SENSORES Y ELECTRODOS PARA INSTRUMENTOS DE ANALISIS DE AGUA
||Sensor Ph, Redox, Oxigeno, Conductividad, Ion
|Sensores y electródos para instrumentos de análisis de agua.
Sensors and electrodes for water analysis instruments.
The pH value is a measure of the alkalinity or acidity of an aqueous solution. The scale reaches from 0 (highly acid) to 14 (highly alkaline). On the basis of this scale, liquids are referred to as acidic (e.g. hydrochloric acid), neutral (e.g. distilled water) and alkaline (e.g. sodium hydroxide). The pH value can be measured by means of electrochemical measuring systems which consist of a measuring and a reference electrode. In the case of a single-rod probe, the two electrodes are integrated in a single housing. When the probe is submerged in the solution to be measured, a voltage is generated between the measuring and the reference electrode which is characteristic of the corresponding pH value. This voltage is detected by the instrument, converted into the pH value and displayed.
- Drinking water monitoring
- Pool water monitoring
- Municipal and industrial sewage plants
- Production and process control (e.g. chemical, paper, textile, paint industries)
- Power plants (boiler feed water, corrosion)
- Food production (dairies, sugar plants, breweries, etc.)
The term redox comes from the two concepts of reduction and oxidation, the designations for chemical reactions of oxygen with other substances. The term relates to reactions in which one part gains electrons (reduction) and one part loses electrons (oxidation).
Everyday redox reactions:
- Iron rusts.
- Verdigris is generated when copper is exposed to the weather.
- Silver tarnishes.
In aqueous solutions, there is a redox voltage between two electrodes consisting of different metals. It is a measure of the different potential of the metals to lose electrons. As a result, the (less noble) metal is dissolved (oxidation) and the nobler metal is separated from the solution (reduction). The redox voltage is measured by means of a redox probe. Just like the pH probe, it consists of a measuring and a reference electrode (single rod probe).
Redox measurements are performed wherever chemical reactions have to be monitored. Examples include denitrification of waste water (determination of redox inflexion point), monitoring the disinfection of cleaning agents or decontamination of electroplating solutions.
More or less oxygen is dissolved in virtually every liquid. For example, water contains approx. 9 mg/l of oxygen at a temperature or 20 °C and an atmospheric pressure of 1013 mbar if it is saturated. Each liquid takes up oxygen until the partial pressures in the liquid and in the air or gas in contact with the liquid have reached equilibrium. The oxygen concentration is determined by means of an electrochemical sensor. The simplest type of oxygen sensor contains two electrodes. Both electrodes are located in an electrolyte system which is separated from the sample by a membrane. At one electrode, oxygen is reduced to hydroxide ions, at the other electrode, ions are oxidised to oxygen. During this electrochemical reaction, a current flows between the two electrodes in the sensor if they are connected. The higher the oxygen concentration in the solution, the greater the current signal. The oxygen measuring instrument uses a solubility function to calculate the oxygen concentration in the solution on the basis of this signal.
- Waster water treatment, e.g. optimisation of the cleaning quality, monitoring of decomposition processes
- Water monitoring, e.g. monitoring the conditions of marine life, e.g. fish and microorganisms
- Drinking water quality, evaluation, prevention of corrosion damage
- Beverage industry, e.g. drink-by dates
- Fish farms, e.g. cost optimisation (oxygen supply)
The electrical conductivity is a measure of the electrical current between two points (electrodes) with different potential (voltage), e.g. in a liquid. The more minerals, acids or bases a solution contains, the greater its conductivity. The unit for conductivity is S/m. The scale for aqueous solutions begins with pure water with a conductivity of 0.05 μS/cm (25 °C) and ends with some bases with a conductivity of 1,000 μS/cm (e.g. potassium solutions). The conductivity of natural water such as drinking water or surface water is in the range of 100–1,000 μS/cm. The conductivity is measured by means of a measuring cell which, in the simplest case, consists of two electrodes. An alternating voltage applied at one electrode causes the ions contained in the solution to move to the electrode. The more ions in the solution, the greater the current between the electrodes. The instrument uses the measured current to first calculate the conductance and then the conductivity level depending on the cell data.
- Boiler feed water monitoring: lime causes damage to the pipes.
- Monitoring of industrial facilities: production of ultra-pure water, drinking water treatment, demineralisation of water for cooling plants, waste water monitoring
- Food industry: milk, beverages, beer, bottle cleaning and CIP systems
Ion-selective electrodes ISE (e.g.: NO3 -, NH4 +, F-, CI-, Ag+/S2-
Ion-selective electrodes enable the determination of activities or concentrations of exactly one type of ion in liquids, irrespective of colouration and turbidity. The measurement is performed by submerging an ion-selective electrode and a reference electrode in the sample solution and measuring the cell voltage. Silver electrodes, silver chloride electrodes and saturated calomel electrodes are primarily used as reference electrodes.
- Water supply and distribution: monitoring of drinking water, process and surface water. Fluoride concentrations in fluorinated drinking water, water hardness, chlorine content, sulphate and sulphide in natural waters.
- Agriculture: examination of the nutrient content and fertiliser requirements in terms of nitrate, chloride, fluoride, sodium, calcium, potassium and iodide.
- Medical technology: fluoride in tooth enamel, chloride, calcium, potassium, sodium and enzymes in the blood, serum and urine as well as chloride in body perspiration.
- Chemical industry: monitoring of base and laboratory chemicals, silver and halogen concentrations in the production of photo emulsions. Checking washing water in the photochemical industry.
- Food industry: measurement of nitrate due to its potential toxicity in food, in particular fresh vegetables, determination of the salt content in meat, fish, milk, vegetable and fruit juices with chloride electrodes.
- Metallurgy: checking etching, flushing and electroplating solutions for silver, copper, fluoride, chloride and cyanide.