Oxidation of [Co(en)2(SOCH2CO2)]+ by S2O82− and by IO4− in binary aqueous solvent mixtures at pressures up to 1.25 kbar

Second order rate constants and activation parameters H, S, and V have been measured for the oxidation of [Co(en)2(SOCH2CO2)]+ by S2O82– and by IO4– in highly aqueous H2O – t-BuOH mixtures. The changes in solvation on going from the initial to the transition state are discussed on the basis of the transfer functions Gto, Hto and Sto. Whereas Gt changes smoothly as the proportion of t-BuOH increases, the plots of Ht and TSt exhibit mirror behaviour and pass through extrema located around x2(t- BuOH)=0.038. Information on the role of solvation is complemented by the determination of activation volumes. These are discussed in terms of intrinsic and solvational contributions. It is proposed that changes in hydrophobic hydration are of principal importance in determining the response of H, S, and V to changes in solvent composition in H2O – t-BuOH mixtures.     

Correlation of volumes and entropies of activation for oxidation of coordinated sulfur in bis-ethylenediaminecobalt(III) complexes by peroxodisulphate,hydrogen peroxide and periodate and for peroxodisulphate oxidation of diimine-cyanide–iron(II) complexes

Activation parameters H , S and V and correlations between S and V are reported for peroxodisulphate oxidation of [Fe(CN)₆]^4–, [Fe(bipy)₃]²⁺ (bipy = (2, 2-bipyridyl), [Fe(phen)₃]²⁺ (phen = 1,10-phenanthroline), cis-[Fe(bipy)₂(CN)₂], [Fe(bipy)(CN)₄]^2–, [Fe(phen)(CN)₄]^2–, [Co(en)₂(glyS)]⁺ (glyS = mercaptoacetate, SCH₂COO^2–), [Co(en)₂(cyS)]⁺ (cyS = cysteinate, SOCH₂CH(COO)NH₂ ^2–) and [Co(en)₂(amS)]²⁺ (amS = ethanesulphenaminate, SCH₂CH₂NH₂ ^–) and for periodate and hydrogen peroxide oxidation of the three last-named complexes. Activation parameters are discussed in terms of electrostriction, solvation and ligand size contributions. Opposite trends for S /V correlations were found for oxidations of Fe^II complexes in comparison with oxidations of coordinated sulphur in the Co^III complexes.     

Molecular structure of N-(2-hydroxyethyl)pyridinium bromide

X-ray diffraction, structural analysis was employed to establish the structure of N-(2-hydroxyethyl)pyridinium bromide, C₇H₁₀NO⁺-Br^–. The plane of the pyridinium ring is twisted by 93 relative to the C-C bond of the side-chain. The N-C-C-O torsion angle is 63.6. The OH bond is oriented toward the heteroaromatic ring. The H-O-C-C torsion angle is 84.4. The molecular packing in the crystal lattice is such that the bromide ions are found between the heteroaromatic rings and the hydrogen bonded to the hydroxyl groups.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2617–2619, November, 1990.     

Synthesis and constants of 6-nitro-2,3-dihydroquinolin-4(1h)-one

In the literature, there has appeared a communication [1] on the synthesis of 6-nitro-2, 3-dihydroquinolin-4(1H)-one (I) with mp 125–126C (2,4-dinitrophenylhydrazone, mp 269–270C) by the reaction of p-nitroaniline with acrylic acid. Quinolinone I with mp 232–234C (2,4-dinitrophenylhydrazone, mp 327–329C) was obtained by us by decarboxycyclization of N-(2-carboxy-4-nitrophenyl)--alanine in acetic anhydride in the presence of potassium acetate [2].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1390–1392, October, 1979.     

Measurement of the specific heat and heat of decomposition of a polymer composite to high temperatures

The specific heat and heat of decomposition of a glass-filled phenolformaldehyde (phenolic) resin have been determined from experimental data obtained using a simultaneous thermal analyzer capable of operation to 1500 C. The measurements were conducted on powdered samples of the polymer to temperatures of approximately 1050 C at a rate of 20 deg min^–1 in an argon atmosphere. Both the mass loss and energetics were measured for the virgin component of the material, while only the energetics were measured for the char component. The combination of these data was used to calculate the specific heat of the virgin, char, and decomposing material, as well as the heat of decomposition. Also, in order to establish the accuracy of the instrument, the specific heat of pyroceram 9606 was measured and compared to previously published values.

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Zusammenfassung Die spezifische Wärme und die Zersetzungswärme eines glasgefüllten Phenol-Formaldehyd-Harzes wurden aus experimentellen Daten bestimmt, die mittels eines simultanen ThermoanalyseGerätes für Arbeiten bis 1500 C gewonnen wurden. Die Messungen wurden an gepulverten Polymerproben bis zu Temperaturen von 1050 C mit einer Aufheizgeschwindigkeit von 20 K min^–1 in Argonatmosphäre durchgeführt. Massenverlust und Energieänderungen wurden für das jungfräuliche Material gemessen, die Energieänderung auch für das verkohlte Material. Zur Prüfung der Messgenauigkeit des Geräts wurde die spezifische Wärmekapazität von Pyroceram 9606 gemessen und mit früher publizierten Daten verglichen.

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, 1500, () . 1050 20/. , — . , , . 9606, .

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On sabbatical leave from the University of Rhode Island, Kingston, Rhode Island 02881 U.S.A.     

Kinetics and mechanism of dissociation of some tri- and tetra-dentate complexes of copper(II) in acid media

Summary Kinetics of the dissociation ofm-phenylene-dibiguanidecopper(II) ion,N-salicylideneglycyl-glycinatocuprate(II) ion andN-salicylideneglycinato-aquocopper(II) in acid media, forming aquo-copper(II) ion, have been studied by the stopped-flow spectrophotometric technique. Dissociation of the complexes occurs in two consecutive steps, the first being faster than the second. For them-phenylenedibiguanide complex each step exhibits second order acid dependance,k _x=k _x [H⁺]² wherek _x is the observed pseudo-first order rate constant. However, the Schiff base complexes show first order acid dependance,k _x=k _x [H⁺], for both steps. The results, with relatively low H and highly negative S values, are consistent with a solvent-assisted dissociative process.     

A new route to the asymmetric Schiff base ligands. The ligand exchange in [Mo(CN)<Subscript>3</Subscript>O(salhy)]<Superscript>2−</Superscript> ion. Kinetics and mechanism

The kinetics of the ligand exchange in (PPh₄)₂[Mo(CN)₃O(salhy)]. 6H₂O (Hsalhy = salicylaldehyde hydrazone) by a solvent molecule and by 2,2-bipyridine (bpy) have been studied in EtOH. For the ligand exchange by a solvent molecule the pseudo-first order rate constant equals k _obs = 3.2 (±0.2) × 10^–3 s^–1 (t=25 °C), H =67 (± 7) kJ mol^–1, S =–75 (±23) J mol^–1 K^–1, while for the exchange by a bpy molecule k _obs=3.5 (±0.2) × 10^–3 s^–1 (t=25 °C), H =56 (±7) KJ mol^–1, S = –104 (±8) J mol^–1 K^–1. It was found, that all reactions proceed via the same mechanism which involves the chelate ring opening cis to the Mo=O bond. The mechanism of the reaction was proposed and was proved by the synthesis of (PPh₄)₂[Mo(CN)₃O(N-pic)]. 2.5H₂O (N-pic denotes that the nitrogen of picolinic acid is trans to Mo=O) by ligand exchange in EtOH, while in aqueous solution the O-pic analogue is formed exclusively.     

Thermal study of synthesis of cryptohalite

The present work represents a thermal study of synthesis of cryptohalite (Ammonium silicon hexafluoride) by sintering of quartz with ammonium fluoride using a derivatograph. The reaction products were identified microscopically and by using a Siemens Crystalloflex diffractometer. The DTA curves indicate that the intensive formation of cryptohalite takes place at 125–155C by an endothermic reaction. Cryptohalite is unstable and dissociates at 320–335C as represented by the sharp and large endothermic peaks at these temperatures.The resulted cryptohalite is colorless in thin sections and crystallizes in cubic system, in the form of octahedral crystals with perfect (111) cleavage. The dimorph bararite is not detected in all runs.

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Zusammenfassung Mittels eines Derivatographen wurde die Synthese von Kryptohalit (Ammoniumsiliziumhexafluorid) durch Sintern von Quarz mit Ammoniumfluorid thermisch untersucht. Die Reaktionsprodukte wurden mikroskopisch und mit Hilfe eines Siemens-Crystalloflex Diffraktometers identifiziert. Die DTA-Kurven zeigen, da\ die intensive Bildung von Kryptohalit in einer endothermen Reaktion bei 125–155C abläuft. Wie durch die scharfen und intensiven endothermen Peaks bei 320–335C gezeigt wird, ist Kryptohalit bei dieser Temperatur instabil und dissoziiert.Das erhaltene Kryptohalit ist in dünnen Schnitten farblos und kristallisiert im kubischen System in der Form von oktaedrischen Kristallen mit perfekter (111) Spaltbarkeit. Das dimorphe Bararit konnte in keinem der Versuche beobachtet werden.

     

Measurement of specific heat and energetic characterization of materials up to high temperatures

A new DSC system has been developed which not only allows quantitative results in the temperature of –160C to 700C, but also allows the quantitative determination of a variety of material properties up to 1500C. For example, the specific heat of materials can be measured to at least 1400C, while enthalpies, etc. can be measured to 1500C.

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Zusammenfassung Ein neuartiges DSC Messystem (Netzsch DSC 404) wurde entwickelt, das sich durch hohe Reproduzierbarkeit der Basislinie, grosse Empfindlichkeit und breiten Temperaturanwendungsbereich (–160C bis 700C resp. bis 1500C) auszeichnet. Die Messanordnung ermöglicht die Verwendung von unterschiedlichen GasatmosphÄren als auch Messungen im Vakuum. Es werden Beispiele der Bestimmung von SchmelzvorgÄngen, der Glasumwandlungstemperatur, der KristallinitÄt und der spezifischen WÄrme, sowohl für Polymere als auch für anorganische Materialien dargestellt und diskutiert.

     

Kinetics and mechanisms of the oxidation of the octacyanoniobate(III)ion by oxyanions in alkaline aqueous media

Summary The kinetics and mechanisms of the oxidation of Nb(CN) _inf8 ^sup5– by the oxyanions S₂O _inf8 ^sup2– , BrO _inf3 ^sup– , and IO _inf4 ^sup– have been investigated in alkaline aqueous media (pH 12). The second-order rate constant for the electron transfer reaction between Nb(CN) _inf8 ^sup5– and S₂O _inf8 ^sup2– at 25.0 °C, I = 0.36m (K⁺), is 11.1± 0.3 m ^–1 s ^–1 with H = 30 ± 2kJmol^–1 and S = - 125 + 7JK^–1 mol^–1. The rate constant for the oxidation of Nb(CN) _inf8 ^sup5– by BrO _inf3 ^sup– at 25.0 °C, I = 0.20m (Na⁺), is 2.39 ± 0.08m ^–1 s ^–1 with H = 28 ± 2kJmol^–1 and S = -139 ± 7JK^–1mol^–1. The oxidation of Nb(CN) _inf8 ^sup5– by IO _inf4 ^sup– proceeds by two parallel pathways involving the monomeric IO _inf4 ^sup– ion and the hydrated dimer H₂I₂O _inf10 ^sup4– . The second-order rate constant for the oxidation of Nb(CN) _inf8 ^sup5– by monomeric IO _inf4 ^sup– at 5.0 °C, I = 0.050m (Na⁺), is (3.3 ± 0.6) × 10³ m ^–1 s ^–1 with H = 75 ± 6 kJ mol^–1 and S = 94 ± 15 J K^–1 mol^–1, while the rate constant for the oxidation by H₂I₂O _inf10 ^sup4– is (1.8 ± 0.1) × 10³ m ^–1 s ^–1 with H = 97 ± 5 kJ mol^–1 and S = 166 ± 16 J K^–1 mol^–1 under the same reaction conditions. The rate constants for each of the oxidants employed display specific cation catalysis with the order of increasing rate constants: Li⁺ < Na+ < NH _inf4 ^sup+ < K⁺ < Rb⁺ < Cs⁺, in the same direction as the electronic polarizability of the cations. The results are discussed in terms of the outer-sphere electron-transfer processes and compared with the corresponding data and mechanisms reported for other metal-cyano reductants.     

Free radicals in γ-irradiated olefinic silica compounds

Results are reported from -irradiation of compounds of structure . each of which gives a characteristic ESR spectrum having a hyperfine structure whose resolution is somewhat reduced by the coupling of the radical to the silica. The splittings are 23–25 Oe (typical of alkyl radicals), except for the phenyl compound. Improved resolution is provided by varying the temperature between –196 and 20C. The bond to the surface gives the radical high thermal stability; contact with air at 20C converts the alkyl radicals to peroxyl ones. Hyperfine splitting from the hydrogen H_p 5000 Oe is observed in all compounds except.     

Diazobicycloalkanes with bridgehead nitrogen atoms in nodal positions. 22. Conformational analysis of benzodiazabicycloalkenes. Iterative evaluation of spectral and thermodynamic parameters

¹H and ¹³C NMR were used to investigate the conformational equilibrium of benzo[f]-1,5-diazabicyclo[3,2.2]nonene and benzo[g]-1,6-diazabicyclo[4.2.2]decene in solution at temperatures from 20 to–110C. Benzo[f]-1,5-diazabicyclo[3.2.2]nonene in this temperature interval undergoes rapid conformational exchange, while the conformation ratio changes from 7327 to 5545. The thermodynamic characteristics of this equilibrium were obtained and some NMR parameters of the individual conformers were estimated. For benzo[g]-1,6-diazabicyclo-[4.2.2]decene it was possible to attain conditions of slow exchange between two conformations whose proportion in the temperature interval studied were almost identical. The kinetics of conformational exchange were investigated and the energy of activation of the process was found to equal 42.3 kJ/mole.For Communication 21 see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1402–1407, October, 1991.     

The kinetics and mechanism of aquation and base hydrolysis of the cis-[Co(cyclen)Cl2]+ cation (cyclen=1,4,7,10-tetraazacyclododecane)

The aquation of cis-[Co(cyclen)Cl2]+ (cyclen=1,4,7,10-tetraazacyclododecane) has been studied over a range of temperatures with 0.1 mol dm–3 HNO3 as solvent. At 25°C, kaq=4.5×10–3 s–1 with H=78 kJ mol–1 and S298=–21 J K–1 mol–1. Base hydrolysis of cis-[Co(cyclen)Cl2]+ is extremely rapid with kOH=2.1×107 dm3 mol–1 s–1 at 25°C and I=0.1 mol dm–3. This is the largest rate constant so far reported for the base hydrolysis of a cis-dichloro-complex of a saturated macrocycle. The activation parameters, H=53 kJ mol–1 and S298=73 J K–1 mol–1 are consistent with a mechanism in which deprotonation of the substrate is rate-determining. This conclusion is confirmed by the observation of general base catalysis by formate ion. The Brønsted value for the reaction is ca. 0.72 and SN1(CB) and E2 mechanisms are considered to account for the kinetic results. Base hydrolysis of cis-[Co(cyclen)(OH)Cl]+ has also been studied in the pH range 6.5 to 8.7. The value of kOH=3.8×102 dm3 mol–1 s–1 at 25°C and I=0.1 mol dm–3 with H=110 kJ mol–1 and S298=171 J K–1 mol–1 are consistent with an SN1(CB) mechanism.     

Kinetics of the displacement of aqua ligands from cis-diaqua(ethylenediamine)-platinum(II)perchlorate by adenosine 5′-monophosphate in aqueous medium

The kinetics of the interaction of adenosine 5-monophosphate (5-AMP) with cis-[Pt(en)(H₂O)₂]²⁺ have been studied spectrophotometrically as a function of [Pt(en)(H₂O)₂]²⁺, [5-AMP] and temperature at pH 4.0, where the substrate complex exists predominantly as the diaqua species. Both N1 and N7 donor sites of 5-AMP are active for coordination to Pt at this pH. Base stacking and metal-induced macrochelate formation of 5-AMP plays a vital role in determining the concentration limit of 5-AMP during kinetics. Substitution occurs in two consecutive steps; both dependent on the 5-AMP concentration. Activation parameters for both steps have been calculated. The low H ₁ (42.76 ± 1.64 kJ mol^–1) and large negative values of S ₁ (–112.1 ± 5.1 J K^–1 mol^–1) as well as H ₂ (58.1 ± 1.4 kJ mol^–1) and S ₂ (–84.2 ± 4.4 J K^–1 mol^–1) indicate associative modes of activation for both ligand substitution processes in the two consecutive steps.     

Kinetics of substitution of aqua ligands from cis-diaqua(ethylenediamine)platinum(II) perchlorate by DL-penicillamine in aqueous medium

The kinetics of the interaction of DL-penicillamine with [Pt(en)(H₂O)₂]²⁺ have been studied spectrophotometrically as a function of [Pt(en)(H₂O)₂]²⁺, [DL-penicillamine] and temperature at pH 4.0. The reaction proceeds via rapid outer sphere association complex formation, followed by two slow consecutive steps. The first is the conversion of the aforementioned complex into the inner sphere complex and the second is the slower chelation step whereby another aqua ligand is replaced. The association equilibrium constant (K _E) for the outer sphere complex formation has been evaluated together with rate constants for the two subsequent steps. Activation parameters have been calculated for both steps using the Eyring equation (H ₁ = 46.5 ± 5.0 kJ mol^–1, S ₁ = – 143.0 ± 15.0 J K^–1 mol^–1, H ₂ = 44.3 ± 1.3 kJ mol^–1, S ₂ = –189.0 ± 4.2 J K^–1 mol^–1). The low enthalpy of activation and large negative entropy of activation values indicate an associative mode of activation for both aqua ligand substitution processes.     

Surface tension as a function of temperature for some molten metals

Results are given for Sn, In, and Ge from the melting points up to 1700 C, for Pb up to 1400 C, for Tl up to 1100 C, for Bi up to 1300 C, and for Cd up to 600 C. In every case the surface tension is a linear function of temperature. Estimates are made of the critical temperatures of Cd and Ge. It is shown that deductions on the structure of the melts can be drawn from surface-tension data.     

Molecular structure of aniline in the gaseous phase: A concerted study by electron diffraction and ab initio molecular orbital calculations

The molecular structure of free aniline has been investigated by gas-phase electron diffraction and ab initio MO calculations at the HF and MP2 levels of theory, using the 6-31G^*(6D) basis set. Least-squares refinement of a model withC _s symmetry, with constraints from MP2 calculations, has led to an accurate determination of the C-C-C angle at theipso position of the benzene ring, =119.0±0.2 (where the uncertainty represents total error). This parameter provides information on the extent of the interaction between the nitrogen lone pair and the system of the benzene ring, and could not be determined accurately by microwave spectroscopy. The angles at theortho, meta, andpara positions of the ring are 120.3±0.1, 120.7±0.1, and 119.0±0.3, respectively. Important bond distances are r _g(C-C)=1.398±0.003 å andr _g(C-N) =1.407±0.003 å. The effective dihedral angle between the H-N-H plane and the ring plane, averaged over the large-amplitude inversion motion of the amino group, is ¦¦=44±4. The equilibrium dihedral angle is calculated to be 41.8 at the HF level and 43.6 at the MP2 level, in agreement with far-infrared spectroscopic information. The MO calculations predict that the differencer(C_ortho-C_meta) -r(C_ipso-C_ortho) is 0.008–0.009 å. They also indicate that the nitrogen atom is displaced from the ring plane, on the side opposite to the amino hydrogens. The displacement is 0.049 å at the HF level and 0.072 å at the MP2 level. The two calculations, however, yield very different patterns for the minute deviations from planarity of the ring carbons.     

Kinetics and mechanism of the interaction of azide with [(H2O)(tap)2RuORu(tap)2-(H2O)]2+ ion at physiological pH

The title reaction has been studied spectrophotometrically in aqueous medium as a function of [substrate complex], [ligand], pH and temperature at constant ionic strength. At the physiological pH (7.4) the interaction with azide shows two distinct consecutive steps, i.e., it shows a non-linear dependence on the concentration of N₃ ^–; both processes are [ligand]-dependent. The rate constant for the processes are: k ₁10^–3 s^–1 and k ₂10^–5 s^–1. The activation parameters calculated from Eyring plots are: H ₁ = 14.8 ± 1 kJ mol^–1, S ₁ = –240 ± 3 J K^–1 mol^–1, H ₂ = 44.0 ± 1.5 kJ mol^–1 and S ₂ = –190 ± 4 J K^–1 mol^–1. Based on the kinetic and activation parameters an associative interchange mechanism is proposed for the interaction process. From the temperature dependence of the outersphere association equilibrium constant, the thermodynamic parameters calculated are: H ₁ ⁰ = 4.4 ± 0.9 kJ mol^–1, S ₁ ⁰ = 64 ± 3 J K^–1 mol^–1 and H ₂ ⁰ = 14.2 ± 2.9 kJ mol^–1, S ₂ ⁰ = 90 ± 9 J K^–1 mol^–1, which gives a negative G ⁰ value at all temperatures studied, supporting the spontaneous formation of an outersphere association complex.     

Diffusion of linear aliphatic esters in polyethylene as a function of chain length of the esters,pressure and temperature

Diffusion of linear aliphatic mono- and diesters (C _N ) havingN main chain atoms (N=13–68) in bulk medium-density polyethylene (MDPE) has been studied under hydrostatic pressures up to 2500 bar at temperatures between 60°C and 125°C. Three triglycerides, phenyl stearate, and p-aminoazobenzene (pAAB, 80°C) as the diffusants and low-density (LDPE) and high-density (HDPE) polyethylenes as polyethylene substrate were used for comparison. Diffusion coefficientD was determined from concentration distribution of the diffusants through stacked PE sheets as substrate. Regarding the linear esters at 90°C, the relationshipD N holds at constant pressures. Under the atmospheric pressure, became –2.10 in accordance with de Gennes's proposal (1971)D N ^–2 as well as with the experimental results reported by Klein and Briscoe (1979) forN larger than 30.D's for the glycerides deviate from the relationshipD N ^–2 toward the smaller values by comparison at the sameN. The exponent is pressure-dependent. It decreases with increasing pressure according to =–2.10–0.000942P, whereP is measured by the unit of bar. Plots of lnD vsP for all the diffusants show linear relationships with negative slopes, from which activation volume for the diffusion V was calculated. At 90°C, V increases slowly with increasingN and increasingV _Ki, the intrinsic molecular volume of the diffusant, from 39.3 cm³/mol for ethyl caprate (C ₁₃,V _Ki=136 cm³/mol) to 76.8 cm³/mol for behenyl behenate (C₄₅,V _Ki=466 cm³/mol). Observed V s are explainable on the basis of the reptation mode of the chain molecule diffusion. V s for C₂₅ and C₄₅ are found to increase with increasing degree of crystallinity where MDPE, heat-treated MDPE, LDPE, and HDPE were used. The results obtained by varying temperature are as follows. V for C₄₅ was always found to be larger than C₂₅. Both decreased linearly with increasing temperature, giving two linear lines with different slopes whose extensions intersected at 132°C, the melting point of the MDPE, where the difference in V disappeared. The apparent activation energiesE _Ds for the diffusion of C₂₅ and C₄₅ increased linearly with increasing pressure, whose slopes are explainable according toE _D=E ₀+PV [1-(dln V /dlnT) _P ].     

Analytische Dichtegradienten-Zentrifugation bei Temperaturen zwischen +4?C und +40?C

Zusammenfassung Es wird gezeigt, da\ die bei der analytischen Dichtegradienten-Zentrifugation benötigten Parameter, die von Hearst u. Mitarb. tabelliert wurden, auch für andere Temperaturen als 25C gültig sind. Das ist von gro\er Wichtigkeit, da hochempfindliche Enzyme nur bei Temperaturen um + 4C lÄnger als 24 h nativ bleiben. Man ist deshalb auf eine Zentrifugation bei dieser Temperatur angewiesen. Weil man oft nur sehr wenig Substanz isolieren kann, bleibt nur die Dichtegradienten-Zentrifugation als Methode der Wahl. Ein Vergleich der gemessenen Molekulargewichte bei 6C und 25C zeigt keine Abweichungen, die über die Me\genauigkeit hinausgehen.

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Analytical density gradient centrifugation at temperatures between +4C and +40 C

It is shown that all the parameters needed for analytical density gradient centrifugation tabulated by Hearst et al. for 25C are valid for other temperatures, too. This will be important in the investigation of highly sensitive enzymes which are only stable at temperatures near +4C over a period of 24 h. Comparison of molecular weights determined at 6C und 25C shows no difference greater than 2%, which will be within the experimental error.

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