Base of logarithm in runtime of Prim's and Kruskal's algorithms
To expand on what David Eppstein said, you can assume it's in any base.

If the log were initially in base $b$, it is asymptotically the same as if it were in base $k$:  

$$O(\log_b n) = O\left(\frac{\log_k n}{\log_k b}\right) = O(\log_k n)$$

because $k$ and $b$ are both constants; $\log_k b = O(1)$.

The logs in any two bases are the same in big-O notation.