While the exclusion of sodium has received the most attention because of its ubiquitous presence and high mobility, {HEAD ~ the other alkali metal ions}, especially potassium and lithium, are also desirably kept to {TAIL ~ acceptable levels} 
desirably kept to
While the exclusion of sodium has received the most attention because of its ubiquitous presence and high mobility , { HEAD ~ the other alkali metal ions } , especially potassium and lithium , are also desirably kept to { TAIL ~ acceptable levels }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL