Equlibrate: fix bug with pH charge balancing; mypy

[rkingsbury]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 75.75758% with 8 lines in your changes missing coverage. Please review.

Project coverage is 81.84%. Comparing base (17f90d2) to head (f7d39e9).
Report is 2 commits behind head on main.

Files	Patch %	Lines
src/pyEQL/engines.py	75.75%	7 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #150      +/-   ##
==========================================
- Coverage   81.95%   81.84%   -0.12%     
==========================================
  Files           9        9              
  Lines        1480     1482       +2     
  Branches      320      321       +1     
==========================================
  Hits         1213     1213              
- Misses        225      226       +1     
- Partials       42       43       +1     

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