Yup, sodium.
I didn't remember the exact amount, so I looked it up in F&C. "The theoretical sodium content is 4.51 mEq (103.6 mg) per gram of ticarcillin/clavulanate. The theoretical total sodium content of the 100 mL premixed solution is 18.7 mEq (429 mg), of which 15.6 mEq (359 mg) is contributed by the ticarcillin component of ticarcillin/clavulanate. Consider this when treating patients requiring restricted salt intake."