Measuring turbidity using a spectrophotometer
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.