PH buffers

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pH buffers (tris/HCl buffer and 2-aminopyridine/HCl buffer) are prepared in synthetic sea water according to DOE (1994)

Contents

Equipment and chemicals

Equipment

(for ½ l of buffer solution)

  • Dessicator with silica gel
  • Precision balance
  • 8 x 100 ml beakers (for chemical weighing)
  • 2 x 600 ml beakers (for final buffer solution mixing)
  • 1 x 200 ml volumetric pipet (for HCl)
  • 2 x magnetic stir bars
  • Small plastic flaks (for storage of buffer solutions)


Chemicals

  • Distilled water
  • NaCl
  • KCl
  • Na2SO4
  • Tris: 2-amino-2-hydroxymethyl-1,3-propanediol
  • AMP: 2-aminopyridine
  • HCl 0.1 N
  • CaCl2 (+2H2O)
  • MgCl2 (+6H2O)



Composition of a buffer solution for pH in synthetic seawater of salinity 35:
(weights based on 500 g of H2O)

      Constituent		Moles			Weight (g)

      NaCl			0.1938			11.3223 g
      KCl			0.0053			0.39420 g
      Na2SO4		        0.0146			2.07815 g
      MgCl2			0.0274			see Mohr’s method*
      CaCl2			0.0137			see Mohr’s method*
      HCl 0.1 N		        0.02			200 ml


 One of :

      Tris			0.04			4.8418 g
      AMP			0.04			3.7615 g


 Total weight of solution containing:

      Tris						522.045 g
      AMP						520.965 g

*Mohr's method


Method


  1. Chemical drying and weighing
  2. Determination of MgCl2 and CaCl2 concentrations
  3. Chemical mixing


Detailed protocol


1. Chemical drying and weighing

  • NaCl, KCl, Na2SO4 and tris: 120 °C for 2 h followed by cooling in a desiccator for 1 h.
  • AMP: 50 °C for 2 h.
  • Double the given weights of chemicals before drying since the desiccation in the oven reduces weights.


2. Determination of MgCl2 and CaCl2 weights
Solutions of MgCl2 and CaCl2 must be analyzed by titrating with a calibrated silver nitrate solution
(see Mohr’s method)

3. Chemical mixing

  • Add Chemicals in 600 ml beakers with magnetic stir bars on a precision balance.
  • Add NaCl, KCl, Na2SO4, tris (or AMP), MgCl2, CaCl2 and HCl with 200 ml volumetric pipet.
  • Add distilled water to bring final weight to 522.045 g for tris and 520.965 g for AMP.
  • Mix with a magnetic stir bar
  • Divide the buffer solutions into plastic flasks and stored them in a cool dark place.
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