The life, sometimes even the survival, of fishes is influenced not only by biotope structures, food availability and predators, but also by the abiotic parameters of their habitat. These include, for example, water hardness, redox values and electrical conductivity as well as ammonium, nitrite, nitrate and phosphate concentrations. Of particular significance, however, is the water’s oxygen content, for the available quantity of oxygen is decisive for the aerobic metabolic processes in the water body. Nearly all organisms, from protozoans to fishes, require oxygen for the internal respiration of their body cells. What is generally not a problem for air-breathing organisms (because the atmosphere contains sufficient oxygen) is much more difficult for aquatic organisms because less oxygen can be dissolved in the water and thus be available for breathing. The oxygen solubility in water decreases with increasing temperature as well as with increasing content of dissolved substances such as salts and other gases. When saturated, fresh water at 0°C can hold up to 14.6 mg O2/l but at 20° C much less at 9.1 mg O2/l.
In order to provide fishes with acceptable living conditions it is necessary to maintain a minimum content of oxygen in the water. Although this value is strongly dependent on the particular fish species oxygen levels below 3 mg / l are mostly considered critical for fishes. A fish has to work hard in order to push the water stream constantly without interruption past its gill plates. Whereas human beings use at most 3.2% of their metabolic rate at rest for breathing a fish uses about 30%, and during strenuous activity even as much as 50%! At the extreme, almost half of the oxygen that a fish absorbs is immediately used again for the breathing process itself.
Long-term measurements are of higher significance
A water body’s oxygen content is the result of numerous oxygen-producing and oxygen-consuming processes and can fluctuate strongly during the day. The main sources of oxygen production are oxygen input from the atmosphere (diffusion), as well as photosynthesis of phytoplankton and water plants which release oxygen in daylight conditions and can lead to O2 oversaturation. Oxygen-consuming processes include not only respiration of animals and at night of plants, but above all the microbial degradation of organic substances in the water. Because this can lead to oxygen deficiency in polluted waters, oxygen content is also a criterion for assessing the ecological situation in a water body (biological oxygen demand BOD, chemical oxygen demand COD).
The daily and seasonal variability and dynamics of the oxygen content in a water body can only be measured with sufficient accuracy by means of continuous measurements. Three measurement methods are generally used to determine the amount of dissolved oxygen in the water: the titrimetric determination according to Winkler, called the Winkler method for short, the polarographic method, and the luminescence method. The Winkler method, developed by the Hungarian chemist and pharmacist Lajos Winkler in 1888, is based on the oxidation of two positively charged manganese ions by the dissolved oxygen, which is fixed in the water sample with two reagents and then titrated in the laboratory as a cinnamon to coffee-brown precipitate. The oxygen content can be computed from the volume of titre. The Winkler method is the most accurate but also the most elaborate method of oxygen determination and is therefore generally only used by specialized measurement laboratories. In practice, electronic oxygen measuring devices are mostly used. These are available in many different variants, designs and measuring ranges. They are suitable for use in the laboratory as well as outdoors as efficient all-round devices of watertight design, with an interface for data transmission or with internal data storage. Some only measure the absolute oxygen content, others also display the respective oxygen saturation.
Physical and chemical basics of oxygen measurement
Technically, the measuring instruments usually operate according to two different methods. In the polarographic method, the basic principle of which was already developed in 1897, electric current is conducted via two electrodes (anode, cathode) which are separated by a semipermeable membrane that is only permeable to oxygen into a measuring cell filled with electrolyte solution and the oxygen is electrochemically reduced at the cathode. The strength of the resulting current flow is proportional to the oxygen concentration in the water and is usually indicated by the measuring device directly in mg O2/l. The best-known use of this relatively simple measuring method, which is not very precise but is normally quite adequate for practical purposes, is the so-called Clark electrode. A disadvantage is, however, that the membrane heads of the measuring electrodes have to be changed regularly and the instruments must be calibrated practically every day prior to measuring. The foundations of the latest and most modern measuring method, the luminescence method, were laid at the end of the 1940s but its practical implementation had to wait until 1987. This highly user-friendly method measures the temporal attenuation of the luminescence radiation of a luminescent material (luminophore) which is stimulated by irradiation with light of different wavelengths. The fading behaviour of the luminescence depends on the material of the luminophore (usually metalloporphyrin-albumin complexes), the wavelength of the irradiated light, and – and this is the interesting part, because it is measurable – on the oxygen concentration in the environment. It is thus possible to draw quantifiable conclusions about the oxygen content from the time sequence of the reflectance curve. This method is very user-friendly because the measuring instrument does not have to be constantly re-calibrated and neither membrane nor electrolyte has to be changed.
Miniaturization and scope of measuring instruments advancing
The advances in electronics and computer measurement technology today make it possible to produce both handheld instruments and fixed installations in many different price classes which are simple to use and sufficiently accurate. Some of them can also optionally be used to measure other water parameters. Since oxygen concentration is strongly dependent on temperature most devices have, for example, a temperature sensor. Temperature compensation for oxygen measurements is usually automatic which guarantees high accuracy. With additional probes some devices can also measure the salt content (conductivity), pH and redox values as well as other parameters. The measured data can be read directly on the display or can be stored in the internal data memory of the devices and accessed later on at the computer via a USB interface. Some manufacturers even provide the necessary software with the device. Modern electronic measuring instruments for important water parameters combine high measuring accuracy, which is indispensable in professional water analysis, with the greatest possible ease of use, which makes them equally suitable for professional and non-professional use. Given such convincing advantages, it is hardly surprising that modern measuring instruments have almost completely replaced the previously used chemical test kits for water analysis.
Manufacturers can today supply measuring instruments that offer higher performance and new characteristics and abilities. They are faster, more sensitive and more accurate, they are lighter and smaller, more convenient and easier to operate than in the past. Some devices have intelligent sensor recognition so that when the measuring sensor is changed the instrument automatically switches to the appropriate parameter and calibrates the sensor. Other devices are equipped with a GPS sensor so that the measured values can be assigned to precise positions based on geographic coordinates. With this additional function, measurements can always be carried out in exactly the same location for comparison with each other. In the case of more recent measuring instruments data can now also be transmitted wirelessly via IR communication interfaces, and anyone who needs measurements in longer time series can with some handsets with autonomous sensors conveniently programme the desired intervals. The sensor then measures and stores the determined data at the predetermined times independently.
The scale of pH values is logarithmic
A further important water parameter which, like oxygen concentration, should be regularly checked in a water body, is the pH value. The pH value is a numeric scale used to specify the acidity or basicity of an aqueous solution thus providing information on how strong and aggressive acids, alkalis or even water are. In its normal state, water contains as many hydroxide ions (OH-) as positively charged hydrogen ions H+, which are present in aqueous solution as "oxonium ions" (H3O+) and react as a weak acid. In chemically unchanged water, one of seven million water molecules is dissociated in OH- and H+. Mathematically, this proportion can also be expressed as 10-7. It marks the neutral point of water because the quantity of bases is the same as the quantity of the acids (H+).Because in practice it would be rather laborious to always specify the proportion of hydrogen ions as a decimal potential it has been internationally agreed to specify only the negative numerical value (minus times minus equals plus) of the exponent. Thus, the pH value is defined as a negative decadal logarithm of the hydrogen ion concentration. The abbreviation "pH" is originally derived from the Latin "pondus hydrogenii" (pondus = weight), but can also be translated as "potentia hydrogen" (potentia = power, force).
Except for a few exceptions, the pH values of almost all substances can be assigned to a scale of between 0 and 14 (the pH value is a dimensionless numerical value so it does not have a special unit of measure). The quantity ratio of the hydroxide and hydrogen ions shifts as a result of addition of acid or basic substances. All pH values above 7 are considered basic, values below 7 are classified as acidic. It should be noted that the pH scale is logarithmic. An increase or decrease in the pH value by the value of 1 thus corresponds to a ten-fold increase in the degree of alkalinity or acidity. A water body with a pH of 6 would thus be ten times, with pH 5 even a hundred times more acid than water with a pH of 7. The same also of course applies to basicity values above pH 7.
Colour change in test strips indicates the pH value
Different methods can be used to determine the pH value. For simple control routines, test sticks with indicator dyes are often sufficient for ascertaining the concrete value by a colour reaction. Known indicator dyes whose colouring changes noticeably at certain pH values are, for example, litmus, methylorange, phenolphthalein and bromothymol blue. Within narrow measuring ranges, usually of two to three pH stages, the sensitivity of individual dyes is sufficient to indicate the pH value by a colour change. For larger measuring ranges one uses universal indicators with dye mixtures that change colour at the respective pH value. The evaluation of the colour change is usually carried out on the basis of colour comparison scales. When measurements have to be particularly accurate, however, the colour display of an indicator dye can also be precisely evaluated using a photometer. Some measuring sticks can even be equipped with temperature compensation.
If the handling of pH test strips is too cumbersome or their accuracy is not sufficient electronic, mostly digital, measuring instruments can also be used. They are available in different price classes for private and professional use. Most pH meters are now not only compact, easy to use, reliable and accurate, but also relatively inexpensive. Nevertheless, if the instrument is to be used for a long time particular attention should be paid to the fact that the measuring electrode can be exchanged simply and without great effort.
Common methods for electrochemical pH determination
Modern measuring instruments do not use acid-base indicators to determine the pH value of a solution but rather electrochemical means. One of the most common methods on which probably most commercially available pH meters are based is potentiometry. These devices have a glass membrane bulb at the tip of the sensor which is filled with a buffer solution and dipped into the relevant liquid for measurement. The hydrogen ions have a tendency to accumulate in a microscopically thin layer on the silicate groups of the glass surface. Since the H+ ions are positively charged, depending on the difference in pH between the inner and the outer side of the bulb a galvanic voltage is generated which can be measured with two reference electrodes. One of the electrodes is located in the glass membrane bulb, the other is in a reference electrolyte.
The accuracy of a pH meter always depends on a number of factors, for example the ambient temperature during the measurements (the device should be equipped with a temperature sensor for temperature compensation), the charge state of the mobile energy source (exhausted batteries or too fast energy consumption can lead to incorrect results) or the exact calibration of the sensor and measuring instrument. In principle, pH measuring devices should be calibrated at least once per application. As a rule, two or three standard solutions with known pH values (usually an acidic pH 5 and a basic pH of 10) are used, to which the device is then calibrated.
It is particularly important that the pH sensor is correctly maintained and stored. It must never be allowed to dry out because the sensitive electrodes would otherwise be damaged and would not provide reliable values. If the electrode is not being used for measurements it should be rinsed clean and kept in pH-regulated water. Some manufacturers supply special storage solutions for the electrodes. When properly maintained, the electrodes of potentiometric pH meters can last for about 6 months. If, despite careful maintenance and calibration of the device, deviations in the measured values occur repeatedly the pH electrode must be replaced. Even in the case of expensive devices deviations up to ± 0.02 pH are often technically caused and tolerable; in the case of inexpensive handheld measuring devices the deviations can be larger. In principle, however, the following applies: the smaller the selected measuring range, the more accurate the pH measurement should be.
An alternative to potentiometric pH measurement with glass membrane electrodes are pH meters which function with ion-sensitive field effect transistors (ISFETs). This young and innovative technology allows very small sensors and has a short response time during the measurements. ISFETs are semiconductor-based chemical sensors, the basic principle of which is approximately the same as that of the glass electrodes. Here, too, the hydrogen ions on the sensitive membrane of the field-effect transistor produce a voltage which changes the conductivity of the transistor which with the appropriate measurement technology can then be displayed as a pH value. The sensitivity and accuracy of the measurement depends on the thin pH-sensitive layer of the ISFET, which in this electronic device replaces the metallic gate electrode of conventional MOS (metal oxide semiconductor) field-effect transistors.