An automatic photometric method to increase the precision of measureme
nt is proposed, which is based on the injection of a very small volume
of sample by rotary valve and the subsequent homogeneous mixing in a
small reaction cell made of glass of the sample with a color-developin
g reagent gradually delivered by a syringe piston pump until the absor
bance of the mixture approaches 0.434, where the lowest relative error
in the concentration locates. This photometric method is characterist
ic of uniformly high precision, low sample and reagent consumption, an
d high sampling rate of 30 samples per h. The other advantages of this
method include easy calibration with only one standard solution and n
o strict requirements on photometric components. While applied for the
determination of cobalt ranging from 0.21 to 1.43 g l(-1), the relati
ve standard deviations are all below 0.4%. (C) 1997 Elsevier Science B
.V.