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Densities of sodium hydroxide solutions at concentrations of 0.1 j m/mol‚kg -1 j 8 have been measured by vibrating-tube densimetry over the temperature range 323 e T/K e 573 at a pressure of 10 MPa. The purposebuilt densimeter, which is capable of measuring densities of corrosive fluids at temperatures up to 573 K and pressures up to 30 MPa, has a platinum-rhodium vibrating tube with inlet/outlet tubes of platinum and an optical detection system. Calibrations were carried out with water and aqueous solutions of sodium chloride at temperatures up to 523 K. At higher temperatures, nitrogen was substituted as the auxiliary density reference because of uncertainties in the NaCl(aq) densities. The apparent molar volumes of NaOH(aq) calculated from the measured densities were fitted with an extended Redlich-Meyer equation. The present results are in good agreement with literature data but are more precise. The current global accuracy in the densities of NaOH(aq) appears to be about 0.1 %, being significantly better at lower concentrations and temperatures but slightly worse at higher concentrations and temperatures. More accurate knowledge of the densities of the reference materials (other than water) used for calibrating vibrating-tube densimeters under extreme conditions is probably required to improve the accuracy of such data.

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L-Cystine is the least soluble of the naturally occurring amino acids, and cystine stones, caused by a genetic disorder, account for between 1±4% of all urinary stones. Since the concentration of cystine in urine is the only factor related to the stone formation, a proper knowledge of cystine solubility in urine is necessary. Some research groups have already reported on the solubility of L-cystine, but the results are scattering. In this work, a systematic investigation of cystine solubility under conditions most pertinent to urolithiasis was carried out. The solubilities of L-cystine were measured at 25.0 and 37.0 C, from pH 1 to 9, and in different media including (i) 0.300 mol Á dm À3 NaCl solution and (ii) arti®cial urine solutions. The Joint Expert Speciation System (JESS) computer package and selected protonation constants of cystine reported in literature were used to model the solubilities of cystine. Excellent agreement was obtained between the experimentally determined solubility data and computer modelling of cystine solubility. The results of this work show that the presence of inorganic components has little in¯uence on the solubility of cystine in urine.Zusammenfassung. L-Cystin ist die am geringsten lo Èsliche natu Èrlich vorkommende Aminosa Èure. Cystinsteine, die durch eine genetische Sto Èrung verursacht werden, machen etwa 1±4% aller Harnsteine aus. Da die Cystinkonzentration im Urin den einzigen fu Èr die Steinbildung verantwortlichen Faktor darstellt, ist die genaue Kenntnis der Cystinlo Èslichkeit in Urin eine notwendige Voraussetzung fu Èr weitere Untersuchungen. Die bisher publizierten Lo Èslichkeitsdaten streuen jedoch sehr stark, so daû in dieser Arbeit eine systematische Studie der Cystinlo Èslichkeit unter Bedingungen durchgefu Èhrt wurde, die denen der Steinbildung mo Èglichst a Èhnlich sind. Die Lo Èslichkeit von L-Cystin wurde bei 25.0 und 37.0 C im pH-Bereich von 1 bis 9 in 0.300 mol Á dm À3 NaCl sowie in ku Ènstlichen Urinlo Èsungen gemessen. Das Joint Expert Speciation System (JESS) sowie ausgewa Èhlte Protonierungskonstanten aus der Literatur wurden zur Modellierung der Cystinlo Èslichkeiten verwendet, wobei eine ausgezeichnete U È bereinstimmung mit den experimentell bestimmten Werten erhalten wurde. Die Ergebnisse dieser Arbeit zeigen, daû anorganische Salze nur einen sehr geringen Ein¯uû auf die Cystinlo Èslichkeit in Urin ausu Èben.