What is the right wavelength to use for the colorimeter?
The trick is to take one of your blue solutions and investigate its properties with YOUR colorimeter. If you have a colorimeter with a knob to control wavelength, then take readings over the scale of the meter. At each wavelength, set the zero with a water blank and then take a reading with your solution. Use the wavelength which shows the greatest absorption – probably in the red end of the spectrum. If your colorimeter has filters, then carry out the above procedure with each filter and then choose the filter which gives the greatest absorption.
