Ions measurement

ISE sensors basic principles

An Ion Selective Electrode (ISE), also known as a specific ion electrode (SIE), is a transducer (or sensor) that converts the activity of a specific ion dissolved in a solution into an electrical potential, which can be measured by a voltmeter or pH meter. The probe includes at least two electrodes, a reference and a measurement electrode. The measurement electrode is equipped with a special membrane, capable of binding specific ions reversibly.

Depending on the activity of the measured ions in the liquid, a varying number of ions will bind to the measurement electrode -- resulting in a varying potential difference between the measurement electrode and the reference electrode, which shows a constant potential in reference to the medium. The measured potential is put in relation to the activity of the measured ion by means of a calibration function.

Figure: Measurement set-up of ion selective electrode (ISE)

The voltage is theoretically dependent on the logarithm of the ionic activity, according to the Nernst equation. The Nernst equation is frequently expressed in terms of base 10 logarithms rather than natural logarithms, in which case it is written, for a cell at 25 °C:

Figure: Nernst equation

The properties of an ion-selective electrode are characterized by parameters like:

Selectivity: The selectivity is one of the most important characteristics of an electrode, as it often determines whether a reliable measurement in the sample is possible or not. The experimental selectivity coefficients depend on the activity and a method of their determination. Different methods of the selectivity determination can be found in the literature. The IUPAC (International Union of Pure and Applied Chemistry) suggests two methods: separate solution method (SSM) and fixed interference method (FIM). The methods proposed by IUPAC with several precautions will give meaningful data.

Slope: of the linear part of the measured calibration curve of the electrode. The theoretical value according to the Nernst equation is: 59.16 (mV/log(a~x~)) at 298 K for a single charged ion or 59.16/2 = 29.58 (mV per decade) for a double charged ion. However, in certain applications the value of the electrode slope is not critical and worse value does not exclude its usefulness.

Range of linear response: At high and very low target ion activities there are deviations from linearity. Typically, the electrode calibration curve exhibits linear response range between 10-1 M and 10-5 M.

Detection limit: According the IUPAC recommendation the detection limit is defined by the cross-section of the two extrapolated linear parts of the ion-selective calibration curve. In practice, detection limit on the order of 10^-5^-10^-6^ M is measured for most of ion-selective electrodes.

Response time: In earlier IUPAC recommendations, it was defined as the time between the instant at which the Ion Selective Electrode and a reference electrode are dipped in the sample solution (or the time at which the ion concentration in a solution is changed on contact with ISE and a reference electrode) and the first instant at which the potential of the cell becomes equal to its steady-state value within 1 mV or has reached 90% of the final value (in certain cases also 63% or 95%).

Ions presence in Water

Ammonium (NH4+)

Ammonium (NH4+), or its uncharged form ammonia (NH3), is a form of nitrogen which aquatic plants can absorb and incorporate into proteins, amino acids, and other molecules. High concentrations of ammonium can enhance the growth of algae and aquatic plants. Bacteria can also convert high ammonium to nitrate (NO3-) in the process of nitrification, which lowers dissolved oxygen.

Bromide (Br-)

Bromide is commonly found in nature along with sodium chloride, owing to their similar physical and chemical properties, but in smaller quantities. Bromide concentrations in seawater are generally in the range of 65 mg/L to well over 80 mg/L in some confined sea area. Concentrations of bromide in fresh water typically range from trace amounts to about 0.5 mg/L. Concentrations of bromide in desalinated waters may approach 1 mg/L. Bromide ion has a low degree of toxicity; thus, bromide is not of toxicological concern in nutrition.

Calcium (Ca2+)

Calcium is an important determinant of water harness, and it also functions as a pH stabilizer, because of its buffering qualities. Calcium is naturally present in water. It may dissolve from rocks such as limestone, marble, calcite, dolomite, gypsum, fluorite and apatite. Seawater contains approximately 400 ppm calcium. One of the main reasons for the abundance of calcium in water is its natural occurrence in the earth\'s crust. Calcium is also a constituent of coral. Rivers generally contain 1-2 ppm calcium, but in lime areas rivers may contains calcium concentrations as high as 100 ppm.

Chloride (Cl-)

Chloride increases the electrical conductivity of water and thus increases its corrosivity. Chloride concentrations in excess of about 250 mg/L can give rise to detectable taste in water, but the threshold depends upon the associated cations. Seawater can contains 20.000 ppm of this ion.

Cupric (Cu2+)

Copper is a naturally occurring metal found in the earth\'s crust. Copper is also generally present in surface waters, with cupric ion (Cu2+) as the primary form in natural surface waters. In freshwater systems, naturally occurring concentrations of copper range from 0.2 µg/L to 30 µg/L, at low concentrations, copper is an essential element to virtually all plants and animals, including humans.

Fluoride (F-)

Fluoride is the simplest anion of fluorine. Its salts and minerals are important chemical reagents and industrial chemicals, mainly used in the production of hydrogen fluoride for fluorocarbons. The MCLG for fluoride is 4.0 mg/L or 4.0 ppm. Exposure to excessive consumption of fluoride over a lifetime may lead to increased likelihood of bone fractures in adults, and may result in effects on bone leading to pain and tenderness.

Fluoroborate (BF4-)

Fluoroborate (or Tetrafluoroborate) is used in industrial applications as thinner, catalysis and batteries. The presence of this element in water is not common, but if dangerous at high concentrations.

Iodide (I-)

Iodine is naturally present in water. Iodine ends up in surface waters naturally through rains and water evaporation. Eventually, it also ends up in groundwater. Other options include weathering of iodine-containing rocks, and volcanic activity (including under-water volcanoes). In nature iodine can be found in reasonably large amounts, but only in compounds. The average concentration in seawater is about 60 ppb, but varies from place to place. Rivers usually contain about 5 ppb of iodine, and in mineral sources some ppm can even be found.

Lithium (Li+)

Lithium (Li+) is present in many minerals. Seawater contains approximately 0.17 ppm lithium. Rivers generally contain only 3 ppb, whereas mineral water contains 0.05-1 mg lithium per liter. Lithium is weakly harmful in water. Lithium is not a very big threat to flora and fauna, nor on the mainland, nor in aquatic environments. It is readily absorbed by plants, causing plants to be an indicator of soil lithium concentrations.

Magnesium (Mg2+)

Magnesium is present in seawater in amounts of about 1300 ppm. After sodium, it is the most commonly found cation in oceans. Rivers contain approximately 4 ppm of magnesium, marine algae 6000-20,000 ppm, and oysters 1200 ppm. Dutch drinking water contains between 1 and 5 mg of magnesium per liter. Magnesium and other alkali earth metals are responsible for water hardness. Water containing large amounts of alkali earth ions is called hard water, and water containing low amounts of these ions is called soft water.

Nitrate (NO3-)

Nitrate is one of the most frequent groundwater pollutants in rural areas. It needs to be regulated in drinking water basically because excess levels can cause methaemoglobinaemia, or "blue baby" disease. The standard for nitrate in drinking water is 10 mg/L nitrate-N, or 50 mg/L nitrate- NO3, when the oxygen is measured as well as the nitrogen. Unless otherwise specified, nitrate levels usually refer only to the amount of nitrogen present, and the usual standard, therefore, is 10 mg/l.

Nitrite (NO2-)

The nitrite ion is an ambidentate ligand, and is known to bond to metal centers in at least five different ways. Nitrite is also important in biochemistry as a source of the potent vasodilator nitric oxide. Nitrite levels above 0.75 ppm in water can cause stress in fish and greater than 5 ppm can be toxic.

Perchlorate (ClO4-)

Perchlorate (ClO4-) salts are mainly used for propellants, exploiting properties as powerful oxidizing agents. Perchlorate contamination in the environment has been extensively studied as it has effects on human health. Perchlorate has been linked to its negative influence on the thyroid gland. Perchlorate is commonly used as an oxidizer in rocket propellants, munitions, fireworks, airbag initiators for vehicles, matches and signal flares. It is naturally occurring in some fertilizers.

Potassium (K+)

Potassium is non-water soluble, but it does react with water as was explained earlier. Potassium compounds may be water soluble. Seawater contains about 400 ppm. It tends to settle, and consequently ends up in sediment mostly. Rivers generally contain about 2-3 ppm potassium. This difference is mainly caused by a large potassium concentration in oceanic basalts.

Silver (Ag+)

Seawater contains approximately 2-100 ppt of silver, and the surface concentration may be even lower. River water generally contains approximately 0.3-1 ppb of silver. Under normal conditions silver is water insoluble. This also applies to a number of silver compounds, such as silver sulphide. Silver is not a dietary requirement for organisms. It may even be lethal to bacteria, and it inhibits fungi reproduction. This is mainly caused by Ag+ ions.

Sodium (Na+)

Sodium compounds serve many different industrial purposes, and may also end up in water from industries. They are applied in metallurgy, and as a cooling agent in nuclear reactors. Sodium nitrate is often applied as a synthetic fertilizer. Seawater contains approximately 11,000 ppm sodium. Rivers contain only about 9 ppm. Drinking water usually contains about 50 mg/L sodium.