The invention relates to an aqueous potassium formate solution, the salt concentration being between 20 and 55 % by weight, and potassium carbonate is added to the aqueous potassium formate solution. At a refrigerant temperature of -15 to -30 DEG C, the salt concentration of the potassium formate is between 20 and 35 % by weight, preferably between 25 and 30 % by weight, and the salt concentration of the potassium carbonate is between 5 and 15 % by weight, preferably 7 and 10 % by weight. At a refrigerant temperature of -30 to -55 DEG C, the salt concentration of the potassium formate is between 25 and 55 % by weight, preferably between 30 and 40 % by weight, and the salt concentration of the potassium carbonate is between 5 and 15 % by weight, preferably 6 and 11 % by weight.
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