Cell expressions for different ways of writing text superscripts in Mathematica: http://stackoverflow.com/q/3860574/421225

H30+

Cell[TextData[{
 "the chemical formula for hydronium is H",
 Cell[BoxData[
  FormBox[
   SubscriptBox["\[Null]", "3"], TraditionalForm]],
  FormatType->"TraditionalForm"],
 "O",
 Cell[BoxData[
  FormBox[
   SuperscriptBox["\[Null]", "+"], TraditionalForm]],
  FormatType->"TraditionalForm"],
 " \ncompare with the formating if the whole expression is in \
mathmode: ",
 Cell[BoxData[
  FormBox[
   RowBox[{
    SubscriptBox["H", "3"], 
    SuperscriptBox["O", "+"]}], TraditionalForm]],
  FormatType->"TraditionalForm"],
 "\nthis is in mathmode, but with italics removed and a negative \
space: ",
 Cell[BoxData[
  FormBox[
   RowBox[{
    SubscriptBox[
     StyleBox["H",
      FontSlant->"Plain"], "3"], "\[NegativeVeryThinSpace]", 
    SuperscriptBox[
     StyleBox["O",
      FontSlant->"Plain"], "+"]}], TraditionalForm]],
  FormatType->"TraditionalForm"]
}], "Text"]