Bugfix in get_viscosity_kinematic

CHANGELOG.md

@@ -5,6 +5,14 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.3.2] - 2025-09-15
+
+### Fixed
+
+- `Solution.get_viscosity_kinematic()`: A recent change to get_salt_dict (#258) created a problem with `get_viscosity_kinetmatic`
+  in which an empty solution would cause the method to return an error. This has been fixed, and unit tests added for both
+  `get_viscosity_kinematic` and `get_viscosity_dynamic`.
+
 ## [1.3.1] - 2025-08-18
 
 ### Fixed

src/pyEQL/solution.py

@@ -651,7 +651,7 @@ def viscosity_kinematic(self) -> Quantity:
         a0 = a1 = b0 = b1 = 0
 
         # retrieve the parameters for the delta G equations
-        params = self.get_property(salt.formula, "model_parameters.viscosity_eyring")
+        params = None if salt is None else self.get_property(salt.formula, "model_parameters.viscosity_eyring")
         if params is not None:
             a0 = ureg.Quantity(params["a0"]["value"]).magnitude
             a1 = ureg.Quantity(params["a1"]["value"]).magnitude
@@ -662,11 +662,16 @@ def viscosity_kinematic(self) -> Quantity:
             temperature = self.temperature.to("degC").magnitude
             G_123 = a0 + a1 * (temperature) ** 0.75
             G_23 = b0 + b1 * (temperature) ** 0.5
+
+            # calculate the cation mole fraction
+            # x_cat = self.get_amount(cation, "fraction")
+            x_cat = self.get_amount(salt.cation, "fraction").magnitude
         else:
             # TODO - fall back to the Jones-Dole model! There are currently no eyring parameters in the database!
             # proceed with the coefficients equal to zero and log a warning
-            self.logger.warning(f"Viscosity coefficients for {salt.formula} not found. Viscosity will be approximate.")
+            self.logger.warning(f"Appropriate viscosity coefficients werenot found. Viscosity will be approximate.")
             G_123 = G_23 = 0
+            x_cat = 0
 
         # get the kinematic viscosity of water, returned by IAPWS in m2/s
         nu_w = self.water_substance.nu
@@ -678,10 +683,6 @@ def viscosity_kinematic(self) -> Quantity:
         # get the MW of water
         MW_w = self.get_property(self.solvent, "molecular_weight").magnitude
 
-        # calculate the cation mole fraction
-        # x_cat = self.get_amount(cation, "fraction")
-        x_cat = self.get_amount(salt.cation, "fraction").magnitude
-
         # calculate the kinematic viscosity
         nu = np.log(nu_w * MW_w / MW) + 15 * x_cat**2 + x_cat**3 * G_123 + 3 * x_cat * G_23 * (1 - 0.05 * x_cat)
 

tests/test_solution.py

@@ -97,7 +97,7 @@ def s6_Ca():
     )
 
 
-def test_empty_solution_3():
+def test_empty_solution():
     # create an empty solution
     s1 = Solution(database=None)
     # It should return type Solution
@@ -118,6 +118,9 @@ def test_empty_solution_3():
     assert np.isclose(s1.pE, 8.5)
     # it should contain H2O, H+, and OH- species
     assert set(s1.components.keys()) == {"H2O(aq)", "OH[-1]", "H[+1]"}
+    assert np.isclose(s1.density.to('kg/m**3').magnitude, 997.0479, atol=0.1)
+    assert np.isclose(s1.viscosity_kinematic.to('mm**2/s').magnitude, 0.8917, atol=1e-3) # 1 cSt = 1 mm2/s
+    assert np.isclose(s1.viscosity_dynamic, s1.viscosity_kinematic * s1.density, atol=1e-8)
 
 
 @pytest.mark.skipif(platform.machine() == "arm64" and platform.system() == "Darwin", reason="arm64 not supported")