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Is your kitten on it's mother or an orphan that you are bottle feeding?
Either way, if it is dehydrated, half/half unflavored pedialyte and water. At a month and a half, you may have to use a syringe, but be careful not to aspirate it. Very little in it's mouth, let it swallow and repeat.
Yes, you can also mix powdered formula with unflavored pedialyte instead of water. This will not only provide needed electrolytes, but also calories and nutrition from the milk.