Calconcarboxylic acid (

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

name 3-hydroxy-4- 2-hydroxy-4-sulfonaphthalen-1-yl)diazenylaphthalene-2-carboxylic acid; commonly called Patton and Reeder's Indicator) is an

azo dye Azo dyes are organic compounds bearing the functional group R−N=N−R′, in which R and R′ are usually aryl and substituted aryl groups. They are a commercially important family of azo compounds, i.e. compounds containing the C- ...

that is used as an indicator for

complexometric titration Complexometric titration (sometimes chelatometry) is a form of volumetric analysis in which the formation of a colored complex is used to indicate the end point of a titration. Complexometric titrations are particularly useful for the determinati ...

s of

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...

with

ethylenediaminetetraacetic acid Ethylenediaminetetraacetic acid (EDTA) is an aminopolycarboxylic acid with the formula H2N(CH2CO2H)2sub>2. This white, water-soluble solid is widely used to bind to iron (Fe2+/Fe3+) and calcium ions (Ca2+), forming water-soluble complexes ev ...

(EDTA) in the presence of

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...

. Structurally, it is similar to eriochrome blue black R, which is obtained from calconcarboxylic acid by

decarboxylation Decarboxylation is a chemical reaction that removes a carboxyl group and releases carbon dioxide (CO2). Usually, decarboxylation refers to a reaction of carboxylic acids, removing a carbon atom from a carbon chain. The reverse process, which is ...

and reaction with

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkal ...

Properties

Calconcarboxlic acid is soluble in water and a variety of other solvents, including

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a h ...

and methanol. It has a violet colour in dissolved form in

. The melting point of calconcarboxylic acid is at approximately 300 °C, where it undergoes

thermal decomposition Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is req ...

Background

Though the determination of calcium and magnesium by

with

standard solution In analytical chemistry, a standard solution is a solution containing a precisely known concentration of an element or a substance. A known mass of solute is dissolved to make a specific volume. It is prepared using a standard substance, such as a ...

s of

disodium dihydrogen tetraacetate Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

, utilising

Eriochrome Black T Eriochrome Black T is a complexometric indicator that is used in complexometric titrations, e.g. in the water hardness determination process. It is an azo dye. Eriochrome is a trademark of Huntsman Petrochemical, LLC. In its deprotonated form, ...

as indicator is widely accepted and quite adequately understood, it, like other complexometric titration methods, suffers from the limitations of having an indistinct

endpoint An endpoint, end-point or end point may refer to: * Endpoint (band), a hardcore punk band from Louisville, Kentucky * Endpoint (chemistry), the conclusion of a chemical reaction, particularly for titration * Outcome measure, a measure used as an e ...

(where a photometric titrator is needed to provide acceptable accuracy) and/or having to separate the metals before

titration Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte (a substance to be analyzed). A reagent, termed the ''titrant ...

can occur. Calconcarboxylic acid was thus adopted as a superior alternative due to its ability to give a good and visual endpoint and its rapid performance even with the presence of

Synthesis

As described by James Patton and Wendell Reeder in 1956, calconcarboxylic acid can be synthesised by coupling diazotized 1-amino-2-naphthol-4-sulfonic acid with 2-hydroxy-3-napthoic acid.

Applications

Calconcarboxylic acid is used for the determination of calcium ion concentration by complexometric titration. Free calconcarboxylic acid is blue colour, but changes to pink/red when it forms a complex with calcium ions. EDTA forms a more stable complex with calcium than calconcarboxylic acid does, so addition of EDTA to the Ca–calconcarboxylic acid complex causes formation of Ca-EDTA instead, leading to reversion to the blue colour of free calconcarboxylic acid. For the complexometric titration, the indicator is first added to the

titrant Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte (a substance to be analyzed). A reagent, termed the ''titrant'' ...

containing the calcium ions to form the calcium ion-indicator complex (Ca-PR) with a pink/red colour. This is then titrated against a standard solution of EDTA. The endpoint can be observed when the indicator produces a sharp, stable colour change from wine red to pure blue, which occurs at pH values between 12 and 14, this indicates the endpoint of the titration, as the Ca-PR complexes have been completely replaced by the Ca-EDTA complexes and hence the PR indicator reverts to its blue colour. The reaction can be given by: :Ca-PR + EDTA^4- → PR + a-EDTAsup>2- The Patton-Reeder Indicator is often used here in the form of a

triturate Comminution is the reduction of solid materials from one average particle size to a smaller average particle size, by crushing, grinding, cutting, vibrating, or other processes. In geology, it occurs naturally during faulting in the upper p ...

. This method of complexometric titration is dependent on the pH of the solution being sufficiently high to ensure that

ions precipitate as

magnesium hydroxide Magnesium hydroxide is the inorganic compound with the chemical formula Mg(OH)2. It occurs in nature as the mineral brucite. It is a white solid with low solubility in water (). Magnesium hydroxide is a common component of antacids, such as mil ...

before the PR indicator is added to prevent interference with the results, as if magnesium were present, the EDTA would also form complexes with it. Concentrated

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which expl ...

is usually added to the solution to this end. The accuracy of this method may also be affected by the presence of metal ions such as

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish ...

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...

cobalt Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, ...

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic t ...

manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy u ...

in sufficiently high concentrations.

References

{{Reflist Azo dyes Chemistry Carboxylic acids Sulfonic acids