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Determination of aluminium by homogeneous precipitation of basic aluminium benzoate
R&tune a 6tudie le comportement polarographique de 35 ions en solution dans le fluorure d’ammonium 1 M ajust il pH 7. Dans cet electrolyte de base le plomb est reduit reversiblement et, sauf lorsqu’il y a des quantites considerablement plus elev5es de cuivre, bismuth, cadmium, antimoine(III) et fer(III), les seuls ions qui interfere& dam le dosage du plomb par polarographie en courant continu sont le vanadium(V), le thallium(I), le telhuium(IV), l’uranium(V1) et l’&ain(II). En amenant le vanadium, le tellurium et l&in aux degreS d’oxydation $4, +6 et +4 respectivement, on peut &miner l’interference de ces elements et disposer dune m&ode polarographique sClective du plomb. Zusannne&ssung-Das polarographische Verhalten von 35 Ionen in 1 M Ammonfluoridlijsung bei pH 7 wurde untersucht. In diesem Triigerelektrolyten wird Blei reversibel reduziert. AuDer betmchtlich griiberen Mengen Kupfer, Wismut, Cadmium, Antimon(III) und Eisen(II1) stiiren die gleichstrompolarographische Bestimmung von Blei nur Vanadin(V), Thallium(I), Tellur(IV), Uran(VI) tmd Zirm(I1). Bringt man Vanadin, Teller und Zinn auf die Oxydationsstufen +4, $6 und f4, ist die Storung durch diese Elemente beseitigt und Blei krum selektiv polarographisch bestimmt werden. REFERENCES 1 D. R. Wood and R. M. Cook, Metallurgia, 1963, 67, 109. * J. B. Headridge, A. G. Hamxa, D. P. Hubbard and M. S. Taylor, Proc. Thirdlnternat. Gong. Polarog. Macmillan and Co. Ltd., London, in press. a J. B. Headridge and M. S. Taylor, Analytical Chemistry 1962, ed. West, Macdonald and West. Elsevier, Amsterdam, 1963, p. 382. d H. P. Raaen, Analyt. Chem., 1962,&t, 1714 s D. J. Ferrett and G. W. C. Mimer, Analyst, 1956, 81, 193
Determination of aluminium by homogeneous precipitation of basic aluminium benzoate (Received 16 June 1965. Accepted 5 August 1965) HOMOGENEOUS methods of precipitation are well established and their uses and advantages have been thoroughly reviewed by Gordon, Salutsky and Willard.’ To give a readily alterable form of aluminium hydroxide Willard and Tang carried out the precipitation in the presence of succinate ion, claiming that this gave the densest precipitate of several anions chosena There is, however, the disadvantage that the basic aluminium succinate forms a tenaciously adherent lilm on the walls of the beaker and this must be dissolved in hydrochloric acid and reprecipitated with aqueous ammonia. The modification described in this paper, namely, the use of benzoate rather than succinate buffer, overcomes this objection and enables the aluminium to be precipitated quantitatively as basic aluminium benzoate in a single operation. The crystalline precipitate is not as dense as the basic succinate but it can be f&red rapidly and washed efficiently; furthermore, any deposit on the walls of the beaker is easily removed mechanically. EXPERIMENTAL Reagents Standard aluminium solution. Pure aluminium (0.8848 g) was dissolved in dilute hydrochloric acid and this solution was diluted to 1 litre. Benzoate bu&. Ammonium chloride (48 g) and benzoic acid (18 g) were dissolved in 2 litres of water.
Short communications
1047
Procedure pH of precipitation of basic aluminium benzoate.
To 350 ml of the buffer solution were added 3 g of urea and 50 ml of the standard aluminium solution which had been neutral&d by adding aqueous ammonia until there was a faint permanent precipitate, followed by just enough dilute hydrochloric acid to redissolve the precipitate. The solution was brought raprdly to boiling point, then kept on a steam bath for the duration of the experiment. Samples (20 ml) were taken at intervals, cooled rapidly and the pH determined at 25” with a glass-electrode system. After filtration the ahnnimum remaining in solution was determined with Aluminon? Determination of aluminium. To 350 ml of the benxoate buffer were added 3 g of urea and the standard aluminit& solution containing 0~01405 g of ahunimum (made just aGd). The solution was brought rapidly to the boil, kept on a steam bath for 1.5-2 hr, filtered through a medium aper and the precipitate washed thoroughly with 1% ammonium nitrate solution. The paper an B pry+ cipitate were then ignited to constant weight at 1250”in a platinum crucible and the product weighed as Al,O* RESULTS AND DISCUSSION As a result of several experiments it was found that precipitation of ahnninium was invariably complete at pH 3.7 and that there was a sufficiently slow increase of pH with time. In a series of experiments -using the same volume of solution, the pH after 60 n&r was remarkably constant. There was some variation depending on the amount of aluminium present: pH 3.7 for 0.08 g of aluminium oxide to pH 4.1 for no aluminium. The results of a typical experiment are given in Table I. TABLEI Time, min 30 50 z 90 120
pH at 25”
Aluminium in solution, mg of Al,O&tOO ml
3.15 3.63 3.70 3.80 4.00 4.08
6 0.2
Similar experiments were carried out using phthalic acid and monosodium phosphate buffers but, although the aluminium could be precipitated quantitatively, there were other disadvantages compared with the benxoate method. A comparison of the well-established ammonia method,’ the succinate method and the benzoate method was carried out. Any precipitate which could not be mechanically removed from the walls of the beaker was ignored, because it was felt that the complication introduced by dissolving and reprecipitating was not warranted. The error occasioned by this procedure was so great with the succinate method with amounts of aluminium less than 0.05 mg of aluminium oxide that the method became useless. The precipitates were ignited to constant weight at 1250°.6 TABLEII Al taken, 8 Of A1~O~
Al,O, found, g Ammonia
Succinate
Benzoate
0.0836
0.0835 0.0833 0.0838
0.0834 0.0827 0.0810
0.0337
0.0332 0.0331 0.0335
0.0167
0.0150 0.0160 0.0166
0.0840 0.0835 0.0841 0.0834 0.0337 0.0337 0.0339 0.0338 0.0168 0.0169 0.0165
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1048
Short communications
The results given in Table II show that the benzoate method is at least as reliable as the ammonia method and is much faster. Effect of beryllium It was hoped that the close control of pH obtainable by the hydrolysis of urea in the presence of the benzoate buffer would permit the determination of aluminium in the presence of beryllium, because the figures quoted by B&tons show that beryllium hydroxide is precipitated at a higher pH than aluminium hydroxide. Using the method described for aluminium it was found that even at pH 4.3 no beryllium hydroxide was precipitated in the presence of benzoic acid and because aluminium hydroxide was completely precipitated at pH 3.7 it seemed possible to effect a separation. Using 330 ml of the benzoate buffer solution, 3 g of urea, 50 ml of standard aluminium solution, 20 ml of standard beryllium chloride solution (equivalent to 0.0360 g of beryllium oxide) and boiling gently for 90 min, the pH rose to 40. The precipitate was filtered, washed with 1% ammonium nitrate solution, adjusted to pH 4.0 and ignited to constant weight at 1250”. Beryllium remaining in the filtrate was determined by precipitation with aqueous ammonia and igniting to ReO. The ignited aluminium precipitate was fused with sodium carbonate, in which beryllium oxide is insoluble. In this way it was shown that OX@38g of beryllium oxide coprecipitated with the aluminium. Treatment of the beryllium precipitate in the same way showed that no aluminium was coprecipitated, i.e., all the aluminium had been precipitated at pH 40. A series of experiments was carried out using various quantities of aluminium and beryllium, but keeping the total volume at 400 ml by taking the appropriate volume of buffer solution. The results are given in Table III. TABLE
III Amount found, g
Amount taken, g Al as AlaOs
Be as Be0
Total
AIBOs
Be0
Total
0.0834 0.0834 0.0834 0.0834 0.0834
0.0360 0.0360 0.0360 0.0360 0.0180
0.1194 0.1194 0.1194 0.1194 0.1014
0.0334 0.0334 0.0167
OGWI 0.0360 0*0900
0.1234 0.0694 0.1067
0.0867 0.0872 0.0854 00869 0.0864 0.0854 0.0363 0.0350 0.0184
0.0327 CO325 0.0338 0.0327 00179 0.0871 0.0342 0.0891
0.1194 0.1197 0.1192 0.1196 0.1043 0.1234 0.0692 0.1075
These results show that, in every case, beryllium is coprecipitated with aluminium and that the benzoate and also, of course, the succinate method cannot be used for the determination of aluminium in the presence of beryllium. It is difficult to explain this coprecipitation of beryllium because the quantity involved appears to bear no simple relationship either to the amount of aluminium or beryllium originally present in solution but it is reasonably constant at 3 mg. Britton found that the titration curve for beryllium sulphate had two distinct curves in the acid zone, and that precipitation did not occur until pH 5.7, the start of the second zone, i.e., when more than one equivalent of base had been added. The Crst zone showed little change of pH as the amount of base was increased. This zone corresponds to the formation of BeSO,.@eOH), and the pH during this state does not rise much above 4. Because the weight of beryllium copreci itated with aluminium is virtually constant, it is felt that this is a pH rather than an adsorption e Bect and that for some reason which is not readily explicable, beryllium hydroxide starts to precipitate as low as pH 3.7, i.e., without the formation of the soluble Be*+.Be(OH), species, iu the presence of aluminium hydroxide. It seems, therefore, that these two elements cannot be separated by any method which relies on the precipitation of their hydroxides. Department of Chemistry Bakersea College of Teihnology London S. W.11, England
R. J.
IRVING
Short communications
1049
Summary-Aluminium can be determined gravimetrically by the homogeneous precipitation of basic aluminium benzoate. The benzoate method is more rapid and has other advantages over the basic succinate and ammonia meihods. It is concluded that aluminium can never be separated from beryllium by any method which relies on precipitation of their hydroxides or basic salts. Zusannnenfassnng-Aluminium kann durch homogene Fallung von basischem Aluminiumbenzoat gravimetrisch bestimmt werden. Die Benzoatmethode geht schneller und hat noch andere Vorteile vor den Bernsteinsilureund Ammoniakmethoden. Ahnninium kann von Beryllium auf keine Art getrennt werden, die auf der F&lhmg von Hydroxyden oder baa&hen Salzen beruht. R&snm&On peut doser l’aluminium gravimetriquement par precipitation en milieu homogene du benzoate basique d’aluminium. La methode au benzoate est plus rapide, et a d’autres avantages par rapport aux methodes au succinate basique et a l’ammoniaque. On conclut que l’ahuninium ne peut jamais bre s&pare du beryllium au moyen dune m&ode reposant sur la precipitation de leurs hydroxydes ou sels basiques. REFERENCES 1 L. Gordon, M. L. Salutsky and H. H. Willard, Precipitation from Homogeneous Solution. John Wiley and Sons, Inc., New York, 1959. s H. H. Willard and N. K. Tang, Znd. Eng. Chem., Analyt., 1937,9,357. * 0. B. Winter, W. E. Thrun and 0. D. Bird, J. Amer. Chem. Sot., 1929,51,2721. 4 W. F. Hillebrand, G. E. F. Lundell, H. A. Bright and J. I. Hoffmann, Applied Inorganic Analysis. John Wiley and Sons, Inc., New York, 1953. 5 0. I. Milner and L. Gordon, TaIanta, 1960,4,115. e H. T. S. Britton, J. Chem. Sot., 1925, 127,212O.
Sur un principe de colorim&rie en milieu non aqueux-IV: L’emploi du 2,4-dinitropl16nol pour le dosage des acides carboxyliques (Regu le 8 Juillet 1965. Acceptd le 8 Aout 1965) LES esters p-nitrophenyles d’acylamino-acides ont pn%demment et& obtenus* par condensation du carboxyle sur lep-nitrophenol en presence de dicyclohexylcarbodiimide. Selon nos es&s,* on accede de m&me aux esters 2&dinitrophenyles par action du 2&dinitrophenol sur les acides carboxyliques. La presence des deux groupes nitr6.s en me’ta permet par ailleurs une identification wlorimetrique en milieu non aqueux. Sur un principe pn%demment enonce,* il est en effet possible, a partir de l’acide carboxylique, de proceder ii la formation de l’ester puis, par I’emploi de solvants wnvenables, de developper d’embl&e une coloration, sans isolement pr6alable de pester ni &nination de l’exc& de reactif. ApreS acylation par le 2&dinitrophenol en nitromethane, et en presence de dicyclohexylcarbodiimide, l’addition d’hydroborure de potassiumt en dimethylformamide dtveloppe une coloration rouge (560 rnp) qui n’est pas observee en l’absence de carboxyle. La loi de Beer Btant satisfaite, des dosages sont possibles sur des prises d’essai de l’ordre de quelques centiemes de milligramme (Tableau I). La reaction est negative avec l’acide formique. Avec les acides essay&s, I’intensite de la coloration obtenue est inversement proportionnelle au poids moleculaire.
Melange ii volumes egaux, prepat% extemporanement, de solution a 0,5 p. cent de 2&iinitrophenol dans le nitromethane et de solution a 5 p. cent dam le meme solvant de N,N’-dicyclohexylcarbodiimide prepark selon Amiard et Heym&s.l * Et%ctues avec la collaboration de Melle M. Pepin. t Le m6me reactif a ttt utilise pour la colorimetrie des groupes 2,4-dinitroph6nylamin&s.4
Determination of aluminium by homogeneous precipitation of basic aluminium benzoate (Received 16 June 1965. Accepted 5 August 1965) HOMOGENEOUS methods of precipitation are well established and their uses and advantages have been thoroughly reviewed by Gordon, Salutsky and Willard.’ To give a readily alterable form of aluminium hydroxide Willard and Tang carried out the precipitation in the presence of succinate ion, claiming that this gave the densest precipitate of several anions chosena There is, however, the disadvantage that the basic aluminium succinate forms a tenaciously adherent lilm on the walls of the beaker and this must be dissolved in hydrochloric acid and reprecipitated with aqueous ammonia. The modification described in this paper, namely, the use of benzoate rather than succinate buffer, overcomes this objection and enables the aluminium to be precipitated quantitatively as basic aluminium benzoate in a single operation. The crystalline precipitate is not as dense as the basic succinate but it can be f&red rapidly and washed efficiently; furthermore, any deposit on the walls of the beaker is easily removed mechanically. EXPERIMENTAL Reagents Standard aluminium solution. Pure aluminium (0.8848 g) was dissolved in dilute hydrochloric acid and this solution was diluted to 1 litre. Benzoate bu&. Ammonium chloride (48 g) and benzoic acid (18 g) were dissolved in 2 litres of water.
Short communications
1047
Procedure pH of precipitation of basic aluminium benzoate.
To 350 ml of the buffer solution were added 3 g of urea and 50 ml of the standard aluminium solution which had been neutral&d by adding aqueous ammonia until there was a faint permanent precipitate, followed by just enough dilute hydrochloric acid to redissolve the precipitate. The solution was brought raprdly to boiling point, then kept on a steam bath for the duration of the experiment. Samples (20 ml) were taken at intervals, cooled rapidly and the pH determined at 25” with a glass-electrode system. After filtration the ahnnimum remaining in solution was determined with Aluminon? Determination of aluminium. To 350 ml of the benxoate buffer were added 3 g of urea and the standard aluminit& solution containing 0~01405 g of ahunimum (made just aGd). The solution was brought rapidly to the boil, kept on a steam bath for 1.5-2 hr, filtered through a medium aper and the precipitate washed thoroughly with 1% ammonium nitrate solution. The paper an B pry+ cipitate were then ignited to constant weight at 1250”in a platinum crucible and the product weighed as Al,O* RESULTS AND DISCUSSION As a result of several experiments it was found that precipitation of ahnninium was invariably complete at pH 3.7 and that there was a sufficiently slow increase of pH with time. In a series of experiments -using the same volume of solution, the pH after 60 n&r was remarkably constant. There was some variation depending on the amount of aluminium present: pH 3.7 for 0.08 g of aluminium oxide to pH 4.1 for no aluminium. The results of a typical experiment are given in Table I. TABLEI Time, min 30 50 z 90 120
pH at 25”
Aluminium in solution, mg of Al,O&tOO ml
3.15 3.63 3.70 3.80 4.00 4.08
6 0.2
Similar experiments were carried out using phthalic acid and monosodium phosphate buffers but, although the aluminium could be precipitated quantitatively, there were other disadvantages compared with the benxoate method. A comparison of the well-established ammonia method,’ the succinate method and the benzoate method was carried out. Any precipitate which could not be mechanically removed from the walls of the beaker was ignored, because it was felt that the complication introduced by dissolving and reprecipitating was not warranted. The error occasioned by this procedure was so great with the succinate method with amounts of aluminium less than 0.05 mg of aluminium oxide that the method became useless. The precipitates were ignited to constant weight at 1250°.6 TABLEII Al taken, 8 Of A1~O~
Al,O, found, g Ammonia
Succinate
Benzoate
0.0836
0.0835 0.0833 0.0838
0.0834 0.0827 0.0810
0.0337
0.0332 0.0331 0.0335
0.0167
0.0150 0.0160 0.0166
0.0840 0.0835 0.0841 0.0834 0.0337 0.0337 0.0339 0.0338 0.0168 0.0169 0.0165
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1048
Short communications
The results given in Table II show that the benzoate method is at least as reliable as the ammonia method and is much faster. Effect of beryllium It was hoped that the close control of pH obtainable by the hydrolysis of urea in the presence of the benzoate buffer would permit the determination of aluminium in the presence of beryllium, because the figures quoted by B&tons show that beryllium hydroxide is precipitated at a higher pH than aluminium hydroxide. Using the method described for aluminium it was found that even at pH 4.3 no beryllium hydroxide was precipitated in the presence of benzoic acid and because aluminium hydroxide was completely precipitated at pH 3.7 it seemed possible to effect a separation. Using 330 ml of the benzoate buffer solution, 3 g of urea, 50 ml of standard aluminium solution, 20 ml of standard beryllium chloride solution (equivalent to 0.0360 g of beryllium oxide) and boiling gently for 90 min, the pH rose to 40. The precipitate was filtered, washed with 1% ammonium nitrate solution, adjusted to pH 4.0 and ignited to constant weight at 1250”. Beryllium remaining in the filtrate was determined by precipitation with aqueous ammonia and igniting to ReO. The ignited aluminium precipitate was fused with sodium carbonate, in which beryllium oxide is insoluble. In this way it was shown that OX@38g of beryllium oxide coprecipitated with the aluminium. Treatment of the beryllium precipitate in the same way showed that no aluminium was coprecipitated, i.e., all the aluminium had been precipitated at pH 40. A series of experiments was carried out using various quantities of aluminium and beryllium, but keeping the total volume at 400 ml by taking the appropriate volume of buffer solution. The results are given in Table III. TABLE
III Amount found, g
Amount taken, g Al as AlaOs
Be as Be0
Total
AIBOs
Be0
Total
0.0834 0.0834 0.0834 0.0834 0.0834
0.0360 0.0360 0.0360 0.0360 0.0180
0.1194 0.1194 0.1194 0.1194 0.1014
0.0334 0.0334 0.0167
OGWI 0.0360 0*0900
0.1234 0.0694 0.1067
0.0867 0.0872 0.0854 00869 0.0864 0.0854 0.0363 0.0350 0.0184
0.0327 CO325 0.0338 0.0327 00179 0.0871 0.0342 0.0891
0.1194 0.1197 0.1192 0.1196 0.1043 0.1234 0.0692 0.1075
These results show that, in every case, beryllium is coprecipitated with aluminium and that the benzoate and also, of course, the succinate method cannot be used for the determination of aluminium in the presence of beryllium. It is difficult to explain this coprecipitation of beryllium because the quantity involved appears to bear no simple relationship either to the amount of aluminium or beryllium originally present in solution but it is reasonably constant at 3 mg. Britton found that the titration curve for beryllium sulphate had two distinct curves in the acid zone, and that precipitation did not occur until pH 5.7, the start of the second zone, i.e., when more than one equivalent of base had been added. The Crst zone showed little change of pH as the amount of base was increased. This zone corresponds to the formation of BeSO,.@eOH), and the pH during this state does not rise much above 4. Because the weight of beryllium copreci itated with aluminium is virtually constant, it is felt that this is a pH rather than an adsorption e Bect and that for some reason which is not readily explicable, beryllium hydroxide starts to precipitate as low as pH 3.7, i.e., without the formation of the soluble Be*+.Be(OH), species, iu the presence of aluminium hydroxide. It seems, therefore, that these two elements cannot be separated by any method which relies on the precipitation of their hydroxides. Department of Chemistry Bakersea College of Teihnology London S. W.11, England
R. J.
IRVING
Short communications
1049
Summary-Aluminium can be determined gravimetrically by the homogeneous precipitation of basic aluminium benzoate. The benzoate method is more rapid and has other advantages over the basic succinate and ammonia meihods. It is concluded that aluminium can never be separated from beryllium by any method which relies on precipitation of their hydroxides or basic salts. Zusannnenfassnng-Aluminium kann durch homogene Fallung von basischem Aluminiumbenzoat gravimetrisch bestimmt werden. Die Benzoatmethode geht schneller und hat noch andere Vorteile vor den Bernsteinsilureund Ammoniakmethoden. Ahnninium kann von Beryllium auf keine Art getrennt werden, die auf der F&lhmg von Hydroxyden oder baa&hen Salzen beruht. R&snm&On peut doser l’aluminium gravimetriquement par precipitation en milieu homogene du benzoate basique d’aluminium. La methode au benzoate est plus rapide, et a d’autres avantages par rapport aux methodes au succinate basique et a l’ammoniaque. On conclut que l’ahuninium ne peut jamais bre s&pare du beryllium au moyen dune m&ode reposant sur la precipitation de leurs hydroxydes ou sels basiques. REFERENCES 1 L. Gordon, M. L. Salutsky and H. H. Willard, Precipitation from Homogeneous Solution. John Wiley and Sons, Inc., New York, 1959. s H. H. Willard and N. K. Tang, Znd. Eng. Chem., Analyt., 1937,9,357. * 0. B. Winter, W. E. Thrun and 0. D. Bird, J. Amer. Chem. Sot., 1929,51,2721. 4 W. F. Hillebrand, G. E. F. Lundell, H. A. Bright and J. I. Hoffmann, Applied Inorganic Analysis. John Wiley and Sons, Inc., New York, 1953. 5 0. I. Milner and L. Gordon, TaIanta, 1960,4,115. e H. T. S. Britton, J. Chem. Sot., 1925, 127,212O.
Sur un principe de colorim&rie en milieu non aqueux-IV: L’emploi du 2,4-dinitropl16nol pour le dosage des acides carboxyliques (Regu le 8 Juillet 1965. Acceptd le 8 Aout 1965) LES esters p-nitrophenyles d’acylamino-acides ont pn%demment et& obtenus* par condensation du carboxyle sur lep-nitrophenol en presence de dicyclohexylcarbodiimide. Selon nos es&s,* on accede de m&me aux esters 2&dinitrophenyles par action du 2&dinitrophenol sur les acides carboxyliques. La presence des deux groupes nitr6.s en me’ta permet par ailleurs une identification wlorimetrique en milieu non aqueux. Sur un principe pn%demment enonce,* il est en effet possible, a partir de l’acide carboxylique, de proceder ii la formation de l’ester puis, par I’emploi de solvants wnvenables, de developper d’embl&e une coloration, sans isolement pr6alable de pester ni &nination de l’exc& de reactif. ApreS acylation par le 2&dinitrophenol en nitromethane, et en presence de dicyclohexylcarbodiimide, l’addition d’hydroborure de potassiumt en dimethylformamide dtveloppe une coloration rouge (560 rnp) qui n’est pas observee en l’absence de carboxyle. La loi de Beer Btant satisfaite, des dosages sont possibles sur des prises d’essai de l’ordre de quelques centiemes de milligramme (Tableau I). La reaction est negative avec l’acide formique. Avec les acides essay&s, I’intensite de la coloration obtenue est inversement proportionnelle au poids moleculaire.
Melange ii volumes egaux, prepat% extemporanement, de solution a 0,5 p. cent de 2&iinitrophenol dans le nitromethane et de solution a 5 p. cent dam le meme solvant de N,N’-dicyclohexylcarbodiimide prepark selon Amiard et Heym&s.l * Et%ctues avec la collaboration de Melle M. Pepin. t Le m6me reactif a ttt utilise pour la colorimetrie des groupes 2,4-dinitroph6nylamin&s.4