Sodium ion migration has been implicated in potential induced degradation of the shunting type. The kinetics of sodium migration in silicon nitride can be investigated using a simple capacitance-voltage characterization technique. However, we show that the charge trapping strongly affects the measurement and must be taken into consideration in the experimental approach. We perform a systematic analysis of charge carrier trapping in silicon nitride, allowing us to correct trapping effects that otherwise prevent sodium signal detection. We extract trapping and detrapping time constants both on the order of several hours. This shows that trapping cannot be ignored as it occurs relatively quickly and is constantly evolving in time. Accurate determination of sodium behavior in dielectric layers will facilitate robust co-optimization of these layers for optical, passivation, and barrier properties.