Tax on Purchase requisition

Hi,

     We have a requirement to start calculating tax on purchase requisitions and POs. We have a tax group that uses a specific account since we are a government entity all of our tax hits this account and then we are reimbursed in the future. I have configured our tax defaults to enable tax calculation on our requisitions however, when tax is calculated the tax distribution shows that it is using the same expense account that was specified on the requisition line instead of the account defined in the tax group. Am I missing a configuration to make sure that