Solve bug in water activity due to undefined formulas at zero concentration for Tang KCl and NaCl

aquasol/solutions/formulas/activity/KCl.py

@@ -27,7 +27,7 @@ default_source = 'Tang'
 
 concentration_types = {'Tang': 'm'}
 
-concentration_ranges = {'Tang': (0, 13)}
+concentration_ranges = {'Tang': (1e-9, 13)}
 
 temperature_units = {'Tang': 'C'}
 

aquasol/solutions/formulas/activity/NaCl.py

@@ -39,7 +39,7 @@ concentration_types = {'Clegg': 'x',
                        'Tang': 'm'}
 
 concentration_ranges = {'Clegg': (0, 0.23),
-                        'Tang': (0, 13)}
+                        'Tang': (1e-9, 13)}
 
 temperature_units = {'Clegg': 'C',
                      'Tang': 'C'}