Use of nickel phthalocyanine sulphonic acid as redox indicator in cerimetry

Summary In part II of this series the titrations of ferrocyanide, hydroquinone and arsenic(III) with cerium(IV) using the indicator nickel phthalocyanine sulphonic acid are studied. The conditions and the limits of reactions are determined.

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Zusammenfassung Im II. Teil dieser Reihe werden die Titrationen von Hexacyanoferrat(II), Hydrochinon und Arsen(III) mit Cer(IV)-Lösung unter Verwendung als Indicators Nickel-phthalocyanin-sulfonsäure studiert. Die Reaktionsbedingungen und ihre Grenzen werden festgelegt.

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Part I: Z. analyt. Chem. 190, 213 (1962).     

Nickel phthalocyanine sulphonic acid as a redox indicator in cerimetry

Summary The use of nickel phthalocyanine sulphonic acid (Ni-PS) as an internal indicator in the cerimetric titration of iron(II) has been investigated. 0.2 ml of a 0.05% solution of the dye serves well as a redox indicator in 50 ml of the titration mixture. The colour change at the end point is from blue to violet. The addition of about 2.5 ml of syrupy phosphoric acid to 50 ml of the titration mixture has been found to be advantageous as it increases the life period of the violet coloured transitory oxidation product of Ni-PS. The Ni-PS indicator is superior to the Cu-PTS indicator previously reported, in that the intermediate violet oxidation product obtained with the former at the equivalence point has a longer life period.It has been observed that in the cerimetric titration of 3–8 ml of about 0.1 N uranium(IV) diluted to 50 ml, the equivalence point is marked by a colour change from turquois blue to colourless, without the appearance of any intermediate violet colour. However, the intermediate violet colour appears, if 3 ml of syrupy phosphoric acid is added before the start of the titration. In the titration of more dilute solutions of uranium(IV) the indicator gives premature and hazy end points. How-ever, in the presence of phosphoric acid, correct equivalence points are obtained.The cerimetric titration of molybdenum(V) using Ni-PS as indicator is recommended to be carried out in 2 N sulphuric acid medium. The addition of phosphoric acid is not necessary. The colour change at the equivalence point is from turquois blue to violet which is stable for 10–15 sec.

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Zusammenfassung Nickel-phthalocyanin-sulfonsäure (NPS) kann als Indicator bei der cerimetrischen Titration von Eisen(II), Uran(IV) und Molybdän(V) mit gutem Erfolg verwendet werden. Auf 50 ml Titrationslösung benötigt man 0,2 ml einer 0,05%igen Lösung des Indicators. Der Farbumschlag am Endpunkt erfolgt von Blau nach Violett. Ein Zusatz von 2,5 ml sirupöser Phosphorsäure auf 50 ml Lösung ist vorteilhaft, da er die Lebensdauer des violett gefärbten Zwischenproduktes der Oxydation von NPS erhöht. Der NPS-Indicator ist wegen der längeren Beständigkeit des erwähnten Zwischenproduktes der früher beschriebenen Kupferphthalocyanin-tetrasulfonsäure überlegen.Bei der cerimetrischen Titration von 3–8 ml ungefähr 0,1 n Uran (IV)-lösung, die auf 50 ml verdünnt wurde, erhält man einen Farbumschlag von Türkisblau nach Farblos, ohne daß das Violett gefärbte Zwischen-produkt auftritt. Wenn man jedoch vor der Titration 3 ml sirupöse Phosphorsäure zusetzt, so erscheint die violette Farbe. Bei der Titration verdünnterer Lösungen ergeben sich vorzeitige und unklare Farbum-schläge. Doch führt auch hier ein Zusatz von Phosphorsäure zu richtigen Werten.Die Titration von Molybdän (V) mit dem beschriebenen Indicator wird am besten in 2n schwefelsaurer Lösung ausgeführt, wobei ein Phosphorsäurezusatz nicht erforderlich ist. Der Farbumschlag erfolgt von Türkisblau nach Violett (Beständigkeit 10–15 sec).

     

2-Nitrodiphenylamine as a versatile high-potential reversible oxidation-reduction indicator

The use of 2-nitrodiphenylamine as a reversible indicator has been investigated in the titration of iron(II) with cerium(IV) sulphate, potassium dichromate and sodium vanadate in sulphuric acid media. Accurate results can be obtained with cerium(IV) sulphate in 0.5–5.0 M acid, with potassium dichromate in 5.0–7.0 M acid, and with sodium vanadate in 5.0–7.5 M acid. With cerium(IV) sulphate the titrations are preferably conducted in 2.0 M sulphuric acid or in a 1.0 M. sulphuric acid-1.0 M pechloric acid medium. Tungstic acid, acetic acid, arsenic(III) and manganese(II) do not interfere. In titrations of iron(II) with dichromate and vanadate, the colour changes at the end-point are much more vivid with 2-nitrodiphenylamine than with ferroin.     

The use of cacotheline as an oxidation-reduction indicator before the volumetric oxidation of iron

Caotheline may be used as an oxidation-ieduction indicator in the removal of stannous tin with cerio sulfate befor the titration of ferrous non     

The calculation of indicator error in oxidation-reduction titrations

Both precise and approximate expressions are derived for the indicator error in oxidation-reduction titrations. Examples are given of the use of these expressions for the choice of the best indicator in titrations with ascorbic acid.     

Oxalate interference in cerimetry

Summary It had been proved that the positive error occurring during cerimetric determination of some substances [e.g. iron(II)-ions, hydroquinone, indigo carmine etc.] in presence of oxalic acid is not of induction character but may be ascribed to a rather rapid competing reaction between cerium(IV) and oxalate ions. Therefore any factor which decreases the rate of this latter reaction diminishes the positive error.     

Use of nitrosochromotropic acid as a metal indicator

Summary Nitrosochromotropic acid and copper give a purple coloured complex in the pH range 5.8–6.5, which is less stable than Cu(II)-EDTA complex. When milligramme quantities of copper solution containing few drops of nitrosochromotropic acid were titrated with EDTA the colour changes to light green or colourless. Copper can be titrated from 15–30° C, but at higher temperature low results are obtained due to dissociation of copper complex. Alkaline earths do not interfere in the titration of copper, but zinc, cadmium, nickel, cobalt, aluminium and iron(III) interfere. The interference of aluminium and iron(III) is masked by sodium fluoride. Microtitration can be carried out using nitrosochromotropic acid screened with xylene cyanol FF as metal indicator.     

Phthalocyanines as oxidation-reduction indicators

Summary Copper phthalocyanine tetrasulphonic acid has been found to work satisfactorily as an inside indicator in the cerimetric titration of hydro-quinone and metol. It has the advantage that the indicator correction in titrations with 0.01 N solutions of ceric sulphate is much less than with the ferroin indicator, amounting to only 0.02 ml for two drops of the indicator solution.Part II: See Z. analyt. Chem. 163, 1 (1958).     

Phthalocyanines as oxidation-reduction indicators

Analytical and Bioanalytical Chemistry -     

Potentiometric titrations in nonaqueous cerimetry

Summary Potassium iodide and hydroquinone can be estimated potentiometrically in nonaqueous medium using ammonium nitrato cerate as oxidant. A platinum indicator electrode coupled with either a glass electrode or an antimony electrode as reference electrode, can be used in nonaqueous medium satisfactorily, for following the potentiometric titration. Direct potentiometric titration of xanthate with ammonium nitrato cerate in nonaqueous medium yields slightly lower values than the actual values in presence of platinum indicator electrode.

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Zusammenfassung Kaliumjodid und Hydrochinon können in nichtwÄ\rigem Medium mit Ammoniumnitratocerat als Oxydationsmittel potentiometrisch bestimmt werden. Dabei wird eine Platin-Indicatorelektrode und eine Glasoder Antimon-Bezugselektrode mit gutem Erfolg benutzt. Die direkte potentiometrische Titration von Xanthat mit Ammoniumnitratocerat in nichtwÄ\rigem Medium gibt mit der Platin-Indicatorelektrode etwas zu niedrige Werte.