Calcon indicator

Calcon indicator Dissolve 0.20 g of the dyestuff (calcon) in 50 cc methanol (ethanol may also be used). Prepare fresh solution weekly. 

Add 10-12 drops of calcon indicator stir the solution and titrate with standard EDTA solution until the colour changes from pink to pure blue. 

Standardise the EDTA solution with standard Ca solution using Eriochrome black-T indicator or calcon indicator (see note below), using the procedure for calcium-estimation. 

If the EDTA solution is prepared by dissolving 2.0 g of the salt in one litre water without addition of 0.05 g magnesium chloride hexahydrate, then use calcon indicator during standardization of EDTA with standard calcium solution. 

Determine a blank for KCl-triethanolamine solution using Eriochrome black T and calcon indicators. 

Ca" can then titrated with EDTA using calcon indicator ... 

Pipet out 50 ml of the hard water sample into a conical flask. Add 3 ml of dicthylauiine and 4 drops of calcon indicator. Keep the flask on a magnetic stirrer and titrate against standard EDTA solution until the colour changes from pink to pure blue. The volume of EDTA used (Dj ml] corresponds to calcium bardness. Magnesium hardness is obtained by subtracting calcium bardness from total hardness. 

Calcon carboxylic acid [3-hydroxy-4-(2-hydroxy-4-sulfo-l-napbtbylazo)napbtbalene-2-carboxylic acid] [3737-95-9] M 428.4, m 300°, X,max 560nm, pKj 1.2, pK2 3.8, PK3 9.26, PK4 13.14. Purified through its p-toluidinium salt. The dye was dissolved in warm 20% aq MeOH and treated with p-toluidine to ppte the salt after cooling. Finally recrystd from hot water. [Itoh and Ueno Analyst (London) 95 583 1970.] Patton and Reeder (Anal Chem 28 1026 1956) indicator and complexes with Ca in presence of Mg and other metal ions. 

Eriochrome Blue Black R (Palatine Chrome Black 6BN, Calcon, 3-hydroxy-4-(2-hydroxy-l-naphthylazo)naphthalene-l-sulfonic acid Na salt] [2538-85-4] M 416.4, pK2 7.0, pKj 13,5. Freed from metallic impurities by three pptns from aqueous soln by addn of HCl. The ppted dye was dried at 60° under vacuum. Indicator for complexometry of Al, Fe and 7i. 

Solochrome dark blue or calcon ( C.1.15705). This is sometimes referred to as eriochrome blue black RC it is in fact sodium l-(2-hydroxy-l-naphthylazo)-2-naphthol-4-sulphonate. The dyestuff has two ionisable phenolic hydrogen atoms the protons ionise stepwise with pK values of 7.4 and 13.5 respectively. An important application of the indicator is in the complexometric titration of calcium in the presence of magnesium this must be carried out at a pH of about 12.3 (obtained, for example, with a diethylamine buffer 5 mL for every 100 mL of solution) in order to avoid the interference of magnesium. Under these conditions magnesium is precipitated quantitatively as the hydroxide. The colour change is from pink to pure blue. 

The titration with EDTA, using solochrome black as indicator, will yield the calcium content of the sample (if no magnesium is present) or the total calcium and magnesium content if both metals are present. To determine the individual elements, calcium may be evaluated by titration using a suitable indicator, e.g., Patton and Reeder s indicator or calcon — see Sections 10.48 and 10.60, or by titration with EGTA using zincon as indicator — see Section 10.61. The difference between the two titrations is a measure of the magnesium content. 

The calcium content may then be determined by titration wth EDTA using either Patton and Reeder s indicator or calcon (Section 10.60), or alternatively by titration with EGTA (see Section 10.61). 

More than 50 such dyestuffs have been proposed as indicators,8 but they have in common at least one o-hydroxy group from which the titratable metal displaces a hydrogen atom and forms a five-membered chelate ring with the azo nitrogens (cf 51) most have two o-hydroxy groups and can form two chelate rings. Of the few that are still used calcon (eriochrome blue black R ... 

Some of the metal ion indicators used for calcium and magnesium are discussed below Solochrome dark blue or calcon... 

X 25g calcon carboxylic acid indicator for metal titration... 

The titration of Ca + in the presence of Mg + is of practical importance. The best results are obtained with complexone(III) as the titrant and with calcein or calcon as the indicator. Murexide, as an indicator, is obsolete since both preceding indicators have been introduced. In any case, titrand solutions must be alkaline for the titration reaction to be sufficiently quantitative. Pb + and Zn + must be masked with 2,3-dimercaptopropanol. Some other heavy metal ions must also be masked with potassium cyanide. Fe + and Mn + can be masked by eomplexation with triethanolamine. Here, again, we find what we have already stressed several times about an interest of the eomplexation phenomenon that can be used to dissimulate disturbing ions. Ba +, Sr +, and Ca + are titrated simultaneously. 

With calcon, the color passes from red wine to blue. The reaction is somewhat less sensitive than the preceding one. With this indicator, however, the selective titration of Ca " " in the presence of Mg " " is possible since the pH value is sufficiently high for Mg + to be quantitatively precipitated as hydroxide Mg(OH)2. EDTA does not react with the precipitated magnesium until free Ca + together with that fixed to the indicator are not yet themselves complexed by EDTA. 

It is easily conceivable that a Ca + and Mg + mixture may be globally titrated in such a way with EDTA. The last edition of the European pharmacopeia prescribed the Ca titration with calcon as the indicator. 

A variation of this methodology consists of replacing Patton and Reeder s indicator by calcon as the indicator in the first titration and then in operating as in the above-mentioned conditions. 

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