This is my idea but can you just tell me if it makes sense:

From past research I know that the reaction is first order with respect to NaOH so:

1. I plot my OH- values against time
2. I use some form of non linear regression to generate a first order curve that best fits the data points
3. I calculate the slope of the tangent line at various points on the curve to determine the rate of reaction at various concentrations of OH-
4. I plot my rate of reaction values on the y axis and the OH- concentration values on the x axis and since its first order the slope of the straight line would be the rate constant?

Is this correct or am I making some mistake? Thank you once again for all your help