The 3S-NA sodium analyzer takes measurements with an Ion Selective Electrode (ISE) sensitive to sodium ions in solution. The response of the electrode is proportional to the sodium concentration in the sample on a logarithmic scale. The instrument can take measurements on (up to) four different input streams. The multiposition switching valve makes the channel selection operations fully automatic, with timing and scheduling programmable by the user. Each sample line has its dedication fast loop reservoir to minimize cross interferences and switching times.