Hydrogen Ion and Carbon Dioxide Measurement Flashcards
Define pH
pH is a measure of the H ion activity in a liquid. H ion activity is not exactly the same as H ion concentration but, for practical purposes in the clinical situation, they may be regarded as equivalent.
pH = - log [H+]
Hence a pH of 9 = [H+] of 10^-9 mol.L^-1
and
pH of 7.4 is 10^-7.4 = 40 nmol/L (normal [H])) at 37 deg C
What is the relationship between pH and [H] over the middle of the clinical range
Practically linear, the pH decreasing by 0.1 for each 10 nmol/L. Thus [H] of 50 nmol/L corresponds to pH 7.3
How can {H} ion be measured in blood?
Using the H+ ion electrode. Most of these devices include a PCO2 and PO2 electrode too.
The H electrode is an example of an ion selective electrode and depends for its operation on the H ion sensitive glass at its tip. A potential develops across the glass which depends on the [H+] difference across it.
- [H] within electorde is maintained at constant value by buffer solution. So potential across glass tip is dependent on H+ gradient across glass tip
- To measure potential difference it is necessary to make electrical contact with the blood and buffer solution. A reference electrode is used which has a membrane at its tip to avoid contamination
How is the electrical circuit completed in a H ion electrode in order to get a current reading and hence an H ion concentration measurement
In both H ion and reference electrode, stability of the metal to solution contact is accomplished through a special electrode of silver in contact with its chloride which is in turn in contact with a solution containing Cl- ions.
The H+ electrode is connected to the reference electrode and the potential difference between these electrodes is measured representing the H ion concentration
How is the stability of the metal to solution contact in both the H+ electrode and Reference electrode achieved
Using a special electrode of silver in contact with its chloride, which is in turn in contact with a solution containing Cl- ions
Describe the mechanical layout of these electrode systems
Sample port containing blood which is in contact with: 1. Reference electrode
- H+ Electrode glass tip
- PO2 electrode
- PCO2 electrode
Which is then attached to a suction control mechanism downstream which moves the sample past the reference electrode and then these electrodes
There is a temperature control unit as with increasing temperature influences the dissociation of H+ ions in solution an will also effect measurements in the Clark Electrode and PCO2 electrode
How is error prevented with the H ion electrode
- Calibration with a fixed concentration of 2 phosphate buffers, for which the H ion concentration has been decided by international agreement
- Maintenance (no holes/protein deposits)
What is the Severinghaus CO2 electrode
- H ion sensitive glass barrier separating two electrodes
- Outer chamber filled with NaHCO3 and in contact with plastic membrane which is in contact with blood.
- CO2 diffuses into outer chamber and combines with water to favour the forward reaction for the CO2 equation. This produces H+ ions. The barrier between the outer and inner chamber only permits H ions.
- The change in H+ concentration is measured by the electrodes and displayed as the PCO@ tension
Why is calibration of PCO2 electrodes NB for dry gas mixtures vs fully humidified mixtures
Must correct for SVP of H2O which is 6.3 kPA. Subtract this from barometric pressure before multiplying by the partial pressure.
Why is the Severinghause electrode slower than the HHydrogen Ion electrode?
The Severing hause electrode response time is 2 - 3 minutes.
The H ion electrode response time is about 1 minute
The Severinghaus electrode takes longer as time is taken for CO2 to diffuse through the plastic membrane.
calculate the partial pressure of 5% CO2 at atmospheric pressure at temperature of 37 deg C in completely humidified gas mixture
5/100 x (100 - 6.3)kPa = 4.7 kPa
6.3 is the saturated vapour pressure of water
What is the principle by which infrared analysers can determine the partial pressure of a gas in a mixture
Gases that have two or more different atoms in the molecule absorb infrared radiation.
By measuring the fraction of radiation absorbed by the by the gas mixture, the partial pressure of a particular gas can be determined.
Each gas absorbs radiation at a characteristic wavelength.
The proportion of gas absorbed also depends on the wavelength
How is intereference from other gases reduced in an infrared analyser reduced
Using a specific wavelength which is characteristically absorbed by the gas of interest. E.g. CO2 at 4.28 nm (minimum intereference from N2O which absorbs 4.35 to 4.5 nm)
How is the specific wavelength of radiation emitted through the sample chamber of an infrared analyzer
Infrared radiation is emitted by a hot wire which passes through a filter which filters all but the wavelength of interest.
How are devices which measure the partial pressures of multiple gases configured
These incorporate variable filters for the various wavelengths of interest. The filters are mounted on a rotating disc (as in spectrophotometry) to permit simultaneous analysis of several sample compounds
