Solubility Flashcards
Define saturated vapour pressure
The pressure exerted by a vapour in equilibrium with the liquid phase of the same substance. It is influenced by temperature and pressure.
Define Henry’s law. When is the application of Henry’s law important
At a particular temperature the amount of given gas dissolved within a given liquid is directly proportional to the partial pressure of the gas in equilibrium with the liquid.
Important application:
Deep-sea divers:
–> N2 and other gases if breathed under pressure, pass into solution in the tissues,. If return to atmospheric pressure is made too rapidly, the N2 comes out of solution as small bubbles in the joints and elsewhere = DECOMPRESSION SICKNESS = THE BENDS
What is the bunsen solubility coefficient?
Volume of gas, corrected to STP, which dissolves in 1 unit volume of the liquid at the temperature concerned, where the partial pressure of the gas above the liquid is 1 standard atmosphere pressure
What is the Otswald solubility coefficient?
is the volume of gas that dissolves in 1 unit volume of liquid at the temperature concerned
What is the advantage of the Otswald solubility coefficient?
the practical advantage of the Otswald solubility co-efficient is that it is independent of pressure
why is the Otswald coefficient independent of pressure?
Pressure does not modify the volume of gas that dissolves in the liquid, provided that VOLUME IS MEASURED AT AMBIENT PRESSURE
Define the PARTITION COEFFICIENT
The ratio of the amount of substance present in one phase compared with another, the two phases being of equal volume and in equilibrium
Why is it blood : gas coefficient and not gas:blood coefficient
Use the example of N2O dissolved in blood
B:G = 0.47 : 1
So 0.47 L of N2O will dissolve into 1 Litre of blood in equilibrium with 1 L of N2O above the 1 L of blood
So 0.47 divided by 1 = 0.47 (B:G)
G:B –> this would be the reciprocal of the above = 2.1
When is the word ‘tension’ used with regard to gas pressures? Give an example
The TENSION of a gas in solution is the partial pressure of the gas which would be in equilibrium with it.
If the partial pressure of N2O in equilibrium with 1 L of blood is 50 kPa then the N2O dissolved in the blood would have a TENSION of 50 kPA.
Compare the blood gas partition coefficients for the anaesthetic agents. How does increased solubility (i.e. higher B:G) affect the onset of anaesthesia and why?
From most soluble to least soluble
AGENT B:G
Ether 12 Halothane 2.3 Enflurane 1.8 Isoflurane 1.4 Sevoflurane 0.7 Desflurane 0.45 N2O 0.47 Xenon 0.14
It is the Partial Pressure of gas in the blood (and brain tissue) that causes anaesthesia. The dissolved anaesthetic agent in blood does not cause anaesthesia.
Increased solubility means more gas dissolved in blood and carried away from alveoli. It therefore causes a prolonged time before FaAG/FiAG reaches 1. The onset of anaesthetic effect is determined by the speed with which this ratio approaches 1 as it reflects the partial pressure of anaesthetic gas in brain tissue.
Why does N2O cause expansion of air filled spaces
N2O B:G is 0.47
N2 B:G is 0.014
N2O is 34 times as soluble in blood versus N2
This means it moves into Gas spaces 34 x faster than Nitrogen can move out –> expansion of gas filled spaces.
List the MAC, O:G, B:G, speed of anaesthesia, % metabolized for the volatile anaesthetic agents
AGENT (MAC | O:G | B:G | Speed | % Metab)
N2O:
Xenon:
Desflurane:
Sevoflurane:
Diethyl Ether:
Enflurane:
Isoflurane:
Halothane:
Methoxyflurane:
AGENT (MAC | O:G | B:G | Speed | % Metab)
N2O: (105 | 1.4 | 0.47 | Fast | 0%)
Xenon: (71 | 1.9 | 0.14 | Fast | 0%)
Desflurane: (6.6 | 29 | 0.45 | Very Fast | 0.02%)
Sevoflurane: (2 | 80 | 0.70 | Fast | 3.5 %)
Diethyl Ether: (2 | 65 | 12.1 | Slow | 10 % )
Enflurane: (1.68 | 98 | 1.8 | Medium | 2% )
Isoflurane: (1.2 | 98 | 1.4 | Medium | 0.2 %)
Halothane: (0.75 | 224 | 2.4 | Medium | 20%)
Methoxyflurane: (0.16 | 950 | 12 | Slow | 50 %)
