standardize_formula: add ammonium sulfate

CHANGELOG.md

@@ -20,6 +20,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `gibbs_mix`: A new keyword argument `activity_correction` was added to `gibbs_mix`. It defaults
   to `True` (no change from prior behavior), but can be set to `False` in order to calculate the
   ideal mixing energy, which is equivalent to only considering entropic effects. (#178, @Jaebeom-P)
+- `standardize_formula`: Improve formatting of ammonium sulfate salts. Aqueous ammonium sulfate previously
+  standardized to `H8S(NO2)2(aq)`, now it will display as `(NH4)2SO4(aq)`.
 
 ### Changed
 

src/pyEQL/utils.py

@@ -137,6 +137,12 @@ def standardize_formula(formula: str):
     elif sform == "C2I2ClO2[-1]":
         sform = "CI2ClCOO[-1]"
 
+    # ammonium sulfate salts
+    elif sform == "H8S(NO2)2(aq)":
+        sform = "(NH4)2SO4(aq)"
+    elif sform == "H4SNO4[-1]":
+        sform = "NH4SO4[-1]"
+
     # TODO - consider adding recognition of special formulas like MeOH for methanol or Cit for citrate
     return sform
 

tests/test_utils.py

@@ -52,6 +52,8 @@ def test_standardize_formula():
     # Cl+Br
     assert standardize_formula("CBrCl2COO-") == "CBrCl2COO[-1]"
     assert standardize_formula("CBr2ClCOO-") == "CBr2ClCOO[-1]"
+    assert standardize_formula("(NH4)2SO4") == "(NH4)2SO4(aq)"
+    assert standardize_formula("NH4SO4-") == "NH4SO4[-1]"
 
 
 def test_formula_dict():