Not all elements are detectable by flame photometry.
When using a flame photometer to detect the concentration of an element, this element must exhibit a specific set of prerequisites to make it suitable for analysis.
The similarities between sodium and hydrogen would suggest that you could measure the concentration of hydrogen ions in solution and the acidity of a solution via flame photometry.
However, this is not the case.
What are the similarities of hydrogen and sodium?
Hydrogen and sodium are both located in period one (the first row of the periodic table).
This means they both have one electron in their outer shell in their atomic form and are relatively small in size.
They both also have a high charge density in their ionic form from the total loss of their outer electron shell (the 1sX orbital), leading to contraction of the ion's diameter.
Can you detect hydrogen using flame photometry?
No. The main factor that prevents hydrogen being analysed by photometry is its lack of electrons.
In fact, it only has a single electron present in its entire structure.
The ionisation of this element to its ionic form would result in a single proton, a proton and a neutron or a proton and two neutrons, dependent on the isotope.
Due to the lack of electrons, there is no possibility for the promotion of an electron to a higher energy state and, therefore, no possibility for the relaxation and release of a photon.
Can you detect sodium using flame photometry?
Sodium has a full 1P and 1S orbital after the ionisation of the element losing its 1s1 electron from the outer shell to form the Na+ ion.
The element also cannot exist in its metallic form when given contact to water.
This ensures all of the sodium present in the solution would be in the ideal form to determine its concentration via photometry.
How does the ionisation of sodium affect its diameter and charge density?
In its elemental form, the diameter of sodium is 154 picometers, in comparison to its ionic form which has a radius of 116 picometers.
This is an overall contraction of ionic radius size of 24.7%.
To summarise, the only reason hydrogen cannot be analysed by photometry due to its lack of electrons.
Comparing hydrogen to a suitable element such as sodium, will hopefully expand your insight into element selection for flame photometry.