Question

I'm working with a delicate experiment whilst trying to test corrosion of stainless-steels in a chloramine environment. I've run into various literature that discusses the formation of different types of chloramines relative to pH and ratios of chlorine to nitrogen (or ammonia). In order to create this environment without being a complete f-bomb and accidentally creating concentrated Nitrogen Trichloride. I'd love some clarification for my non-chemical-based knowledge.

I've read that monochloramine dominates when the chlorine to nitrogen weight ratio in <5:1 (or 1:1 molar ratio). Separate ratios exist for other chloramines but let's focus on this for the purpose of my question.

Firstly, I'm a little confused how to convert between these units. I imagine the 'weight ratio' is measure as PPM or mg/L, while molar ratio is g/mol? I feel like this should be easy but my head's in a twist.

Secondly, separate literature use similar figures for 'chlorine to nitrogen ratio [Cl:N]' whilst other reference 'chlorine to ammonia ratio [Cl:NH3]' (or even chlorine to ammonia-nitrogen). Are these synonymous or should weight of the included hydrogen (albeit minimal) be included?

Tertiary , I'd assume the ratios mentioned above refer to 'free chlorine residual' not total chlorine, which includes chloramines (chorine already bonded to ammonia). Sounds obvious but just checking.

Answer 1

To convert between weight ratios (mg/L) and molar ratios (g/mol) for the purpose of creating chloramine in a controlled experiment, the molar masses of the substances involved must be considered. The chlorine to nitrogen weight ratio differs from the chlorine to ammonia ratio because the latter includes the hydrogen's mass.

Step-by-step explanation:

When converting between weight ratios (often expressed in parts per million or mg/L) and molar ratios (expressed in g/mol), it is important to take into account the molar masses of the substances involved.

In the context of chloramine formation, the weight ratio refers to the ratio of chlorine to nitrogen by mass, which can be converted to a molar ratio by using the molar masses of chlorine and nitrogen to find the corresponding number of moles for each. Once you have the number of moles, you can establish the molar ratio by simply comparing them.

Regarding the chlorine to ammonia ratio [Cl:NH3] vs. the chlorine to nitrogen ratio [Cl:N], these are not synonymous. The former includes the mass of hydrogen atoms in ammonia. However, for the purpose of creating monochloramine, you can typically disregard the mass of hydrogen as it is minimal compared to chlorine and nitrogen.

Lastly, in the context of water treatment and chloramine formation, the ratios mentioned typically do refer to 'free chlorine residual' and not total chlorine.

This is because free chlorine residual refers to the amount of chlorine available to react with nitrogen (or ammonia) to form chloramines, whereas total chlorine includes both free chlorine and chlorine that has already reacted to form compounds such as chloramines.