Aqueous Dilution Method

A known quantity of sample is added to a known volume of a universal buffer solution of sufficient capacity and of known pH. The amount of sample must be sufficient to cause precipitation to occur in the formed saturated solution. After waiting for a period of time to allow the saturated solution to reach the desired steady state, the solution is filtered to remove the solid and obtain a clear solution, whose spectrum is then taken by the UV spectrophotometer. Mathematical treatment of the spectral data yields the area-under-the-curve of the filtered sample solution, AUQ. 

A reference solution is prepared by a dilution method. A known quantity of sample is dissolved in a known volume of the system buffer of known pH the amount of sample is X times less than in the above case in order to avoid precipitation in the formed solution. The spectrum is immediately taken by the UV spectrophotometer, to take advantage of the possibility that solution may be supersaturated (i.e., solid should have precipitated, but because not enough time was allowed for the solid to precipitate, the solution was temporarily clear and free of solid). Mathematical treatment of the spectral data yields the AUC of the reference sample solution, AUQ . The ratio R = AUCS/AUCS is used to automatically recognize the right conditions for solubility determination when the reference has no precipitate, and the sample solution is saturated with precipitate. Under these conditions, solubility is determined from the expression 

TABLE 6.1 Intrinsic Solubility S0, Corrected for the Drug DMSO/Drng Aggregation Effects 

The results of a pH 4-9.5 solubility assay of chlorpromazine are shown in Fig. 6.10. The horizontal line represents the upper limit of measurable solubility (e.g., 125 pg/mL), which can be set by the instrument according to the requirements of the assay. When the measured concentration reaches the line, the sample is completely dissolved, and solubility cannot be determined. This is automatically determined by the instrument, based on the calculated value of R. When measured points fall below the line, the concentration corresponds to the apparent solubility Sapp. 

---

Succinyl coenzyme A trisodium salt [108347-97-3] M 933.5. If it should be purified further then it should be dissolved in H2O (0.05g/mL) adjusted to pH 1 with 2M H2SO4 and extracted several times with Et20. Excess Et20 is removed from the aqueous layer by bubbling N2 through it and stored frozen at pH 1. When required the pH should be adjusted to 7 with dilute NaOH and used within 2 weeks (samples should be frozen). Succinyl coenzyme A is estimated by the hydroxamic acid method [J Biol Chem 242 3468 1967]. It is more stable in acidic than in neutral aqueous solutions. [Methods Enzymol 128 435 7956.]... 

Higher concentrations of DMDO can be obtained by extraction of a 1 1 aqueous dilution of the distillate by CH2C12, CHC13, or CC14.88 Another method involves in situ generation of DMDO under phase transfer conditions.89... 

Grant et al. (2002) designed a parallel system employing two HTLC columns (Cyclone, 50 x 1 mm, Cohesive Technologies) connected to one analytical column (Zorbax SB-C18, 50 x 2 mm, Hewlett Packard) on a 2300 HTLC. A polyarylethyl ketone (PAEK) six-port Valeo (Valeo Instruments, Texas) was used to increase switching speed and reduce carry-over. Peak focusing was used when the analyte was flushed from the TFC column into the analytical column by aqueous dilution. Compared to the dual column method, the overall time reduction was 1.5 to 4 min per sample with comparable data quality at the linear range of 0.1 to 100 ng/mL. 

The second example in Table 10 demonstrates the advantages of this silica approach [308]. Using a mobile phase of methanol-water (75 25) buffered with ammonium phosphate at pH = 7.8, various syrups and tablets were analyzed for antihistamines, antitussives, and decongestants. A comparison between reversed-phase and silica methods of similar cough syrups clearly demonstrates that peak responses obtained by the aqueous silica method are more symmetric than the reversed-phase methods (compare Figure 5.8 with Figure 5.9). In addition, the sample preparation procedures in the silica method are relatively simple, requiring dilution for syrup formulations and dissolution for tablets. 

Kilic et al. selected five different antibiotics, including ciprofloxacin, and reported a titration method based on the use of tetrabutylammonium hydroxide as the titrant in a non-aqueous assay method [6]. Ciprofloxacin was dissolved in water and diluted to 25 mL with pyridine. A 20 mL portion of the resulting solution was titrated with 0.04 M tetrabutylammonium hydroxide in methanol/propan-2-ol at 25°C under nitrogen in a jacketed glass reaction cell. End-point detection was performed with an Orion 720A digital pH ion-meter, equipped with a combination pH electrode that contained a saturated solution of anhydrous methanolic KC1 in the reference. The method was applied to pharmaceutical preparations, and enabled recoveries in the range of 99.95-101.53%, with a RSD of 0.5-1.09%. 

The standard addition method was also applied to the extracted micellar layers containing iron(III). The results obtained with both procedures were found in good agreement, as well as which obtained from DC-plasma spectrometry after analysis of the aqueous dilute phases. 

A common method for the electrodeposition of Am is from isopropanol solutions containing small quantities of dilute acid stock solutions of Am ions. Aqueous deposition methods have also been employed, but the organic electrolyte medium is more advantageous in that it tends to produce more uniform coatings [151[. Zhi etal. prepared relatively thick targets of Am from a mixture of isopropanol and dilute (0.1 N) nitric acid stock solutions pf [152]. The electrolysis... 

A number of techniques using similar concepts were developed over the years to treat leathers that had already deteriorated. Aqueous solutions of potassium lactate were routinely applied to the surface of bookbindings and other leather objects. Unfortunately, in many cases, the deleterious effects of the liquid water solvent on acidic leathers took place before the buffer salt neutralised the acid, resulting in dark, seriously embrittled leather. In order to avoid the effects of aqueous solutions, methods were tried employing ammonia vapour. The object was placed in an enclosed chamber over an open dish of ammonium hydroxide solution. However, ammonia vapour is a strong alkali and there is a danger that even with the use of dilute solutions, leathers that were too acidic would be transformed into ones which were too alkaline. Another alternative was to use an organic base such as imidazole in a non-aqueous solvent. However, this was also liable to result in leathers that were too alkaline. 

Esteban, N.V. Yergey, A.L. Liberate, D.J. Lougfalin, T. Loriaux, D.L. Stable isotope dilution method using thermospray liquid chromatography/mass spectrometry for quantification of daily cortisol production in humans. Biomed.Environ.Mass.Spectrom., 1988, 15, 603-608 [thermospray LC-MS] Sheikh, S.U. Touchstone, J. HPLC of steroids in non-aqueous mobfle phase at subambient temperature. J.Liq.Chromatogr., 1987, 10, 2489-2496 [column temp -50 simultaneous cortisone, desoxycorticos-terone, estradiol, estrone]... 

Hexen-l-yn-3-one, 1-methoxy spiroanellation with, 74 Hexonamides, TV-alkyl- aqueous suspensions various micellar aggregates, 352-353 L-Hexoses total synthesis, 264-265 2-Hexulosides by redox glycosylation , 272 High-dilution methods macrocycle syntheses by, 145-146, 240-241, 329, 338 Hindrance, steric. See Steric hindrance Hippuric acid See Glycine, iV-bcnzoyl-HMDS. See Silanamine,. .. 

Aqueous viscosity was measured at room temperature with a Brookfield and Ubbelohde viscometer. Intrinsic viscosities were measured by a five-point dilution method no shear-rate corrections were made for the data. Interfacial tension was measured with a DuNouy ring tensiometer against toluene at various polymer concentrations. The formulations used to evaluate the HMHECs for latex paints have been described elsewhere (5). 

The gasoline samples were digested with nitric acid in a microwave digestion oven. The resulting acidic solution was diluted with de-ionized/distilled water down to about 5-10 % HNO3 and analyzed by aqueous ICP method (Table 6). Sample dilution in the aqueous analysis is about 1 to 50, unlike organic analysis, which is 1 to 100. 

---