[Babcock_Hall]

The recipes I have seen for the bicinchoninic acid call for 0.16% disodium tartrate dihydrate (I don't yet have the original 1985 reference).  I have sodium potassium tartrate.  Using this would produce a concentration of potassium ions of 7 mM.  So far I have not seen anything to suggest that potassium interferes with this particular assay.  However, the Lowry assay is also based on copper ions, and 30 mM potassium phosphate is listed as the highest acceptable concentration (Bollag et al., Protein Methods, 2nd ed. 1996).  On the other hand one recipe I consulted (Ninfa et al. Fundamental Laboratory Approaches for Biochemistry and Biotechnology) for the Lowry assay calls for use of sodium potassium tartrate.  ThermoFisher's website suggests that sodium potassium tartrate is present in both the Lowry assay and the BCA assay.  Their website notes, "The Lowry assay reagent forms precipitates in the presence of detergents or potassium ions. When potassium ions are the cause, the problem can sometimes be overcome by centrifuging the tube and measuring the color in the supernatant. "  There is also a copper-based method called the Biuret assay, and one recipe (Ninfa et al.) calls for sodium potassium tartrate.
https://www.thermofisher.com/us/en/home/life-science/protein-biology/protein-biology-learning-center/protein-biology-resource-library/pierce-protein-methods/chemistry-protein-assays.html

Does anyone know if there is likely to be a problem with this concentration of potassium ions?

[Yggdrasil]

According to https://tools.thermofisher.com/content/sfs/brochures/TR0068-Protein-assay-compatibility.pdf the BCA assay is compatible with up to 3M potassium thiocyanate, so 7 mM potassium should not be problem.

[Babcock_Hall]

Thank you very much.  I missed that particular page at the ThermoFisher web site, but it is quite useful.