Table of Contents
Advertisement
2. Photometric Test Kits
2.2.2 Influence of pH
Chemical reactions follow an optimal course only
within a certain pH range. The reagents contained in
the test kits produce an adequate buffering of the
sample solutions and ensure that the pH optimal for
the reaction in question is obtained.
Highly acidic (pH < 2) and strongly alkaline (pH > 12)
solutions can prevent the pH from being adjusted to
an optimal range, since under certain circumstances
the buffering capacity of the test-kit reagents may
not be sufficient. In such cases the pH must be
corrected by adding diluted sulfuric acid (0.5 mol/l;
lowers the pH) or diluted sodium hydroxide solution
2.2.3 Influence of Temperature
The temperature of the sample solution and reagents
has a varying influence on the colour reaction and
thus on the measurement result. The typical tempe-
rature course is illustrated in the figure at the right.
If the sample temperature is lower than 15 °C, false-
low results must be reckoned with. Temperatures
exceeding 30 °C generally influence the stability of
the compound that is formed in the reaction. The
optimal temperature for the colour reaction is stated
in the package inserts of the respective Spectro-
®
quant
test kits.
Attention! After thermic decomposition proce-
dures, the determination of COD or total contents
of nitrogen, phosphorus, or metal, a sufficient
waiting time must be allowed for to permit the
solution cool to room temperature.
2.2.4 Time Stability
Most of the colour reactions require a certain time to
reach the maximum colour intensity. The solid curve
in the figure at the right gives a schematic impression
of a typical time course. The behaviour of relatively
instable colour reactions with time is shown by the
dotted curve.
The reaction time specified in the working instruc-
tions refers to the period of time from the addition of
the last reagent until the actual measurement. In
addition, the package inserts for the individual test
kits also state the time interval in which the measure-
ment value does not change. The maximum time
interval is 60 minutes; this time should not be ex-
ceeded, even in the case of stable colour reactions.
Release 07/03
(1 mol/l; raises the pH) dropwise, testing the pH with
suitable indicator strips after each drop is added.
The addition of the acid or lye results in a dilution of
the test solution. When up to five drops are added to
10 ml of sample, the change in the volume can be
neglected, since the resultant error is lower than 2 %.
The addition of larger quantities should be duly con-
sidered by adjusting the sample volume accordingly.
The specified pH values for the sample solution and,
wherever applicable, for the measurement solution
are defined in the respective package inserts and in
the analysis instructions in chapter 3 of the manual.
10
20
Temperature (°C)
30
Reaction time (minutes)
30
40
60
V
Advertisement
Chapters
Table of Contents
