Orthophosphate reacts with molybdate in acidic medium to yield 12-molybdophosphoric heteropolyacid; subsequently, detection is undertaken either on the molybdophosphate reduction product (molybdenum blue method) or on the yellow vanadomolybdate complex [2,5,6]. The molybdenum blue method became broadly used due to its higher sensitivity and lower susceptibility to interferences [4,7,8], than the vanadophosphomolybdate method. Nevertheless, the low concentration levels imposed by legislation are difficult to achieve. Therefore, the development of new methodologies capable of monitoring trace phosphate levels in environmental samples with adequate precision, has gained importance [9]. Flow injection analysis (FIA) is a widespread and well established approach to determine phosphorus. In fact, the first publication on FIA is dedicated to phosphate determination [10] and discusses both the yellow vanadomolybdate and the molybdenum blue method. Compared to the blue molybdenum method, the yellow vanadomolybdate analytical procedure can present some advantages that make it more suitable for flow systems: (i) the use of a single reagent mixture with extended stability makes the method suitable for field portable applications and for unattended operation in water monitoring [11,12] and (ii) the formation of a fine precipitate, accompanying the development of the blue colour, does not occur. However, maybe because of its limited sensitiv

A mathematical model that combines oxygen uptake from the outside environment with oxygen consumption by oxidative reactions, in a liquid packed food during storage, was developed. The model was applied to orange juice aseptically packaged in Tetra Brik Aseptic cartons, during storage of up to 5 months at 4, 8, 20, 30, 40 and 50°C. The parameters of the model, the oxygen mass transfer coeficient and the rate constant of consumption reactions, were estimated by fitting the model to the experimental data. The value of the rate constant estimated for the system tested in this work, was three orders of magnitude greater than the value of the oXygen mass transfer coeficient. The influence of temperature on the reaction rate was well described by an Arrhenius type equation, with an activation energy of 46 Wlmole. This model was further tested with data reported in literature and it was found that it adequately describes the dissolved oxygen concentration changes during storage.

Optimal experimental designs for maximum precision in the estimation of diffusivities (D) and mass transfer coefficients (K c ) for solute transport from/to a solid immersed in a fluid were determined. Diffusion in the solid was considered to take place according to Fick's second law. It was found that the optimal design was dependent on the Biot number. In the range of Biot numbers tested (0.1±200), the first sampling time corresponded to values of fractional loss/uptake between 0.10 and 0.32, and the second sampling time corresponded to values of fractional loss/uptake between 0.67 and 0.82. Pseudo-experimental data were simulated by applying randomly generated sets of errors, taken from a normal distribution with 5% standard deviation, to data calculated using given values of the model parameters. Both optimal and heuristic designs (for which the sampling times corresponded to values of fractional loss/uptake from 0.30 to 0.95) were analyzed. The accuracy and precision of the estimates obtained by non-linear regression were compared. It was confirmed that optimal designs yield best results in terms of precision, although it was concluded that the joint estimation of D and K c should, in general, be avoided. For intermediate values of the Biot number, reasonably precise and accurate estimates can however be obtained if the experimental error is small. # 1998 IMACS/Elsevier Science B.V.

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