Solochrome black indicator

Solochrome black indicator mixture. Triturate 0.20 g of the solid dyestuff with 50 g of potassium chloride. 

Analyses, (a) Original zinc-ion solution. Dilute 2.00 mL (pipette) to 100 mL in a graduated flask. Pipette 10.0 mL of the diluted solution into a 250 mL conical flask, add ca 90 mL of water, 2 mL of the buffer solution, and sufficient of the solochrome black indicator mixture to impart a pronounced red colour to the solution. Titrate with standard 0.01 M EDTA to a pure blue colour (see Section 10.59). 

With calcium ions alone, no sharp end point can be obtained with solochrome black indicator and the transition from red to pure blue is not observed. With magnesium ions, a somewhat less stable complex is formed ... 

Traces of many metals interfere in the determination of calcium and magnesium using solochrome black indicator, e.g. Co, Ni, Cu, Zn, Hg, and Mn. Their interference can be overcome by the addition of a little hydroxylammonium chloride (which reduces some of the metals to their lower oxidation states), or also of sodium cyanide or potassium cyanide which form very stable cyanide complexes ( masking ). Iron may be rendered harmless by the addition of a little sodium sulphide. 

Palladium(II) compounds can be determined by a similar procedure, but in this case, after addition of the cyanonickelate, excess of standard (0.01 M) EDTA solution is added, and the excess is back-titrated with standard (0.01 M) manganese(II) sulphate solution using solochrome black indicator. 

Procedure. To a 50 mL sample of the water to be tested add 1 mL buffer solution (ammonium hydroxide/ammonium chloride, pH 10, Section 10.54) and 30-40 mg solochrome black indicator mixture. Titrate with standard EDTA solution (0.01 M) until the colour changes from red to pure blue. Should there be no magnesium present in the sample of water it is necessary to add 0.1 mL magnesium-EDTA solution (0.1 M) before adding the indicator (see Section 10.54). The total hardness is expressed in parts of CaC03 per million of water. 

Procedure. Prepare a manganese(II) sulphate solution (approx. 0.05M) by dissolving 11.15 g of the analytical-grade solid in 1 L of de-ionised water standardise the solution by titration with 0.05 M EDTA solution using solochrome black indicator after the addition of 0.25 g of hydroxylammonium chloride — see below. 

Ignite 2 g in a silica dish, gently until all the fat has burned and then at about 800° to a white ash. Dissolve in 10 ml dilute hydrochloric acid, heating on a water-bath to assist solution. Dilute to 100 ml with water. To 20 ml add 10 ml of ammonia buffer solution and titrate immediately with 0-05M EDTA using solochrome black indicator. 1 ml 0-05M = 0 002016 g MgO. 

For magnesium To 25 ml of the prepared solution add 1 g of ammonium chloride, swirl to dissolve and then add sufficient triethanolamine to dissolve the precipitate that first forms. Without delay, dilute to about 200 ml with water and add 5 ml of ammonia buffer solution and sufficient solochrome black indicator to give a good red colour. Titrate immediately wdth 0 05M EDTA to the formation of a full blue colour. 1 ml 0-05M - 0-002016 g MgO. 

Dissolve about 0-3 g of mercuric chloride, accurately weighed, in 100 ml of water, add about 40 ml of 0 05M EDTA (this solution need not be accurately standardised), 5 ml of ammonia buffer solution and 0 5 ml of solochrome black indicator and titrate with 0 05M zinc solution until the blue colour changes to purple (do not overshoot the endpoint) add 3 g of potassium iodide, swirl to dissolve, allow to stand for two minutes and then continue the titration with zinc solution to the same end-point as before. Each ml of zinc solution required after addition of the potassium iodide = 0 01358 g HgCl2. 

Sulphate may be determined by precipitation as barium sulphate or as lead sulphate. The precipitate is dissolved in an excess of standard EDTA solution, and the excess of EDTA is back-titrated with a standard magnesium or zinc solution using solochrome black as indicator. 

Add excess of standard EDTA and back-titrate with standard Mg solution using solochrome black as indicator. This gives the sum of all the metals present. 

The indicator colour change is affected by the hydrogen ion concentration of the solution, and no account of this has been taken in the above expression for the formation constant. Thus solochrome black, which may be written as H2In , exhibits the following acid-base behaviour ... 

Solochrome black (eriochrome black T). This substance is sodium 1-(1-hydroxy-2-naphthylazo)-6-nitro-2-naphthol-4-sulphonate, and has the Colour Index reference C.I. 14645. In strongly acidic solutions the dye tends to polymerise to a red-brown product, and consequently the indicator is rarely applied in titrations of solutions more acidic than pH = 6.5. 

This colour change can be observed with the ions of Mg, Mn, Zn, Cd, Hg, Pb, Cu, Al, Fe, Ti, Co, Ni, and the Pt metals. To maintain the pH constant (ca 10) a buffer mixture is added, and most of the above metals must be kept in solution with the aid of a weak complexing reagent such as ammonia or tartrate. The cations of Cu, Co, Ni, Al, Fe(III), Ti(IV), and certain of the Pt metals form such stable indicator complexes that the dyestuff can no longer be liberated by adding EDTA direct titration of these ions using solochrome black as indicator is therefore impracticable, and the metallic ions are said to block the indicator. However, with Cu, Co, Ni, and Al a back-titration can be carried out, for the rate of reaction of their EDTA complexes with the indicator is extremely slow and it is possible to titrate the excess of EDTA with standard zinc or magnesium ion solution. 

Calmagite. This indicator, l-(l-hydroxyl-4-methyl-2-phenylazo)-2-naphthol-4-sulphonic acid, has the same colour change as solochrome black, but the colour change is somewhat clearer and sharper. An important advantage is that aqueous solutions of the indicator are stable almost indefinitely. It may be substituted for solochrome black without change in the experimental procedures for the titration of calcium plus magnesium (see Sections 10.54 and 10.62). Calmagite functions as an acid-base indicator ... 

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. 

Procedure. Prepare the indicators by grinding (a) 0.5 g HHSNNA with 50 g potassium chloride, and (b) 0.2 g solochrome black with 50 g potassium chloride. The following solutions will also be required ... 

Buffer solution. Add 55 mL of concentrated hydrochloric acid to 400 mL de-ionised water and mix thoroughly. Slowly pour 310 mL of redistilled monoethanolamine with stirring into the mixture and cool to room temperature (Note 2). Titrate 50.0 mL of the standard magnesium chloride solution with standard (0.01M) EDTA solution using 1 mL of the monoethanolamine-hydrochloric acid solution as the buffer and solochrome black as the indicator. Add 50.0 mL of the magnesium chloride solution to the volume of EDTA solution required to complex the magnesium exactly (as determined in the last titration), pour the mixture into the monoethanolamine-hydrochloric acid solution, and mix well. Dilute to 1 litre (Note 3). 

The sulphate is precipitated as barium sulphate from acid solution, the precipitate is filtered off and dissolved in a measured excess of standard EDTA solution in the presence of aqueous ammonia. The excess of EDTA is then titrated with standard magnesium chloride solution using solochrome black as indicator. 

Solar energy, 6, 488 surface modified electrodes, 6, 30 Sol-Gel process fast reactor fuel, 6, 924 Solid state reactions, 1, 463-471 fraction of reaction, 1, 464 geometric, 1, 464 growth, 1, 464 nucleation, 1, 464 rate laws, 1,464 Solochrome black T metallochromic indicators, 1,555 Solubility... 

For example, if the indicator being used was solochrome black, the metal-indicator solution would be red while the colour of the free indicator would be blue (in the pH range 7-11). The reaction takes place if the EDTA displaces the indicator from the metal-indicator complex. Therefore the metal-EDTA complex must be more stable thermodynamically than the metal-indicator complex. 

Fig. 23.3 Examples of metal-ion indicators solochrome black and calmagite.

The titration with EDTA, using Solochrome Black (Eriochrome Black T) as indicator gives the total calcium plus magnesium content. To determine the individual elements, calcium may be estimated by titration using Patton and Reeder s indicator. The difference between the two gives the estimate of magnesium. 

The first indicator in this group was proposed by Schwarzenbach and Biedermann in 1948. This was eriochrome black T (solochrome black T, erio T, 52) which behaves as an acid—base indicator with changes from H2ln (red) to Hln - (blue) to In (orange) at pH 6.5 and 11.5, respectively. However in a buffer of pH 10 erio T forms a wine-red 1 1 complex with cadmium, calcium, magnesium, manganese and zinc the colour change is well marked at as low as 10 to 10 M Mg-". Erio T cannot be used for the direct titration of Al, Co, Cu or Ni because it is blocked ... 

For most work in pharmaceutical analysis classical methods have now been superseded by complexometric titration. Determination of calcium by itself presents little difficulty and two basic procedures are applicable, (i) titration at pH 10 in ammonia buffer using solochrome black as indicator and (ii) titration at pH 12 to 13, either in diethylamine using alizarin black (diadem chrome black) or in potassium hydroxide solution using Patton and Reeder s indicator. Provided a small quantity of complexed magnesium is included for titration (i) sharp and satisfactory end-points are obtained by these methods and there is no significant difference in the results obtained at the two pH values if a pure calcium compound is titrated. Difference in the two results would be expected if the calcium salts were contaminated with magnesium, when the titration at pH 10 would include both ions and that at the higher value the calcium only. 

Dissolve 1 g in 10 ml of concentrated hydrochloric acid by warming on a water-bath. Dilute with 50 ml of water, cool, transfer to a 250 ml graduated flask and dilute to volume with water. Take 25 ml in a 500 ml flask, add 30 ml of 0 05M EDTA, 10 ml of ammonia buffer solution and 100 ml of water and titrate with 0-1N zinc to solochrome black as indicator. 1 mlOOSM FDTA - 0 005170 g Ca CPO ).. ... 

Transfer a 20 ml aliquot of solution A to a flask, dilute to about 50 ml with water and neutralise with 20 per cent sodium hydroxide solution. Add 5 ml of ammonia buffer solution and titrate with 0-05M EDTA using solochrome black as indicator. Let the ml 0 05M EDTA required be a ml. 

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