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Selective chelatometry of copperII with ethylenediamine- N,N,N',N'-tetra-n-propionic acid
Talanta, 1962, Vol. 9, pp. 71 to 73. aWgamon Ptwa Ltd. Prlnrcd In Nartkm
PRELIMINARY
Ireland
COMMUNICATIONS
Selective chelatometry of coppeG with e~ylen~i~n~ N,N,N’,N’-tetra-n-propionic acid (Received 26 October 1961) IN a recent review of some aspects of the selectivity of chelatometric
titrations it has been indicated that, without recourse to auxihary complexing agents, selectivity of titration with reagents of the complexan group is only possible by the simultaneous availability of selective chelating agent and selective pM indicator.Z In the same paper, the high selectivity of ethylenediamine-N,N,N’,N’-tetra-npropionic acid, referred to subsequently as EDTP, and the -N,N’-di-n-propionic acid (EDDP) towards copper11 ion* was stressed. The strength of the copperi* chelates of these reagents relative to their chelates with magnesium, cadmium, etc. was attributed to the (relatively) uncommon square-coplanar habit of the copperrrion and the presence in both reagents of the
\
/ N-CHaCHBN
chain.
The
/ \ replacement of acetic acid groups (EDTA) by n-propionic acid groups critically weakens the strength of the primary chelate ring for most metal ions and results in low stability constants for the EDTP and EDDP chelates of magnesium (lo’**, lo’*‘), cadmium (10 ‘.‘, lo’.“), etc.,a whereas the steric disposition of the copper11 bonding and its well known tendency to react well with molecules containing /
\
groups ensures efficient chelation of copper (1016**1016.Z).
N-CH,-CH,--N /
\
The above findings suggested to us that these reagents should serve as useful tin-ants for coppern in the presence of several other ions; but such a reagent is of no avail with conventional metallochromic indicators since these will respond to the presence of other metal ions such as magnesium, etc. However, two virtually specific indicators for copper I1 have been described previously viz. Fast Sulphon Black F3 and Ponceau 3R.’ The first of these indicators responds only to coppern and nickel and the second also to copper and nickel. In both cases the quality of the nickel end-point is poor_ In bur preliminary experiments, of which this communication is a brief report, the EDTP/Ponceau 3R system was explored using 10-*&f solutions of copperIt, other metals and the complexan. The titrations were carried out using an Agla micrometer syringe burette, with a total volume of titration medium of approximately 1 ml. Ponceau 3R works most effectively at pH * 9 in an ammoniacal medium, when the colour change is from yellow to red. The quality of the end-point in the EDTP titration of copperrI is only slightly inferior to that with the considerably stronger complexan FDTA. Interference studies were carried out with ca. 60 ,q of copperlrand varying concentrations of magnesium, calcium, barium, strontium, cadmium, manganese, zinc, lead, cobalt, nickel and aluminium. When EDTA was used as titrant all of these titrated additively with the copper. With EDTP, however, no interference was found from 10 molar ratios of magnesium or calcium or from 5 molar ratios of strontium, barium and cadmium. No attempt was made to find the upper limit of non-interference of these five ions. Manganesei did not interfere up to a 2 molar ratio, but larger amounts promoted sluggish end-points and low copper recoveries, presumably caused by copr~ipitation of some copper with manganese hydroxide. Cobalt interfered, of course, by virtue of its colour and zinc and lead could not be held in solution at pH 9. A slight raising of the pH to 10 allowed the titration of copper in the presence of a 1 molar ratio of zinc, though the quality of the end-point was considerably impaired at this pH. Nickel titrated as copper with a sluggish end-point while aluminium could be maintained harmlessly in solution by the addition of fluoride ion. 71
12
Preliminary communications
The procedure was applied successfully to the small-scale analysis of copper in a standard brass sample (70.8 % of Cu, 24.17 % of Zn, 1.85 % of Sn, 2.52 % of Pb, 0.31% of Fe, 0.12 % of Mn, 0.17 % of Ni). The brass was dissolved in the usual way, the solution diluted, the pH adjusted to 9-10 and the copper titrated directly with EDTP without significant interference from the other elements present. Further more detailed experiments will shortly be initiated withFast SulphonBlackF* as indicator. This indicator functions at pH = 11 where aluminium, lead, zinc, etc. can be held in solution. The indicator action of Ponceau 3R is rather dependent on electrolyte concentrations and pH; but it was used in these preliminary small-scale experiments because it is rather more sensitive. EXPERIMENTAL Reagents 10-2M EDTP: Prepared by condensation of 3-chloro-n-propionic acid with ethylenediamine. 10-2M CuSO,: Standardised against EDTA. Indicator: 0.25 % aqueous Ponceau 3R. E.@r: 54 g of NH&l plus 22 ml of NH, solution (sp. gr. 0.88), diluted to 1 litre. I. P. SANDERSON T. S. WEST
Chemistry Department University, Edgbaston Birmingham 15, England
Summary--A highly selective complexometric titration of copperrr is reported using the complexan ethylenediamine-N,N,N’,N’-tetra-n-propionic acid and a selective metallochromic indicator.
Zusammenfassung-Es wird eine Methode fur eine hoch-selektive complexometrische Kupfer(II) mittels. Athylendiamin-N,N’-tetra-n-propionsatire Indikator angelgeben.
Titrierung von und ein selektives metallochromisches
ResunG-On d&it une methode complexometrique pour le dosage de cuivre (II) par l’acide ethylene&amine-N,N,N’,N’-tetra-n-propionique avec un indicateur metallochromique. REFERENCES 1 T. S. West, Analyt. Chim. Acta, 1961, 25, 301. 2 R. C. Courtney, S. Chaberek and A. E. Martell, J. Amer. Chem. Sot., 1953, 75, 4814. s R. Belcher, R. A. Close and T. S. West, Chem. and Znd., 1957, 1647. 4 Zdem, Chemist-Analyst, 1957, 46, 86.
Analytical aspects of the oxidation of organic nitrogen compounds with chromii: acid (Received 4 September 1961.) THE
oxidation of a series of organic nitrogen compounds from different classes with chromic acid in a medium of dilute sulphuric acid has been systematically investigated. From the consumption of the oxidising agent, oxidation numbers have been calculated and defined as the amount of oxygen in grams consumed by the oxidation of 100 grams of the substance. The oxidation numbers calculated from the experimental data have been compared with the theoretical ones that had been calculated from the oxidation equations for the tested compounds. In most cases a good agreement was found, the following processes being taken into consideration : Carbon and hydrogen are oxidised to carbon dioxide and water, respectively, nitro and nitroso groups (attached to carbon, nitrogen or oxygen) being oxidised to nitric acid: two nitrogen atoms attached to each other as in azo compounds, hydrazines, pyrazoles, etc., are split off in the form of elementary nitrogen and aromatic amino groups are converted into ammonia. From halogen compounds, chlorine and bromine are liberated in the form of the elements, while iodine is oxidised to iodic acid. All groups containing sulphur are ultimately oxidised to sulphuric acid.
PRELIMINARY
Ireland
COMMUNICATIONS
Selective chelatometry of coppeG with e~ylen~i~n~ N,N,N’,N’-tetra-n-propionic acid (Received 26 October 1961) IN a recent review of some aspects of the selectivity of chelatometric
titrations it has been indicated that, without recourse to auxihary complexing agents, selectivity of titration with reagents of the complexan group is only possible by the simultaneous availability of selective chelating agent and selective pM indicator.Z In the same paper, the high selectivity of ethylenediamine-N,N,N’,N’-tetra-npropionic acid, referred to subsequently as EDTP, and the -N,N’-di-n-propionic acid (EDDP) towards copper11 ion* was stressed. The strength of the copperi* chelates of these reagents relative to their chelates with magnesium, cadmium, etc. was attributed to the (relatively) uncommon square-coplanar habit of the copperrrion and the presence in both reagents of the
\
/ N-CHaCHBN
chain.
The
/ \ replacement of acetic acid groups (EDTA) by n-propionic acid groups critically weakens the strength of the primary chelate ring for most metal ions and results in low stability constants for the EDTP and EDDP chelates of magnesium (lo’**, lo’*‘), cadmium (10 ‘.‘, lo’.“), etc.,a whereas the steric disposition of the copper11 bonding and its well known tendency to react well with molecules containing /
\
groups ensures efficient chelation of copper (1016**1016.Z).
N-CH,-CH,--N /
\
The above findings suggested to us that these reagents should serve as useful tin-ants for coppern in the presence of several other ions; but such a reagent is of no avail with conventional metallochromic indicators since these will respond to the presence of other metal ions such as magnesium, etc. However, two virtually specific indicators for copper I1 have been described previously viz. Fast Sulphon Black F3 and Ponceau 3R.’ The first of these indicators responds only to coppern and nickel and the second also to copper and nickel. In both cases the quality of the nickel end-point is poor_ In bur preliminary experiments, of which this communication is a brief report, the EDTP/Ponceau 3R system was explored using 10-*&f solutions of copperIt, other metals and the complexan. The titrations were carried out using an Agla micrometer syringe burette, with a total volume of titration medium of approximately 1 ml. Ponceau 3R works most effectively at pH * 9 in an ammoniacal medium, when the colour change is from yellow to red. The quality of the end-point in the EDTP titration of copperrI is only slightly inferior to that with the considerably stronger complexan FDTA. Interference studies were carried out with ca. 60 ,q of copperlrand varying concentrations of magnesium, calcium, barium, strontium, cadmium, manganese, zinc, lead, cobalt, nickel and aluminium. When EDTA was used as titrant all of these titrated additively with the copper. With EDTP, however, no interference was found from 10 molar ratios of magnesium or calcium or from 5 molar ratios of strontium, barium and cadmium. No attempt was made to find the upper limit of non-interference of these five ions. Manganesei did not interfere up to a 2 molar ratio, but larger amounts promoted sluggish end-points and low copper recoveries, presumably caused by copr~ipitation of some copper with manganese hydroxide. Cobalt interfered, of course, by virtue of its colour and zinc and lead could not be held in solution at pH 9. A slight raising of the pH to 10 allowed the titration of copper in the presence of a 1 molar ratio of zinc, though the quality of the end-point was considerably impaired at this pH. Nickel titrated as copper with a sluggish end-point while aluminium could be maintained harmlessly in solution by the addition of fluoride ion. 71
12
Preliminary communications
The procedure was applied successfully to the small-scale analysis of copper in a standard brass sample (70.8 % of Cu, 24.17 % of Zn, 1.85 % of Sn, 2.52 % of Pb, 0.31% of Fe, 0.12 % of Mn, 0.17 % of Ni). The brass was dissolved in the usual way, the solution diluted, the pH adjusted to 9-10 and the copper titrated directly with EDTP without significant interference from the other elements present. Further more detailed experiments will shortly be initiated withFast SulphonBlackF* as indicator. This indicator functions at pH = 11 where aluminium, lead, zinc, etc. can be held in solution. The indicator action of Ponceau 3R is rather dependent on electrolyte concentrations and pH; but it was used in these preliminary small-scale experiments because it is rather more sensitive. EXPERIMENTAL Reagents 10-2M EDTP: Prepared by condensation of 3-chloro-n-propionic acid with ethylenediamine. 10-2M CuSO,: Standardised against EDTA. Indicator: 0.25 % aqueous Ponceau 3R. E.@r: 54 g of NH&l plus 22 ml of NH, solution (sp. gr. 0.88), diluted to 1 litre. I. P. SANDERSON T. S. WEST
Chemistry Department University, Edgbaston Birmingham 15, England
Summary--A highly selective complexometric titration of copperrr is reported using the complexan ethylenediamine-N,N,N’,N’-tetra-n-propionic acid and a selective metallochromic indicator.
Zusammenfassung-Es wird eine Methode fur eine hoch-selektive complexometrische Kupfer(II) mittels. Athylendiamin-N,N’-tetra-n-propionsatire Indikator angelgeben.
Titrierung von und ein selektives metallochromisches
ResunG-On d&it une methode complexometrique pour le dosage de cuivre (II) par l’acide ethylene&amine-N,N,N’,N’-tetra-n-propionique avec un indicateur metallochromique. REFERENCES 1 T. S. West, Analyt. Chim. Acta, 1961, 25, 301. 2 R. C. Courtney, S. Chaberek and A. E. Martell, J. Amer. Chem. Sot., 1953, 75, 4814. s R. Belcher, R. A. Close and T. S. West, Chem. and Znd., 1957, 1647. 4 Zdem, Chemist-Analyst, 1957, 46, 86.
Analytical aspects of the oxidation of organic nitrogen compounds with chromii: acid (Received 4 September 1961.) THE
oxidation of a series of organic nitrogen compounds from different classes with chromic acid in a medium of dilute sulphuric acid has been systematically investigated. From the consumption of the oxidising agent, oxidation numbers have been calculated and defined as the amount of oxygen in grams consumed by the oxidation of 100 grams of the substance. The oxidation numbers calculated from the experimental data have been compared with the theoretical ones that had been calculated from the oxidation equations for the tested compounds. In most cases a good agreement was found, the following processes being taken into consideration : Carbon and hydrogen are oxidised to carbon dioxide and water, respectively, nitro and nitroso groups (attached to carbon, nitrogen or oxygen) being oxidised to nitric acid: two nitrogen atoms attached to each other as in azo compounds, hydrazines, pyrazoles, etc., are split off in the form of elementary nitrogen and aromatic amino groups are converted into ammonia. From halogen compounds, chlorine and bromine are liberated in the form of the elements, while iodine is oxidised to iodic acid. All groups containing sulphur are ultimately oxidised to sulphuric acid.