PFK has added a new Nitrite Toxicity Calculator to the Advanced Water Chemistry section of the Fishkeeping Calculators page.
The new program, which was written by Technical and Website Editor Matt Clarke, determines the amount of chloride required to decrease the blood level of methaemoglobin to the lowest possible level during a nitrite crisis.
Nitrite oxidises the iron molecule in the haemoglobin of fish blood to form methaemoglobin which has no oxygen carrying capacity. When methaemoglobin levels rise too high the fish will die.
When the pH of the water is over about 5.0, nitrite becomes concentrated in the blood as it is taken up by the gill anion uptake mechanism used for chloride uptake.
The mechanism can't distinguish between chloride and nitrite, because the ions are of similar size and share the same negative charge. This means that by adding chloride to the water it's possible to out-compete the nitrite uptake, which reduces the percentage of methaemoglobin forming in the blood.
The program uses the system volume, nitrite concentration and hardness to determine how much, if any, chloride is required in order to make conditions safer. It then determines how much salt (sodium chloride) is required to increase the chloride concentration to the target level.
Of course, while the addition of chloride can make conditions safer in the event of a nitrite crisis, the first measures to take must include making a large partial water change and trying to determine why the water quality has deteriorated in the first place.