Page 11 - 3_Chemical industry

Sum parameter – Total Organic Carbon

TOC – Determination in sodium

hydroxide solution
SCA-130-305

Organic contaminants in basic chemicals may Several mL of ultrapure water were placed in
lead to impurities in the products. Therefore, a 50 mL volumetric flask. Subsequently, 5 mL
quality control of the reactants is necessary. of the concentrated sodium hydroxide was
pipetted into the flask. Finally, concentrated
sulfuric acid was added until the solution has
reached a pH < 2. The flask was then filled
with ultrapure water up to the mark. The
addition of sodium hydroxide, as well as the
addition of sulfuric acid to the water must be
done with the utmost care and caution, as a
violent chemical reaction occurs.

Compound Conz.
(Concen- Dilution [%]
tration)
The TOC determination in sodium hydroxide 1 : 10 diluted
can lead to various problems. The catalyst approx.
and the combustion tube wear out very Sodium (5 ml / 50 ml) add. 5%
rapidly. This, in turn, will lower the sensitivity hydroxide 1-2 drops Sulfuric
at an equally fast rate and leads to very poor solution acid until pH<7 + Sulfuric
reproducibilities. (50%) (Note: generation of acid
heat)
NaOH can also absorb CO 2 from the
environment. As air contains approximately
400 ppm CO 2, direct TC determination in
sodium hydroxide can lead to much higher Calibration of the TOC-L system was carried
values. The NPOC method is, therefore, out using the automatic dilution function
recommended for the determination of within the range of 0.5 mg/L to 10 mg/L.
organic contaminations in sodium hydroxide.
The sample should also be diluted as much
as possible.

In the present case, a 50% sodium hydroxide
solution was analyzed. The purity criterion
and the required limit of detection was < 10
mg/L.

■ Sample Preparation Fig. Multi-point calibration
The sample was first manually diluted to a with dilution function
ratio of 1:10 with ultrapure water and a
suitable amount of acid.