2. ENVIRONMENTAL ChEMISTRy & TEChNOLOGy 2.1. Lectures

Chem. Listy, 102, s265–s1311 (2008) Environmental Chemistry & Technology
P49 SPECTROPhOTOMETRIC
MICRODETERMINATION OF PhOSPhATE
bASED ON ThE ION ASSOCIATION COMPLEx
wITh RhODAMINE b IN wATER
MARTIn MOOS and LUMíR SOMMER
Brno University of Technology,Chemistry and Technology of
Environmental Protection, Purkyňova 118, 612 00 Brno,
xcmoos@fch.vutbr.cz
Introduction
Phosphate may be a serious problem for the ecosystem 1,2
since it is the main reason for the eutrophication of natural
surface waters. In the presence of phosphate, a considerable
growing of anabaena is observed which release toxins after
their extinction. Moreover, the phosphates cause a considerably
growing of water plants which consume the oxygen
contents in water, and interfere with the aquatic life. The
visual spectrophotometry 3,4 often based on the interaction of
molybdatophosphate or molybdatovanadato-phosphate with
some basic dyes is often used for its determination. 5–8 The
formation of an ion associate with the sensitive Rhodamine B
(Tetraethylrhodamine) was studied in detail in this paper.
Experimental
C h e m i c a l s
All chemicals used were in analytical grade quality.
0.01 mol dm –3 standard solution of phosphate was prepared
from 0.3402 g KH 2 PO 4 (Lachema, Brno, Czech Republic)
in 250 ml, previously dried 1h at 130 °C.
0.3 mol dm –3 solution of sodium molybdate, (Lachema,
Brno, Czech Republic) and 1 × 10 –3 mol dm –3 , solution of
Rhodamine B (Tetraethylrhodamine), (Merck, Darmstadt,
SRn) in milli Q water were stock solutions.
Brij 35 (Aldrich, Steinheim, SRn), Triton X 100 (Calbiochem
Co., San Diego, USA) and Polyvinylalcohol (PVA)
(Sigma, Steinheim, SRn) surfactants were in 1 % wt. aqueous
solutions.
Astasol standard solutions with 1 g dm –3 of SiO 3 2- , Ca 2+ ,
Al 3+ , Fe 3+ , K + , na + , Mg 2+ , As 3+ , Cl – , SO 4 2– , nH4 + , nO3 – ,
nO 2 – (Analytika, Praha, ČR), were used for studying interferences.
R e a l W a t e r s S a m p l e s
Surface water from the River Sázava, mineral water
Korunní, drinking water from the Brno water supply and sea
water from the Mediterranean Sea were sampled. The water
samples were filtered by using membrane filter with pore size
0.45 µm.
I n s t r u m e n t
Spectrophotometer Spectronic UnICAM UV 500
(Spectronic Unicam, UK, Cambridge).
s430
C a l i b r a t i o n p l o t s a n d l i m i t s o f
d e t e c t i o n
All linear calibration plots were evaluated according
to the standard ČSn ISO 8466-1 10 characterizing necessary
statistical characteristics for evaluation of linear calibration
plots (variation range homogeneity test and linearity test).
The detection limits were expressed according to Graham
9 , Miller 11 and to IUPAC 13 .
The method of continuous variation 12 was used for the
evaluation of the mol ratio of components.
Results
The sequence of mixing components has an important
effect for the sensitivity and reproducibility of the
method. The maximal absorbance was reached for the following
order of mixied components: phosphate → nonionic
surfactant → sodium molybdate → sulphuric acid →
Rhodamine B. The absorbance of the ternary species of
12–molybdatophosphate with Rhodamine B reaches its
maximum value for 8.3 × 10 –5 mol dm –3 Rhodamine B and
0.03 mol dm –3 sodium molybdate after 20 min. at 572 nm.
The higher concentrations of both components are responsible
for the absorbance decreases.
E f f e c t o f A c i d i t y
In the following range 0.1–3.0 mol dm –3 , the absorbance
considerably decreased with the increasing concentration of
sulphuric acid. 1 mol dm –3 of H 2 SO 4 was optimal for obtaining
stable absorbance of the associate in time. Hydrochloric
acid has a similar effect but 1.5 mol dm –3 was used for further
measurements.
E f f e c t o f s u r f a c t a n t s
Three non-ionic surfactants were used, Brij 35, Triton
X 100 and PVA respectively, from which 0.01 % wt.
of Brij 35 was optimal. In the presence of Triton X 100 the
adsorption of the ion associate on glass surface was observed.
PVA 30,000–70,000 mol. weight does not prevent turbidity
in solution.
C a l i b r a t i o n P l o t s a n d D e t e c t i o n
l i m i t s ( L O D )
The strictly linear calibration plots were evaluated for six
concentration levels between1 × 10 –6 –7 × 10 –6 mol dm –3 . The
points were measured in triplicate for the optimal conditions
1 mol dm –3 sulphuric acid, 0.01 % wt. Brij 35, 0.03 mol dm –3
sodium molybdate and 8.3 × 10 –5 mol dm –3 Rhodamine B for
the evaluation were used.
e f f e c t o f i o n s
1,000 : 1 Mg 2+ , K + , na + , nH 4 + , nO3 – , SO4 2– , Cl – , HCO3 –
did not interfere the determination of 1 × 10 –6 mol dm –3
H 2 PO 4 and 100 : 1 Al 3+ , Fe 2+ , Ca 2+ . As (III, V) and nO 2 –
interfered above concentrations only which are not present
in natural waters. The SiO 3 2– was successfully masked with
6.7 × 10 –4 mol dm –3 tartaric acid.