Frequently Asked Questions about Density Gradient Column Fillers
- Why do I need the H&D Fitzgerald column filler?
- Which liquids can be used?
- How long does the filler take to build the column?
- I am building a column from propanol and water. Why does it appear ‘milky’?
- How was the H&D Fitzgerald column filler developed?
Why do I need the H&D Fitzgerald column filler?
The microprocessor controlled density gradient column filler allows the user to build accurate and consistent columns quickly. It overcomes the problems associated with ‘hand-filling’ by replacing the often inaccurate and extremely time consuming methods previously used with an automated and repeatable method.
Which liquids can be used?
Any two miscible liquids can be used to provide the heavy and light densities. The table below shows some suggested liquids and their density ranges.
|Mixture||Density Range (kg/m³)||Density Range (g/ml)|
|2-propanol & water||785 – 997||0.785 – 0.997|
|2-propanol & di(ethylene glycol)||785 – 1130||0.785 – 1.130|
|ethanol & water & zinc chloride||790 – 1700||0.790 – 1.700|
|water & sodium bromide||997 – 1400||0.997 – 1.400|
|water & calcium nitrate||997 – 1600||0.997 – 1.600|
|water & caesium formate||997 – 2350||0.997 – 2.350|
|ethanol & tetrachloroethylene||790 – 1620||0.790 – 1.620|
|toluene & tetrachloroethylene||870 – 1620||0.870 – 1.620|
|tetrachloroethylene & 1, 3-dibromopropane||1620 – 1985||1.620 – 1.985|
|1, 3-dibromopropane & 1, 2-dibromoethane||1985 – 2180||1.985 – 2.180|
|tetrachloroethylene & bromoform||1620 – 2890||1.620 – 2.890|
|1, 2-dibromoethane & bromoform||2180 – 2890||2.180 – 2.890|
In some circumstances, it will be necessary to use a three component mixture, normally because the sample is hydrophobic, and cannot be wetted by water. An example of such a system is the use of 8% of 1-methyl-2-pyrrolidinone in water/sodium bromide columns when testing certain plastics.
How long does the filler take to build the column?
A ≈1m column can be built in around 20 minutes. As the process is fully automated, the filler can be left unattended during this time, leaving the operator free to undertake other tasks and duties.
I am building a column from propanol and water. Why does it appear ‘milky’?
This is an entirely natural phenomenon. Exactly the same thing happens when water and IPA is mixed in any other vessel.
IPA, and other alcohols, can contain very large amounts of air. When you mix water with alcohol, the alcohol molecules prefer to interact / ‘bond’ with water molecules, rather than air molecules, so this air is released as extremely small micro-bubbles, which make the solution cloudy. The mixed solution, once in the column, is still saturated with air, and as you add more water from your water bottle, more air will be released.
The solution can stay super-saturated with air until a float or a piece of plastic is dropped into the column – this will act as a nucleation centre, and become coated with bubbles; hence you will get the wrong density. To attempt to get round this problem, you should make up two water / IPA solutions, one heavier than the desired density at the bottom of the column, and the other lighter than the top density. Deair these two solutions by just bringing them to the boil, and then allow them to cool in closed containers. The act of boiling will change the densities – you’ll have to determine experimentally what densities your two starting solutions will need to be. These liquids will not be saturated with air, and should therefore reduce or eliminate the formation of micro bubbles. Once the column is made, the small area at the top of the liquid will absorb air back into solution, but this should be relatively small.
Remember that when you mix two propanol / water solutions, you will still see some shrinkage, but less than when you mix pure propanol & pure water. The amount of shrinkage will depend on the densities of the two solutions.
How was the H&D Fitzgerald column filler developed?
At H&D we supply calibrated density gradient floats which are used to calibrate density gradient columns. The raw uncalibrated floats are initially supplied to us with densities that are within a nominal density range. We established that sorting these floats into narrower nominal ranges would greatly speed up our calibration process, and decided to sort them by setting up our own density gradient columns.
After setting up the column tubes in a vertical water bath, we needed a way to fill them. However our requirements for the final density gradient included a better linearity and repeatability than was normally achieved by traditional hand-filling methods. We also required a wide range of densities. Unable to find a commercially available way of achieving this, we designed our own specific apparatus. This project gave rise to the column filler which we sell today.