Highlight and explain the interest of an harmonic feature from the 2nd mvt. of Schubert's Piano Trio in Bb.

A sense of threat, and possibly even death, is evoked in the movement’s harmonic workings through the tritone. The first explicit tritone interval is exploited in bars 23-4, between A natural and Eb, when the piano takes over the main subject. Again, this significant interval is heard first in the piano, the accompanying instrument, that which holds the music and solo instruments together. It is ironic, therefore, that what should be a site of core stability is actually infected. Two further references to tritone intervals are condensed into bars 31-2, again the in the right hand of the piano, between Cb and Eb, and Bb and E natural. As we hear this start to pervade the inner harmonic language of the movement, it can also be magnified onto the harmonic structure as a whole: the relationship between E natural (bar 88) and Ab (bar 82), between Cb minor (bar 61, enharmonically respelled as B natural) and Eb, the movement’s tonic, and between C (bar 48) and Gb (bar 29). Thus, the tritone embodies some of the tonal organisation of the movement, governing the mind already overwrought with competing voices in the form of different instrumental lines and intrusive harmonic inflections.