Voltammetric experiments were used to demonstrate the possibility to rapidly obtain stability constants, E degrees values and kinetic parameters of Fe(III) complexes with 1,6-dimethyl-4-hydroxy-3-pyridinecarboxylic acid (DQ716) at pH 2.3 and 4-hydroxy-2-methyl-3-pyridinecarboxylic acid (DQ2) at pH 3. Fe(III) diffusion coefficient (D(Fe)= 5.5.10(-6) cm(2)/s), heterogeneous electron transfer kinetic constant (k degrees = 2.7.10(-4)cm/s), symmetry coefficient (alpha= 0.57) and Fe(III)/Fe(II) standard reduction potential (E degrees = 0.53 V vs. SCE) were determined beforehand and used to obtain all the other results. Digital simulation together with potentiometric data were used to define the whole reaction system in terms of thermodynamic and kinetic parameters. In particular, E degrees and the dissociation kinetic constant, k(b), of the 1:1 (E degrees = 0.22 V vs. SCE, k(b)= 0.032 s(-1)), 1:2 (E degrees = 0.098 V vs. SCE; k(b)= 0.22 s(-1)) and 1:3 (E degrees < or =-0.29 V vs. SCE, k(b)= 157.9 s(-1)) Fe(III)/DQ716 complexes, were estimated. Stability constants of the Fe(II) complexes were computed from these values. The voltammetric data were also interpreted with two independent formalisms: (1) solution of an equation system and (2) a curve fitting method based on the Koutecky-Levich equation. Both approaches allowed us to obtain the speciation of a Fe(III)/DQ716 solution at pH 2.3. Moreover, the second approach allowed the evaluation of the kinetic contributions, the stepwise stability constant of Fe(III)L(2) (7.65 +/- 0.07), and to define the mathematical formalization of the experimental result which link some key-points of the voltammetric curve (inflection points and plateaux) to D(Fe), k degrees , alpha(j) and E degrees . This approach was also successfully applied to obtain the speciation of a Fe(III)/DQ2 solution at pH 3.

l es substances susceptibles d'être utilisées contre les stades libres des Schistosomes, certains agents de surface, du type bétaïne, montrent une activité remarquable à des doses faibles (0,1-5 ppm) aussi bien contre l es miracidiums que contre l es cercaires. La laurylamidopropylbétaïne a fait en particulier l'objet d' une série de tests concluants, démontrant une bai sse très significative de l'infection des hôtes (mollusques ou souris) par l es larves libres ayant été à son contact. D' après plusieurs essai s faits au Niger, ce produit possède une rémanence satisfaisante dans l es eaux tropicales, malgré la biodégradation et l es phénomènes d'absorption aux particules en suspension. Il est envi sagé de l'introduire dans les savons domestiques à la dose de 5 %, ce qui assurerait un renouvellement régulier du produit actif dans certains sites et pourrait réduire fortement la transmission. Potential use of amphoteric surface agents to combat agents of Schistosome transmission.

RÉSUMÉ. L'efficacité d'un agent de surface, la laurylamidopropylbétaïne a été testée dans des conditions de terrain sur les miracidiums de Schistosoma mansoni. Ces expériences ont eu lieu dans des mares artificielles de 20 m3 reproduisant le plus fidèlement possible certains types de sites de transmission. La dégradation de la laurylamidopropylbétaïne est assez lente et son efficacité sur les miracidiums de Schistosoma mansoni est très nette : à la concentration de 2 ppm, le pourcentage de Mollusques infectés est égal à zéro. Il est suggéré d'incorporer le produit actif dans les savons afin de limiter la transmission des schistosomes dans les lieux de lavage et de baignade.Mots clés : Agents de surface amphotères ; Lutte chimique ; Miracidiums ; Schistcsoma mansoni ; Stades infectants ; Niger. Efficiency of an amphoteric surface agent against Schistosoma mansoni miracidia in field-like conditions in Niger.SUMMARY. The laurylamidopropylbétaïne, a surface active substance, has been tested in field conditions, in Niger, against Schistosoma mansoni miracidia. Experiments were carried out in artificial ponds of 20 m3, in which conditions were similar, as far as possible, to conditions found in transmission sites. The product showed a satisfactory remanence and the percentage of infected molluscs dropped down to zero with a low (2 ppm) concentration. It is suggested that laurylamidopropylbetai'ne could be used into soaps in order to renew constantly the product in washing and bathing places and reduce schistosome transmission.