PH buffers

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

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
'''Composition of a buffer solution for pH in synthetic seawater of salinity 35: (weights based on 500 g of H2O)''' Constituent		Moles			Weight (g)
 * Distilled water
 * NaCl
 * KCl
 * Na2SO4
 * Tris: 2-amino-2-hydroxymethyl-1,3-propanediol
 * AMP: 2-aminopyridine
 * HCl 0.1 N
 * CaCl2 (+2H2O)
 * MgCl2 (+6H2O)

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 '''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
 * 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.
 * 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.