PH buffers

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

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.