So, firstly, HC-110 is relatively low toxicity because the amount of developing and other agents it requires is low. Follow the instructions and take reasonable care but don’t be too paranoid. Remember that exposure is key. So if you work in a darkroom you need to be more and routinely careful than if you develop one tank of films a month. These chemicals also have environmental toxicity and should be disposed of responsibly (what ‘responsibly’ means depends on where you are).
In the table above:
The first component is hydroquinone, a developing agent. In its pure form it is a white crystalline powder.
The second is monoethanolamine (MEA) - a viscous liquid at room temperature, but 1-3% MEA is not enough to make a mixture ‘syrupy’. It is alkaline in solution, and is one of the alkali/activator components of HC-110.
The third component is diethylenetriaminepentaacetic acid (DTPA) - a chelating agent to manage calcium, magnesium and other nasties in your water. It is like a longer molecule version of EDTA. It is also a powder in pure form.
The fourth, potassium bromide, is a restrainer, and is another powder.
The fifth is dimezone, a developing agent like phenidone but more resistant to oxidation. Also a powder.
The last is catechol, another developing agent that is a powder at room temperature.
MDS/MSDS documents are a terrible source of information about formulations - they do not need to show non-toxic/non-hazardous components, and they do not need to show anything that is in the product at less than 2%. I also do not know if German MDS documents state the perfectage in the syrup or the working solution. My guess is the former, but I wouldn’t guarantee it.
So the short answer is probably that none of those make it syrupy, but what does is something like MEA but is non-toxic or generally recognized as safe (GRAS). Both diethanolamine (DEA) and triethanolamine (TEA) are used in cosmetics and may be used in HC-110, as are propylene glycol or dimethylsulfoxide, which are also largely inert but very good solvents. HC-110 is complex, and there is also probably a non-active carrier, which may be part of what makes it thick.
I may write a long HC-110 rant ( like my
Long Xtol rant) some time (when I have time) if anyone is interested.