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)
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
Method
- Chemical drying and weighing
- Determination of MgCl2 and CaCl2 concentrations
- 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 weight of chemicals since the desiccation in the oven reduces weights.
2. Determination of MgCl2 and CaCl2 concentrations
see Mohr’s method
3. Chemical mixing
- Add Chemicals in 600 ml beaker with magnetic stir bar 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 for AMP.
- Mix with a magnetic stir bar
- Divide into plastic flasks the buffer solution and stored the flasks in a cool dark place.