TITRATION OF ISOLATED CELL-WALLS OF LEMNA-MINOR-L

A theoretical model was built to bypass the equation of titration of the cell wall. This equation, which is an extension of the Henderson-Hasselbalch equation, underlines the importance of the exchange constant, the ionic strength and 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 strongly dependent on the valency of the base used. Experimental results showed that isolated cell walls bear at least 2 kinds of sites. The first sites which are titrated after a short time of equilibration are attributed to polyuronic acids (capacity: 0.3 meq/g fresh cell walls). The 2nd sites are obtained after a long time of equilibration (capacity: 1.2-1.3 meq/g, fresh cell walls). Titrations were 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 Ca2+ 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.