Preparation of the solution

Before recrystallizing a small amount of material it is advisable to carry out a few trials in a micro testtube to find a suitable solvent and the optimal concentration. The general rules for choice of a solvent apply also for microtechniques, except that hot-saturated solutions should not be used. Solvent mixtures, such as ethanol-ether, benzene-light petroleum, and ethanol-water are favored if the substance has different solubilities in the two components of the solvent mixture, it can be dissolved in one in the cold, that solution can be clarified or filtered, and crystals then obtained by adding the second solvent. 

In general work on a micro scale, hot-saturated solutions are used only when filtration and clarification are unnecessary or when the substance has little tendency to crystallize. However, to prevent crystallization during filtration, it is better to use solutions that are less than saturated even at room temperature after clarification and filtration, the dilute solution is concentrated until saturated hot or, alternatively, the dilute solution is evaporated to dryness and the residue is dissolved in a fresh small amount of the solvent. 

It sometimes happens that recovery of very small amounts of a substance by evaporation of a dilute solution in a relatively large flask yields the material as a thin film on the wall of the vessel in such cases quantitative recovery of the substance can be achieved by means of a suction ball. 

This suction ball is a plum-shaped 0.5-20 mm glass ball with capillaries of medium internal diameter (length 100-350 mm, diameter 0.3-2.5 mm) attached at diametrically opposite sides.62 The principle of operating a suction ball is to seal one capillary, warm the ball so that air or solvent vapor is driven out of the other end, and then to dip the open end into the liquid which is then sucked into the ball as the latter cools. The suction ball can be emptied either analogously by warming or by cutting off the sealed end of the capillary. The suction ball has many advantages over the micropipette with rubber cap used for similar purposes. 

It is also possible to repeat several times the evaporation and condensation of the solvent added to the larger vessel, before introducing this solution into the suction ball in this way the lower part of the vessel is washed by the condensed solvent that runs down the walls. 

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The solute molecular weight enters the van t Hoff equation as the factor of proportionality between the number of solute particles that the osmotic pressure counts and the mass of solute which is known from the preparation of the solution. The molecular weight that is obtained from measurements on poly disperse systems is a number average quantity. 

Preparation of the solutions was similar to that of niobium-containing solutions, i.e. by dissolving tantalum metal powder in hydrofluoric acid, HF, at a concentration of about 40% weight. 

An initial solution was prepared by the hydrofluoride method, i.e. melting of a mixture of ammonium hydrofluoride and tantalite, followed by the digestion of soluble components with water and separation of the solution by filtration. The prepared initial solution contained no free HF or any other acid, and had a pH 3. In order to obtain an optimal acidity level, sulfuric acid was added to the solution. Concentrations of Ta2Os (50-60 g/1) and Nb205 ( 30 g/1) were kept approximately constant during the preparation of the solutions. Extraction was performed using a polypropylene beaker and a magnetic stirrer. 

Details for the preparation of the solutions referred to in the table are as follows (note that concentrations are expressed in molalities) all reagents must be of the highest purity. Freshly distilled water protected from carbon dioxide during cooling, having a pH of 6.7-7.3, should be used, and is essential for basic standards. De-ionised water is also suitable. Standard buffer solutions may be stored in well-closed Pyrex or polythene bottles. If the formation of mould or sediment is visible the solution must be discarded. 

In contrast, in HPLC assays the chromatographic separation and the integration of the resulting analyte peak normally are just as or even more error-prone than is the preparation of the solutions here it would be acceptable to simply reinject the same sample solution in order to obtain a quasi-independent measurement. Two independent weighings and duplicate injection for each solution is a commonly applied rule. 

The light-absorbing impurities of oxytetracycline hydrochloride and oxytetracycline dihydrate [2,4,6] are detected using a UV-spectrophotometric method. In all the compendia, the absorbance of a solution of 2.0 mg/mL in a mixture of 1 volume of hydrochloric acid solution (0.1 mol/L) and 99 volumes of methanol at 430 nm not greater than 0.50 is required the absorbance of a solution of 10 mg/mL in the same solvent at 490 nm is not greater than 0.20. The measurements are carried out within 1 h of the preparation of the solutions. 

For the preparation of the solution of sodium ip liquid ammonia, compare part A. 

Procedure Dissolve the specified quantity for the substance in DW, or prepare a solution as directed in the text and transfer to a Nessler cylinder. Add 10 ml of dilute nitric acid, except when it is used in the preparation of the solution, dilute to 50 ml with DW, and add 1 ml of AgN03 solution. Stir immediately with a glass rod and allow to stand for 5 minutes. The opalescence produced is not greater than the standard opalescence, when viewed transversely. 

A highly efficient reagent for removing electrophilic and nucleophilic impurities is neutral aluminium oxide, which is used immediately before preparation of the solution or during the voltammetric measurement ... 

A further method for the specification of the composition of a solution or mixture, related to the molar scale, is the volume fraction of the solute, ( ). This takes into account any change in the volume of the system that has taken place on the preparation of the solution-the volume (change) of mixing. Therefore for a solute I ... 

The submitters used the inverse addition procedure for preparing the methanolic sodium methoxide, as follows. In a thoroughly dry 500-ml., three-necked, round-bottomed flask fitted with a mechanical stirrer, dropping funnel, and a reflux condenser carrying a calcium chloride drying tube is placed 13.8 g. (0.60 g. atom) of sodium freshly cut into small pieces. To this is added slowly 140 ml. of anhydrous methanol at such a rate as to maintain vigorous reflux. If all the sodium does not dissolve during the addition of the methanol, the mixture may be heated on the steam bath until solution is effected or additional methanol (up to 25 ml.) may be added. The preparation of the solution of sodium methoxide requires about 30 minutes. 

Accuracy and Interpretation of Measured pH Values. To define the pH scale and pertnil the calibration of pH measurement systems, a scries of reference buffer solutions have been certified hy the U.S. National Institute of Standards and Technology iNIST). The acidity function which is the experimental basis for the assignment of pH. is reproducible within about O.IKl.I pH unit from It) to 40T. However, errors in the standard potential of the cell, in the composition of the buffer materials, and in the preparation of the solutions may raise the uncertainty to 0 005 pH unit. The accuracy of ihe practical scale may he furthei reduced to (I.Ot)X-(l.(ll pH unit as a result of variations in the liquid-junction potential. 

Preparation of the Solutions Polarimetric Tests.—100 c.c. of the wine are neutralised in a porcelain dish with caustic potash solution, care being taken not to render the liquid alkaline.1 The alcohol is evaporated off on a water-bath and the residue introduced into a 200 c.c. flask, into which also the dish is rinsed several times with water. A slight excess (about 5 c.c.) of basic lead acetate solution is added, the precipitate formed being allowed to settle and sufficient saturated sodium sulphate solution added, drop by drop, to precipitate the excess of lead. When further addition of the sodium sulphate solution fails to produce a precipitate, the liquid is made up to 200 c.c. with water, shaken, allowed to settle and filtered by decantation through a dry filter. 

Preparation of the Solution.—The concentration of the solution of the substance should be such that it contains as nearly as possible 4 grams of tanning material per litre and in any case between 3-5 and 4-5 grams.1... 

DNA-Cu(II)-quercetin interactions can be followed electrochemically using a DNA-electrochemical biosensor [29,35]. This knowledge about the electrochemical behaviour of the dsDNA incubated with quercetin-Cu(II) complexes at GC electrode [45] is an important feature to understand quercetin-DNA interactions at a DNA-electrochemical biosensor. The preparation of the solutions and the quercetin-Cu(II) complex used during the characterization of in situ electrochemical DNA damage promoted by the quercetin-Cu(II) complex using a DNA biosensor is described (see Procedure 29 in CD accompanying this book). 

A test solution having a concentration of 40 mg/mL in 15 N formic acid is prepared, and the optical rotation determined according to General Test <781 S>. The specific rotation is to be between +14.5° and +16.5°, determined at 20°C within 30 minutes after preparation of the solution. 

Fig. 2. Methyl proton resonances of (CeHihShChfacac) in dichloromethane at various temperatures, (a) The solution was freshly prepared below —30°. The figures in parentheses represent the time (in minutes) elapsed from the preparation of the solution to the measurements, (b) The solution was kept at room temperature for 24 hours, and ample time was allowed for each measurement.

Unless otherwise prescribed, use phosphate buffer solution pH 7.4 R containing 3.0% ntfV of bovine albumin K for the preparation of the solutions and dilutions used in the assay. 

MALDI is achieved in two steps. In the first step, the compound to be analysed is dissolved in a solvent containing in solution small organic molecules, called the matrix. These molecules must have a strong absorption at the laser wavelength. This mixture is dried before analysis and any liquid solvent used in the preparation of the solution is removed. The result is a solid solution deposit of analyte-doped matrix crystals. The analyte molecules are embedded throughout the matrix so that they are completely isolated from one another. 

The addition of salt to the egg yolk solution during the preparation of the solutions disrupts the granules and provides additional surface active material for adsorption, such as a- and yS-lipovitellins and phosvitin. In this regard, Chang et al. (1972) observed with an electron microscope a continuous speckled layer around oil drops and also a fibrous membrane on the surface of drops after they were washed with water. The electron-dense particles in the speckled layer were interpreted to be egg yolk lipoproteins and the fibrous material to be livetin-phosvitin complex. 

It is necessary to study stability in solution in the solvent used to prepare sample solutions for injection in order to establish that the sample solution composition, especially the analyte concentration, does not change in the time elapsed between the preparation of the solution and its analysis by HPLC. This is a problem for only a few types of compound (e.g. penicillins in aqueous solution) when the sample solution is analysed immediately after the preparation of the sample solution to be injected. The determination of stability in solution is more of an issue when sample solutions are prepared and then analysed during the course of a long autosampler run. While the acceptance criteria for stabUity in solution may be expressed in rather bland terms by making a statement such as, e.g. the analyte was sufficiently stable in solution in the solvent used for preparing sample solutions for reliable analysis to be carried out , in practice it has to be shown that within the limits of experimental error, the result of the sample solution analysis by the HPLC method is the same for injections at the time for which stability is being validated as for injections immediately subsequent to the sample solution preparation. While this may be done by a subjective assessment of results with confidence limits, strictly speaking a statistical method known as the Student s t-test should be used. 

When chloride was added to the system either as sodium chloride (Table I, lines 3a and b) or as palladium chloride (lines 4r-6) a drastic change in the reaction path occurred. The source of the halide is unimportant apparently a rapid scrambling of anionic ligands occurs during the preparation of the solutions prior to olefin addition (c/., Table I, lines 3a and 4a). However, in the presence of chloride, temperature effects become pronounced. In all cases, the higher reaction temperatures greatly favor I-substitution, so much so that it was possible to reverse completely... 

The first step is preparation of the solution. Aqueous solutions are prepared by dissolving either soluble salts in solvents (usually water) or metals in acids. For multicomponent systems, the mutual solubility of the various components must be considered. For example, a solution for lead zirconate cannot be prepared from lead nitrate and zirconium sulfate, both of which are soluble in water, because lead sulfate, which is insoluble, will precipitate. A solution of nitrates of both cations is satisfactory. 

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