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Journal of Solution Chemistry

, Volume 27, Issue 1, pp 73–87 | Cite as

Thermodynamics of the Second Dissociation Constant and Standards for pH of 3-(NMorpholino) Propanesulfonic Acid (MOPS) Buffers from 5 to 55°C

  • Rabindra N. Roy
  • Dawn R. Mrad
  • P. A. Lord
  • Julie A. Carlsten
  • William S. Good
  • Paul Allsup
  • Lakshmi N. Roy
  • Kathleen M. Kuhler
  • W. F. Koch
  • Y. C. Wu
Article

Abstract

The second dissociation constant pK2 of 3-(N-morpholino)propanesulfonic acid (MOPS) has been determined at eight temperatures from 5 to 55°C by measurements of the emf of cells without liquid junction, utilizing hydrogen electrodes and silver–silver chloride electrodes. The pK2 has a value of 7.18 ± 0.001 at 25°C and 7.044 ± 0.002 at 37°C. The thermodynamic quantities ΔG°, ΔH°, ΔS°, and ΔC p o have been derived from the temperature coefficients of the pK2. This buffer at ionic strength I = 0.16 mol-kg−1 close to that of blood serum, has been recommended as a useful secondary pH standard for measurements of physiological fluids. Five buffer solutions with the following compositions were prepared: (a) equimolal mixture of MOPS (0.05 mol-kg−1) + NaMOPS, (0.05 mol-kg−1); (b( MOPS (0.05 mol-kg−1) + NaMOPS (0.05 mol-kg−1) + NaCl (0.05 mol-kg−1); (c) MOPS (0.05 mol-kg−1) + NaMOPS (0.05 mol-kg−1); + NaCl (0.11mol-kg−1); (d) MOPS (0.08 mol-kg−1) + NaMOPS (0.08 mol-kg−1); and (e)MOPS (0.08 mol-kg−1) + NaMOPS (0.08 mol-kg−1) + NaCl (0.08 mol-kg−1).The pH values obtained by using the pH meter + glass electrode assembly are compared with those measured from a flow–junction calomel cell saturated with KCl (cell B), as well as those obtained from cell (A) without liquid junction at 25 and 37°C. The conventional values of the liquid junction potentials Ej have been obtained at 25 and 37°C for the physiological phosphate reference solution as well as for the MOPS buffers (d) and (e) mentioned above.

Dissociation constant buffers NaMOPS temperature dependence emf Gibbs energy enthalpy entropy heat capacity zwitterion liquid junction 

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Copyright information

© Plenum Publishing Corporation 1998

Authors and Affiliations

  • Rabindra N. Roy
    • 1
  • Dawn R. Mrad
    • 1
  • P. A. Lord
    • 1
  • Julie A. Carlsten
    • 1
  • William S. Good
    • 1
  • Paul Allsup
    • 1
  • Lakshmi N. Roy
    • 1
  • Kathleen M. Kuhler
    • 1
  • W. F. Koch
    • 2
  • Y. C. Wu
    • 2
  1. 1.Hoffman Department of ChemistryDrury CollegeSrpingfield
  2. 2.Chemical Science and Technology LaboratoryNational Institute of Standards and TechnologyGaithersburg

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