Solubility measurement procedure

Solubility measurement at a single pH [37-39] under equilibrium conditions is largely a labor-intensive procedure, requiring long equilibration times (12h-7 days). It s a simple procedure. The drug is added to a standard buffer solution (in a flask) until saturation occurs, indicated by undissolved excess dmg. The thermostated saturated solution is shaken as equilibration between the two phases is established. After microfiltration or centrifugation, the concentration of the substance in the supernatant solution is then determined using HPLC, usually with UV detection. If a solubility-pH profile is required, then the measurement needs to be performed in parallel in several different pH buffers. 

Various procedures have been reported in literature to investigate the sol-ubihty of compoimds in SCCO2 [34], They can be divided into static [34, 35] and dynamic [36,37] methods. Most recently, also parallel techniques have been developed for solubility measurements [38]. All methods, except for the static synthetic one (see below), require analytical measurements to quantify the amount of solute dissolved in SCCO2. Mainly spectroscopic, chromatographic, and gravimetric techniques are applied for the analytical measurements. 

To allow for solubility measurements by a dynamic procedure, equilibrium conditions have to be established in the extraction cell. If a sufficiently low flow rate is adjusted, the CO2 passing the extraction cell is loaded with an equilibrium substance amount in the steady state. 

The determination of the purity of a homogeneous solid from solubility measurements is an example of this application of the phase rale. The experimental procedure... 

Apparatus and Procedure for Solubility Measurement. Chemicals used were ethyl alcohol of special grade reagent and qulzalofop-ethyl crystal of purity 99.5%, specific rotatory power 99.5% e.e. a-form crystal of qulzalofop-ethyl was prepared by cooling crystallization at 283K from the qulzalofop-ethyl solution (5 gr. of qulzalofop-ethyl was dissolved In 100 gr. of ethyl alcohol.) 8-form crystal was also prepared In the same way at 313K from the solution In which 25 gr. of qulzalofop-ethyl was dissolved In 100 gr. of ethyl alcohol. 

Apparatus and Procedure for Supersolublllty Measurement. Same apparatus used In solubility measurement was also used In this experiment. Various amounts of qulzalofop-ethyl was dissolved Into 100 gr. of ethyl alcohol. In order to dissolve the compound completely, the solution was heated up to lOK higher than Its saturation temperature. After keeping the solution as such for about 20 minutes, the solution was cooled at the rate of 0.5K/mln. As well known. If the solution was supercooled beyond Its saturation temperature, crystallization did not occur Immediately. But as soon as a few crystals began to deposit, many crystals precipitated succeed-Ingly almost at the same time. The temperature at which crystals began to deposit was observed. Thus produced crystals were withdrawn and analyzed for Ofr-form content by using DSC and X-ray. 

The procedures outlined above have a practical use, but it should be realized that the parametric models are almost entirely empirical. Experimental uncertainties are also involved since solubility measurements are not very accurate. Solubility loops described by the models only indicate the limits of compatibility and always Include doubtful observations. 

It should be noted that this is a crude method of solubility determination and takes no account of the kinetic aspects of the dissolution processes involved in solubility measurements. To more accurately determine the concentration of a saturated solution of a compound, the following procedure can be used. A known volume of the solvent, water or buffer is pipetted into a scintillation vial, and the compound of interest is added until saturation is observed to occur. The solution is then stirred or shaken for approximately 1 h at the desired temperature. If the compound dissolves, then more compound is added and the experiment restarted. It is recommended that the experiment be conducted at least overnight, but longer time periods may be required if the compound has a very low solubility for example, saturation of morphine in water at 35°C was obtained only after approximately 48 h (Roy and Flynn 1989). 

Third, a basis has been presented for a general approach for estimating and reporting selectivity for a sample In which a large number of unsuspected and unknown Interferences may occur. The approach depends upon ii-dlmenslonal screens of the properties of the sample Itself (e.g., solubility) and of those Involved In the isolation and measurement procedures. 

Kragten and Decnop-Weever (1978, 1979, 1980, 1982, 1983a, b, 1984, 1987) conducted a series of solubility measurements over a wide range of pH (ca. 6 15). The dependence of the precipitation on pH allowed calculation of formation constants of mononuclear and polynuclear species which were internally consistent. The accuracy of the calculated constants was estimated to yield uncertainties of ca. +0.1-0.2 log units. The accuracy of successive stability constants (as p and/or q increased) decreased as a result of error accumulation in the fitting procedure. Nevertheless, the results, in general, indicate that the hydrolysis constants for stepwise formation of higher hydroxo species are of the same order of magnitude as those for the formation of Ln(OH) +. 

At the beginning of the experiments, the cells were thoroughly cleaned. The procedure described previously for the solubility measurements was followed to saturate the triglycerides with CO2 in the high-pressure view cell. Before the mixing period was started the valves between the two cells were opened, so that some triglyceride could flow through the capillary into the second cell. This was done to ensure that the CO2 atmosphere in the second cell was saturated with the... 

Other New Methods. Because the values obtained are dependent on the conditions of measurement, standard test procedures are under review by ISO for determination of cold-water solubiUty of water-soluble dyes (38) determination of the solubiUty and solution stabiUty of water-soluble dyes (39) and determination of the electrolyte stabiUty of reactive dyes (40). 

Mancozeb is a dithiocarbamate pesticide with a very low solubility in organic and inorganic solvent. In this work we have developed a solvent free, accurate and fast photoacoustic FTIR-based methodology for Mancozeb determination in commercial fungicides. The proposed procedure was based on the direct measurement of the solid samples in the middle infrared region using a photoacoustic detector. A multivariate calibration approach based on the use of partial least squares (PLS) was employed to determine the pesticide content in commercially available formulations. 

To determine of Ce(IV) in acid soluble single crystals, a simple and sensitive method is proposed. The method is based on the reaction of tropeoline 00 oxidation by cerium(IV) in sulfuric acid solution with subsequent measurement of the light absorption decrease of the solution. The influence of the reagent concentration on the analysis precision is studied. The procedure for Ce(IV) determination in ammonium dihydrophosphate doped by cerium is elaborated. The minimal determined concentration of cerium equal to 0.04 p.g/ml is lower than that of analogous methods by a factor of several dozens. The relative standard deviation does not exceed 0.1. 

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