Titration of Isolated Cell Walls of Lemna minor L.
A theoretical model has been built to bypass the equation of titration of the cell wall. This equation, which is an extension of the Henderson-Hasselbach equation, underlines the importance of the exchange constant, the ionic strength as well as the rate of neutralization. The model is restricted to the case when the ionization degree is equal to the neutralization degree. The shape of the titration curve is shown to be strongly dependent on the valency of the base used.Experimental results have shown that isolated cell walls bear at least two kinds of sites. The first sites which are titrated after a short time of equilibration are attributed to polyuronic acids (capacity: 0.3 milliequivalents per gram fresh cell walls). The second sites, are obtained after a long time of equilibration (capacity: 1.2 to 1.3 milliequivalents per gram, fresh cell walls). Titrations have been performed with different bases [KOH, NaOH, and Ca(OH)(2)] and under different ionic strengths.The results obtained with NaOH and KOH do not exhibit any difference of selectivity. Conversely, the sites have a much bigger affinity for the Ca(2+) ions than for the monovalent ones. The apparent pKa of the uronic acids was estimated to lie between 3.0 and 3.4; this is consistent with the values obtained with polyuronic acid solutions.
Morvan
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Laboratoire de Nutrition minérale, Faculté des Sciences, Université de Rouen, 76130 Mont-Saint-Aignan, France.
Demarty
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Thellier
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