sequences and series – Find an equation for a curve

How would I go about trying to fit an equation to the data points shown in the chart below. The chart was generated in Excel. Excel generates the solid line when the chart type selected is scattered with smooth line. The dotted line is the best equation that Excel can fit using “Add Trendline”. The curve is a plot of umbelliferone fluorescence versus hydroxyl ion concentration. The smooth curve that Excel generated is approximately correct based on literature data that shows the fluorescence of umbelliferone versus pH (I chose to plot versus hydroxyl ion concentration and the x axis is a logarithmic scale).

So how could I go about trying to generate an approximate equation for the solid curve shown that would give a better fit than the logarithmic equation that Excel can come up with.

enter image description here