Can potassium ions depolarise neuron membranes?
Yes, this is a very basic neuroscience question, something that would probably come up in the first week of a neurophysiology class or unit.
The polarization of the membrane at rest is due to cells being primarily permeable to potassium at rest, and due to a concentration difference of potassium inside vs. outside the membrane. Inside, potassium is high, outside potassium is low.
Therefore, there is a concentration gradient such that if a cell were at 0 mV, potassium would be flowing out of the cell. There is a membrane voltage where this net flow would stop. We refer to this as the &quot;reversal potential&quot; or &quot;equilibrium potential&quot; for an ion. For potassium in a typical cell, this tends to be around -90mV. The larger the ratio of concentrations in versus out, the larger the potential. You can calculate this potential with the Nernst equation if you know the concentrations (image from Wikipedia):

From this equation you can see that the reversal potential is negative for a positive ion if [outside] is small compared to [inside]. By applying higher potassium outside than usual, the ratio of [outside]/[inside] gets closer to 1, so the reversal potential for potassium is closer to 0mV, and typical neurons that are mainly permeable to potassium at rest will depolarize.