Acid-base indicator is a weak acid or weak base, such as litmus, methyl orange or phenolphthalein, which changes colour when it gains or loses an H+ ion.
Universal indicator is an indicator which undergoes several colour changes over a wide range of pH. The colour is used to indicate pH directly. Universal indicators are usually mixtures of several indicators.
Indicator is a substance used to show the presence of a chemical substance or ion by its colour. Acid-base indicators are compounds, such as phenolphtaleine and methyl orange, which change colour reversibly, depending on whether the solution is acidic or basic. Oxidation-reduction indicators are substances that show a reversible colour change between oxidised and reduced forms.
Indicator electrode is working in one of the electrodes in some classical two-electrode cells, e.g., in a potentiometric electroanalytical setup where the potential of the measuring electrode (against a reference electrode) is a measure of the concentration (more accurately activity) of a species in the solution.
By use of suitable radioactive isotopes biochemical processes can be observed in plants, animals and humans, by measuring radioactive radiation of radioactive indicator. Artificial radioactive isotopes have the same chemical properties as natural ones, which enable us to mark those natural isotopes with addition of artificial ones and in this way follow the path of those elements during a chemical reaction. One of the most important radioactive indicators is the radioactive carbon 14C.
Acid is a type of compound that contains hydrogen and dissociates in water to produce positive hydrogen ions. The reaction for an acid HA is commonly written:
In fact, the hydrogen ion (the proton) is solvated, and the complete reaction is:
This definition of acids comes from the Arrhenius theory. Such acids tend to be corrosive substances with a sharp taste, which turn litmus red and produce colour changes with other indicators. They are referred to as protonic acids and are classified into strong acids, which are almost completely dissociated in water, (e.g. sulphuric acid and hydrochloric acid), and weak acids, which are only partially dissociated (e.g. acetic acid and hydrogen sulphide). The strength of an acid depends on the extent to which it dissociates, and is measured by its dissociation constant.
In the Lowry-Brønsted theory of acids and bases (1923), the definition was extended to one in which an acid is a proton donor (a Brønsted acid), and a base is a proton acceptor (a Brønsted base). An important feature of the Lowry-Brønsted concept is that when an acid gives up a proton, a conjugate base is formed that is capable of accepting a proton.
Similarly, every base produces its conjugate acid as a result of accepting a proton.
For example, acetate ion is the conjugate base of acetic acid, and ammonium ion is the conjugate acid of ammonia.
As the acid of a conjugate acid/base pair becomes weaker, its conjugate base becomes stronger and vice versa.
A further extension of the idea of acids and bases was made in the Lewis theory. In this, a G. N. Lewis acid is a compound or atom that can accept a pair of electrons and a Lewis base is one that can donate an electron pair. This definition encompasses "traditional" acid-base reactions, but it also includes reactions that do not involve ions, e.g.
in which NH3 is the base (donor) and BCl3 the acid (acceptor).
Litmus paper is an unsized paper treated with litmus for the use as an acid-base indicator, in an acid it turns red, and in a base it turns blue.
Acid-base titration is an analytical technique in volumetric analysis, where an acid of known concentration is used to neutralise a known volume of a base, and the observed volume of the acid required is used to determine the unknown concentration of the base. An acid-base indicator is used to determine the end-point of the titration.
Artificial radioactive isotopes are formed when an atom is bombed with an accelerator or exposing it to slow moving neutrons in a nuclear reactor. In this way isotopes (radionuclides) are obtained which are non-existent in nature because of their unstability and radioactive transition into stable isotopes. Most important radioactive isotopes are:
Radioactive isotope of cobalt is formed when ordinary metal cobalt is bombed with neutrons in a nuclear reactor.
Radioactive isotope of phosphorus is formed when ordinary phosphorus is bombed with deuterons produced in cyclotron.
radioactive isotope of carbon is formed when a nitrogen is bombed with slow moving neutrons in a nuclear reactor. It is mostly used as a radioactive indicator.
Originally chlorinity (symbol Cl) was defined as the weight of chlorine in grams per kilogram of seawater after the bromides and iodides had been replaced by chlorides. To make the definition independent of atomic weights, chlorinity is now defined as 0.3285233 times the weight of silver equivalent to all the halides.
The Mohr-Knudsen titration method served oceanographers for more than 60 years to determine salinity from chlorinity. This modification of the Mohr method uses special volumetric glassware calibrated directly in chlorinity units. The Mohr method uses potassium chromate (K2CrO4) as an indicator in the titration of chloride ions chloride (plus a small amount of bromide and iodide) with a silver nitrate (AgNO3) standard solution.
The other halides present are similarly precipitated.
A problem in the Mohr titration was that silver nitrate is not well suited for a primary standard. The Danish physicist Martin Knudsen (1871-1949) suggested that a standard seawater (Eau de mer Normale or Copenhagen Normal Water) be created and distributed to oceanographic laboratories throughout the world. This water was then used to standardize the silver nitrate solutions. In this way all chlorinity determinations were referred to one and the same standard which gave great internal consistency.
The relationship between chlorinity Cl and salinity S as set forth in Knudsen's tables is
In 1962, however, a better expression for the relationship between total dissolved salts and chlorinity was found to be
Generalic, Eni. "Indicator." Croatian-English Chemistry Dictionary & Glossary. 29 June 2022. KTF-Split. {Date of access}. <https://glossary.periodni.com>.
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