RESUMEN
The following values of the enthalphy of solution of well-characterized samples of guanine were obtained from measurements in an adiabatic solution calorimeter: [Table: see text] The following enthalpies of protonation were calculated for guanine: [Table: see text].
RESUMEN
The value for the enthalpy of solution of SRM 1655 (KCl), ΔH°(500 H2O, 298.15 K) = (235.86±0.23)J·g-1 or (17.584±0.017)kJ·mol-1, was obtained from measurements in an adiabatic calorimeter, and confirmed by measurements in an isoperibol calorimeter. Enthalpy of solution measurements are reported in the temperature range 296 K to 358 K, at molalities between 0.005 and 0.18 mol-kg-1, for various ranges of particle size up to 1000 µm, and in CO2- saturated solutions. Observations were also made relative to the hygroscopicity, removal of occluded H2O, and homogeneity of SRM 1655. Between 296 K and 303 K, ΔC p =-(2.076±0.087) J·g-1·K-1or -(154.8±6.4) J·mol-1·K-1 for the reaction of SRM 1655 in 500 H2O.
RESUMEN
Systematic errors in an isoperibol calorimeter of a widely-used design, amounting to about 0.5 percent of the endothermic enthalpy of solution of SRM 1655 (KCl) in H2O, were found to be the result of errors in heat leak corrections due to inadequate stirring and commonly used calorimetric procedures. Other systematic errors were found in measurements of the enthalpy of solution of the exothermic reaction of tris(hydroxymethyl)aminomethane in aqueous HCl solution. Recommended procedures are summarized.
RESUMEN
The following values were obtained from measurements of the enthalpy of solution of a well characterized sample of crystalline adenine in various solvents: [Table: see text] Using these measured values the following enthalpies of protonation and proton dissociation were calculated: [Table: see text].
RESUMEN
Enthalpy of solution measurements of four potassium salts in H2O were made in either an adiabatic or an isoperibol calorimeter or both. The following summarizes the measured and recommended values: [Table: see text] The value for KIO4 has been corrected for the hydrolysis of the periodate ion. The ΔC p = -(82.5 ± 4.3)J·mol-1·K-1 for the unhydrolysed reaction. For the reaction of KBr in H2O, ΔC p was measured as -(166.6 ± 7.2) J·mol-1·K-1 in the temperature range 298 K to 319 K. Measurements with the isoperibol calorimeter are also reported for the endothermic reaction of tris(hydroxymethyl)aminomethane, SRM 724a, in aqueous NaOH (0.05 mol·L-1). Comparisons of measurements by different calorimeters on the same samples reveal unidentified calorimetric errors for endothermic reactions which are greater than the imprecision of the measurements.
RESUMEN
An adiabatic solution calorimeter was used to measure the enthalpy of solution in water of various adenine samples for which a large amount of analytical information is reported. The experimental imprecision of the measurements was 1.1 percent. However, it was necessary to assign an overall uncertainty of 3 percent because of impurity uncertainties. Thus, the best value for the enthalpy of solution is Δ H ∘ ( ∞ , 298.15 K ) = ( 33.47 ± 1.00 ) k J m o l - 1 ΔC p = (78.7 ± 10.4) J·mol-1·K-1 in the range 298 to 328 K at 5 mmol·kg-1, and the enthalpy of dilution is -(316 ± 208) kJ·mol-1 (mol·kg-1)-1 in the range 1 to 7 mmol·kg-1. The entropy of solution for adenine was calculated to be ΔS° (298.15 K) = (72.1 ± 3.9) J·mol-1·K-1, and the partial molar heat capacity at infinite dilution, C° p2 = (226 ± 11) J·mol-1·K-1. The density of adenine was measured by displacement as 1.47 g·mL-1 with an estimated uncertainty of 1 percent.
RESUMEN
An adiabatic solution calorimeter was used to measure enthalpies of solution in water of 7 uracil samples in a concentration range of 3 to 45 mmol · kg-1 and over a temperature range of 298 K to 325 K. Analytical data for the samples are given. Our best value for the enthalpy of solution is Δ H 0 ( ∞ , 298.15 K ) = ( 29.33 ± 1.2 ) kJ mol - 1 and for the change in heat capacity for the reaction with temperature, Δ C p 0 = ( 57 ± 13 ) J mol - 1 K - 1 No change in the enthalpy of solution with concentration was found in this range. Values were calculated for the entropy of solution, ΔS° = (68.1 ± 4.2) J · mol-1 · K-1, and for the apparent molal heat capacity at infinite dilution, C p 2 0 = ( 178 ± 15 ) J m o l - 1 K - 1 .
RESUMEN
An adiabatic solution calorimeter was used to measure enthalpies of solution in water of 8 samples of thymine for which analytical data are reported. Our best values for the enthalpy of solution and the change in heat capacity are Δ H ∘ ( ∞ , 298.15 K ) = ( 24.32 ± 0.70 ) kJ â mol - 1 Δ C p ∘ = ( 106 ± 26 ) J â mol - 1 â K - 1 , 298 K < T < 328 K . These were used to calculate ΔS° = (53. 1 ± 3.6) J · mol-1 · K-1 for the entropy of solution, and C p 2 ∘ = ( 256 ± 26 ) J · mol-1 · K-1 for the apparent molal heat capacity at infinite dilution. No change in the enthalpy of solution with concentration was observed in the range of 5 to 35 mmol · kg-1.
RESUMEN
An adiabatic solution calorimeter was used for measuring enthalpies of solution in water of seven samples of cytosine for which analytical data are given. Our best values are: Δ H ∘ ( ∞ , 298.15 K ) = ( 27.2 ± 4.0 ) kJ â mol - 1 , and Δ C p = ( 76 ± 21 ) J â mol - 1 â K - 1 , 298 K < T < 324 K at ( 24 ± 2 ) mmol â kg - 1 . Evidence is given for an unidentified side reaction at low concentrations which is responsible for the large uncertainty assigned to the enthalpy of solution at infinite dilution. An approximate value of (1.44 ± 0.08) g · mL-1 for the density of cytosine was also measured. Values are calculated for ΔG° and ΔS° for the solution reaction.
RESUMEN
An adiabatir solution calorimeter was used to measure enthalpies of solution and dilution of sodium nitrite in water in the concentration range of 5 to 200 mmol · kg-1. For the solution reaction where molality, m = 100 mmol · kg-1, ΔC p = -1.394 ± 0.014 J · g-1 · K-1 (-23.0 ± 0.2 cal · mol-1 · K-1). Other ΔC p values for some dilution reactions were also measured. The value selected for the enthalpy of solution at infinite dilution is Δ H ∞ ∘ ( 298.15 K ) = 14.006 ± 0.015 kJ â mol - 1 = 3.347 ± 0.004 kcal â mol - 1 Values for the relative apparent molal heat content Ï L , are tabulated and the enthalpy of transition and of fusion derived from differential thermal analysis measurements are also given.
RESUMEN
An adiabatic solution calorimeter was used to measure the enthalpy of solution of the NBS Standard Reference Material No. 1654 (α-quartz or low quartz) for use as a reference standard for HF solution calorimetry. The enthalpy of solution of this material at a concentration of 5 grams in 1000 cm3 of 24.4 weight percent HF(aq) is - Δ H soln ( 353.15 K ) = 2362.2 + 1.1 J â g - 1 . This value also applies to the enthalpy of solution of pure low quartz within the uncertainty limits given. The enthalpy of solution of quartz (in J · g-1) as a function of the temperature, T, of reaction in 24.4 weight percent HF in the range 298 to 358 K, is - Δ H soln ( T ) = 2275.0 + 1.586 ( T - 298.15 ) . Thus, the average ΔCp for the reaction over the range, 298 to 358 K, is - 1.586 ± 0.025 J · g-1 · K-1. As a function of the concentration of HF solution in the range, 18 to 30 weight percent, - Δ H soln ( 353.15 K ) = 2362.10 + 1.429 ( wt % HF - 24.40 ) + 0.069 ( wt % HF - 24.40 ) 2 . From measurements of the enthalpies of solution of several samples of quartz and fused silica, the enthalpy difference between low quartz and silica glass at 298.15 K was determined to be 162.2 ± 4.9 J · g-1 (2.330 ± 0.070 kcal · mol-1).
RESUMEN
This paper discusses in detail the design, construction, and performance of the vacuum jacketed, adiabatic solution calorimeter which was used for the work in the certification of two NBS Standard Reference Materials for solution calorimetry. As a test for the calorimetric precision the following enthalpies of reaction were measured (referred to the mass or moles of H 2 SO 4 · 8H 2 O): H 2 S O 4 â 8 H 2 O + 2.36 ( N a O H â 2620 H 2 O ) = ( N a 2 S O 4 â 0.36 N a O H â 6193 H 2 O ) soln Δ H r ( 298.15 K ) = - 590.692 ± 0.101 J â g - 1 = - 34 , 194.1 ± 5.9 cal â mol - 1 H 2 S O 4 â 8 H 2 O + 2.6 ( N a O H â 714 H 2 O ) = ( N a 2 S O 4 â 0.6 N a O H â 1866 H 2 O ) soln Δ H r ( 298.15 K ) = - 590.828 ± 0.193 J â g - 1 = - 34 , 203.4 ± 11.1 cal â mol - 1 .
RESUMEN
The enthalpy of reaction of tris(hydroxymethyl)aminomethane, NBS Standard Reference Material 724a, measured in an adiabatic solution calorimeter at 298.15 K in 0.1 N HCl solution is -245.76 ±0.26 J · g-1, and in 0.0500 N NaOH solution is 141.80 ± 0.19 J · g-1. The conditions applicable and the factors included in the overall uncertainties are discussed in detail. For the reaction in 0.1 N HCl in the range, 293 to 303 K, ΔCp = 1.435 ± 0.023 J · g-1 · K-1, and in 0.0500 N NaOH in the range, 295 to 303 K, ΔCp = 1.025 ± 0.025 J · g-1 · K-1. Possible sources of error in measurements of the reactions are discussed. A summary of other enthalpy measurements of the reaction in 0.1 N HCl is given.
RESUMEN
The enthalpies of precipitation of silver halides and the enthalpies of solution of AgNO3, KCl, and KBr in H2O were measured in an adiabatic solution calorimeter. From the enthalpy measurements of KCl(c) and KBr(c) in AgNO3(aq), and of AgNO3(c) in KCl(aq), in KBr(aq), and in KI(aq), we calculated (in kJ · mol-1) -65.724, -84.826, and -111.124 for ΔH° pptn(298.15 K) for the averages of the chloride, bromide, and iodide reactions, respectively. A reevaluation of the data for the enthalpy of solution of AgNO3(c) has resulted in our selected best value, Δ H ° ( ∞ ) ( 298.15 K ) = 22.730 + 0.084 kJ â mol - 1 = 5.433 ± 0.020 kcal â mol - 1 A table of enthalpies of dilution of AgNO3(aq) is also given. The average standard entropy for the aqueous silver ion at 298.15 K is found to be S ° [ Ag + ( aq ) ] = 73.42 ± 0.20 J â mol - 1 â K - 1 = 17.55 ± 0.05 cal â mol - 1 â K - 1 . .