To maximize the performance of the MPT-2 titrator it is necessary to degas the titrants on a daily basis. Without this step titrations can be slow and sometimes may cease to progress at all.
Methods of degassing and the improvements to be expected are detailed in this document, along with examples of titrations of a coffee whitener to show the repeatability possible when the titrants are degassed.
An automatic degassing unit is the most efficient and convenient method of degassing
There are a number of ways of degassing, from boiling the titrants to ultrasonication under vacuum, however the best and most convenient method is to use an automatic vacuum degassing system of the type used by HPLC systems. This requires no attention or maintenance, and only PTFE is in contact with the sample.
The autodegassing unit is compatible with both the MPT-1 and MPT-2.
The unit requires a separate power outlet to be available, and the tubing to fit it between the titrant container and syringe pump inlet is included.
The degasser is switched on after the tubes have been inserted into the titrants, after which no more interaction with it is required. All operations of the Zetasizer and MPT-2 remain the same as before.
The unit has three channels, one for each of the titrants.
Experience has already shown the importance of priming the titrants before starting each titration. Even with this precaution, after a period of use, tests have shown that the accuracy of dispensing a low volume dose depends on how well degassed the titrant is.
All solvents, particularly water, contain dissolved air, and this will come out of solution when a vacuum is applied, as in the case of the MPT-2 when the syringe pumps aspirate the titrants. This can build up in the syringe pumps and valves.
The effects of gas accumulating in the pumps is gradual, in that the dispense volume efficiency of small doses around 2 micro liters reduces from 100% over time, sometimes to 0%. In extreme cases, no titrant will be dispensed, so the titration will not progress. Since the system assumes that the correct volume has been dispensed, this will cause errors in the addition calculation, causing longer titration times and inaccurate pH point spacing. This effect is particularly noticeable around pH 7, when small volumes of titrant are usually dispensed.
These errors and slow operation can be eliminated or reduced, depending on the method used, by degassing the titrants.
|Non-degassed titrants||Degassed titrants|
|Time taken (minutes)||56.5||40.5|
|Calculated dose (ml)||0.54||0.288|
|Accuracy (average deviation between target and achieved pH)||0.1||0.07|
A titration was performed twice on a coffee whitener, once with degassed titrants, once without. The titrants were primed in each case before starting. The results are summarized in table 1, where a titration pH range 9 to 3 in 0.5 pH steps, with a requested tolerance of 0.05. Base 0.25M NaOH. Acids 0.25M and 0.025M HNO3 were used.
The use of degassed titrants clearly improved the performance in all aspects. In this test, the titrants were degassed by filtering through a 0.4 micron filter membrane under reduced pressure, using a filter funnel and a vacuum pump.
There are a number of ways to degas the titrants:
1. Heating to just below boiling point while stirring, then cooling is very efficient, if inconvenient. Heating and stirring under a vacuum will allow the boiling point to be reached at a lower temperature.
2. Filtering under vacuum is less efficient, but more convenient.
3. Ultrasonication with stirring is less effective.
4. Trickling helium continuously through the titrants was used as a standard technique for degassing HPLC eluents, but has now been superseded by method 5.
5. Automated degassing units are available for a range of flow rates. These can have several channels that apply a vacuum to the outside of a length of porous tubing that is inserted between the titrant and the pump.
In cases 1,2 and 3, the degassing effect will only last a few hours. The titrants will return to their 'gassy' state completely after 24 hours in an open container.
Table 2 shows the results of dispensing 2000 doses of 2µL.
|Method||Dispense efficiency (%)|
|15 min ultrasound||40|
|5 min ultrasound||35|
The following graphs show repeat measurements of two batches of coffee whitener, using the automatic degasser, showing that the repeatability of the IEP is good enough to distinguish easily between the two batches.