Measuring turbidity using a spectrophotometer

I am conducting a lab investigating on the effect of aluminium sulfate concentration on turbidity. In my high-school, we only have a spectrophotometer. Is there a way to measure turbidity with a spectrophotometer? Do I need to dye my solution or alter it in some other way? How do I prepare my blank?




You can use a spectrophotometer to measure turbidity since the suspended particles will scatter light and so a reduced amount of light is detected dependent upon how much scatter occurs. The relationship is $I\_{trans}=I\_0\exp(-k\cdot C)$ where *k* is a proportionality constant depending on the particle size and shape, particle refractive index, sample path-length and *C* is the mass concentration in what ever units you choose. Usually in a spectrophotometer the log is produced by the machine as optical density ( *OD* ). Thus $OD = -\log\_{10}(I\_{trans}/I\_0)= kC$.


It is essential when comparing samples that the particle size is the same otherwise *k* varies between measurements and as *C* is also varied this confuses any measurements. Gum arabic or gelatin are often used to stabilise a suspension. The more finely divided is the suspension the greater is the ability to scatter light. Often experiments are made by comparison to a standard suspension say of Ca oxalate, $\ce{BaSO4}$, or Zn ferrocyanide.


 2