Can the carbonate hardness be higher than the total hardness?
I have checked the water hardness with the sera GH-Test kit and with the sera KH-Test kit. They turned out to be 7 °dGH and 14 °dKH. How is this possible? I thought the KH value must in any case be lower or at the maximum equal to the GH value, since the carbonate hardness is only a part of the total hardness. Am I wrong?
the reason for this difference is as follows: From a 'chemical' viewpoint, the total hardness ist the total amount of bivalent metal ions. In practical aquarium terms, this can without hesitation be simplified to calcium and magnesium, all other bivalent ions (e.g. strontium or barium) are present in such low concentrations that they are negligible.
The carbonate hardness is the part of the total hardness present as (hydrogen) carbonate. This leads to the conclusion (just as you suggested completely correctly), that the KH can indeed become equal the GH at the maximum.
However, there are also (hydrogen) carbonates of monovalent ions such as potassium or sodium. These are not monitored by GH test kits (which monitor only bivalent metal ions), but they are detected by carbonate hardness test kits. Strictly speaking, you would have to call it "acid binding capacity up to pH 4.3" instead of KH - which hardly anybody within the aquarium hobby does. In other words: When we talk about "KH" within the aquarium hobby, we actually mean the acid binding capacity.
This is why the measured "KH" can actually be higher than the GH. This is, by the way, not a "constructed laboratory example", but this situation actually occurs in nature. Lakes Malawi and Tanganyika are well known examples which are also relevant in aquarium terms: The measured total hardness is about 5 °dGH in Lake Malawi, the "carbonate hardness" is about 7 °dKH. In Lake Tanganyika they are about 11 °dGH and 16 °dKH.
Dr. Bodo Schnell