Serial dillutions and oclorimetry

• What are serial dilutions


  they are created by taking repeated dilutions starting from a stock solution of known concentration
• What happens to the quantity of the solution in serial dilutions 


  it decreases by the same quantity between each test tube
• Describe a method for serial dilutions for a 10cm^3 of 2M glucose solution 


  Use a syringe to transfer 5cm^3 of the 2M stock solution to tube 1

  Add 5cm^3 of distilled water to tube 1 (10cm^3 in total)

  Shake tube/mix contents

  repeat the process for tubes 2, 3 and 4
• How is the required solution of concentration achieved in serial dilutions?


  By taking a specific volume of the stock solution each time and topping this up with distilled water
• How to calculate the volume of stock solution required?


  (Desired concentration/concentration of stock) x volume wanted
• Why is establishing the reducing sugar concentration of a solution subjective?


  Means that results can be bias or affected by human error
• What is a colorimeter?


  a piece of equipment that beams a specific wavelength (colour) of light through a sample and measures how much of this light is absorbed by the sample ( or how much is transmitted )
• What does a higher absorbance (low transmission) mean in colorimeters


  a greater concentration
• What does a lower absorbance or high transmission mean in colorimeters?


  lower concentration
• Why should we repeat readings with colorimeters?


  Enables you to remove anomalous results and calculate a mean
• The more decimal places the higher the resolution
• What are colour filters used to determine?


  the light wavelength being emitted by the colorimeter
• How is a colorimeter calibrated?


  by placing a corvette of distilled water ( a blank) into the colorimeter setting it to zero absorbance (no light being absorbed)
• What does calibrating a colorimeter ensure?


  all values are measured to the same standard and are compatible accurate and valid
• What are calibration curves used for?


  to plot the absorbance of known concentrations. The calibration curve (line of best fit) can be used to determine an unknown concentration from its absorbance
• Describe a method to find the unknown concentration of a solution?


  Set the colorimeter to zero

  Using the cuvette of distilled water (a blank)

  Select the colour of filter (usually red)

  Use known concentrations of solutions by producing serial dilutions

  make a calibration curve (graph)X axis = concentration, Y axis = absorbance

  Test unknown sample using the same method to establish its absorbance

  Follow the absorbance value/ value on y axis across to line of best fit/ calibration curve and then down to the concentration/ x axis

  Find the concentration that corresponds to this absorbance
• Describe how someone would create 4 different enzyme concentrations using tenfold serial dilutions. They started by using a syringe to transfer 1 cm^3 of the 1% trypsin solution into tube 1. With a clean syringe they then added 9cm^3 of distilled water to tube 1. (2)


  • shake tube
  • remove 1cm^3 from tube 1 and add 9cm^3 of distilled water to tube 2
  • repeat processes for tubes 3 and 4