Spectrophotometric determination of thorium with a new reagent 3- (4-chloro-2-phos-phonophenylazo-6- (2
6-dibromo-4-nitrophenylazo) -4
5-dihydroxy-2
7-naphthalene disalfonic acid (DBNCPA) is described. In nitric acid medium (0.08—0.5 mol)
DBNCPA reacts with thorium ion to form a purple complex
which composition is Th:DBNCPA= 1:4. The maximum absorption peaks of DBNCPA and its thorium complex are at 543 and 638nm respectively. The molar absorption is 6.64×10
4
1/mol·cm. It obeys Beer's law for thorium ranged 0—15
µ
g/25ml
and most of metallic ions do not interfere the determination.