe walls which can limit its effectiveness in delivering water and limit its service life. A low pH can eat away at metal piping causing it to corrode and leech metal into the water which can lead to health concerns. The efficiency of chlorine as a disinfectant is also diminished when the pH exceeds a certain range resulting in an increased potential for microbial organisms to thrive in drinking water. For these reasons, the EPA recommends a pH value of 6.5 to 8.5.
Method of Detection
One of the most common and accurate ways of measuring pH is through the use of a pH meter. The pH meter consists primarily of three main components: a measuring electrode, a reference electrode, and the pH meter. The combination electrode, which contains a glass electrode (measuring electrode) and a reference electrode in a single enclosure, is a widely used configuration of the pH meter.
The glass electrode is constructed using a special type of glass membrane that is highly sensitive to hydrogen ions and possesses a low electrical resistance. Within the glass electrode is a buffer solution with a constant pH and a silver wire that attaches to the electrode's connector. The buffer solution is usually a known concentration of Potassium Chloride (KCl) maintained at a pH of 7. When the glass electrode is placed into a test solution, an electrical potential is developed on both surfaces of the glass. The presence of hydrogen ions in the test solution produces an electrical potential of approximately 59.2mV/pH at 25Â°C. The KCl solution acts as a medium or connection for the voltage to travel from the glass membrane to the silver wire. The silver wire, coated in silver chloride (AgCl), carries the voltage from the solution to the electrode's connector cable which then enters the pH meter.
A reference electrode is used in conjunction with the measuring electrode for the purpose of facilitating the measurement of the difference in electric potential generated between it and the measuring electrode. The measuring electrode is a half cell and as a result measuring the potential across it is not possible without the inclusion of another half cell to complete a galvanic cell which the reference electrode provides. Similar to the glass electrode, the reference electrode contains a silver wire coated in AgCl and submerged in a KCl solution. However unlike the glass electrode the reference electrode does not use a specialized glass membrane but rather an inert one. The reference electrode requires a junction or a porous frit which allows the internal KCl solution to make contact with the sample solution. This contact between the two solutions is necessary in completing the cell. Several variations of junctions have been developed to facilitate this contact. Junctions are broken up and classified into 4 categories:
Pinhole- Simplest of all the junctions, provides an opening of only a few microns and has loss of buffer solution.
Sleeve- Easier to perform maintenance cleaning on but a higher loss of buffer solution is expected.
Ceramic- Limited loss of buffer solution however there are problems with adhering to the sample solution.
Double Junction- A combination of the pinhole and ceramic junction that attempts to make use of the advantages from both.
The pinhole and sleeve type junctions are open junctions because there are no obstructions between the reference solution and sample solution. The direct contact between the two solutions allows for faster and more accurate readings to be taken. However because there is an opening, the risk of contaminating the reference solution due to the sample seeping into the electrode is much greater. In addition to possible contamination of the reference electrode, the loss of the buffer solution is much greater, prompting frequent refills of the solution. The ceramic type junction uses a porous ceramic diaphragm which has the advantage of being fabricated to produce a precise flow rate. The ceramic junction is ideal for liquids and is versatile enough to measure rapid fluctuations in pH but requires frequent cleaning because precipitates tend to form on the diaphragm causing it to clog.
The pH meter is essentially a high impedance voltmeter that is able to measure the small changes in voltage received through the electrodes. Although voltmeters are commercially available most do not possess the sensitivity required to measure the minute changes in voltage from the electrodes. A pH voltmeter is able to overcome this by incorporating in its design an increase in internal resistance resulting in an amplification of voltage so that it becomes a readily useable value.
The value of pH can be calculated using:
In a pH meter, the total electric potential is determined by calculating the difference between electric potential created by the reference electrode and the measuring electrode. The reference electrode develops a constant electric potential while the electric potential developed in the measuring electrode is dependent on the concentration of hydrogen ions in a sample solution.
The Nernst equation is the governing equation for representing the electrical potential developed in a pH meter.
The ln(aC) term in the equation can be rewritten in terms of log10 by multiplying 2.303, the conversion factor for natural log to log which becomes . Representing in terms of pH can be done by expressing it in terms of hydrogen ions or the pH formula which results in:
Rearranging the equation to solve for pH the final format becomes:
This equation however is specific to a temperature of 25Â°C. Compensation for the effects of temperature on pH can be applied by increasing the value by 0.003 pH/pH Unit/Â°C assuming the meter has been calibrated at a pH of 7 at 25Â°C. The table below demonstrates the adjustment.