Henryk Zbigniew WREMBEL, Zbigniew FRĄCKOWIAK, Krzysztof

Henryk Zbigniew WREMBEL, Zbigniew FRĄCKOWIAK,
Krzysztof K IPO
U n iv ersity of E ducation in S łu p sk
DETERMINATION OF TRACE AMOUNTS OF MERCURY
IN WATER BY GRAPHITE-SPARK SPECTROSCOPY
C o n t e n t s : 1. Introduction, 2. Experim ental, 3. D iscussion of results, 4. Con. elusions; Streszczenie; References
1. INTRO D U CTIO N
M ercu ry com pounds a re com m on in n a tu re , b u t th e y are h ig h ly d isp e r'
sed, h o w ever [3, 6 , 16], T he p ro b lem of p o llution of th e n a tu ra l en v i­
ro n m e n t w ith m e rc u ry em erg ed d ram a tic ally (acute poisonings w ith
m e rc u ry com pounds) in various regions of th e w orld in re c e n t years.
R ap id ly grow ing pollu tio n of th e aquatic e n v iro n m en t w ith m ercu ry , as
side effects of th e econom ic activ ity of m an, c o n stitu te s an increasing
th re a t to life on th e E a rth [2, 5, 15], In view of th e h ig h ly effective
bio co n cen tratio n of m e rc u ry in aquatic food chains, assaying low le ­
vels of th e e le m en t in w a te r is becom ing in creasin g ly im p o rta n t [13,
17, 22, 26],
T he pro b lem of assaying low m e rc u ry levels in th e aquatic e n v i­
ro n m e n t h as been d e a lt w ith in m an y articles. Those of Stock and Zirnerm ann, w ho developed th e m icrom etric m ethod of assaying m e r­
cury, and th a t of H atch and O tt, w ho em ployed flam eless atom ic ab­
sorption sp e ctro m e try to assay m e rc u ry in w a te r [7, 8 ], provide m ilestones. H ow ever, w ork is still co n tin u ed in th e search for n ew and
effective m ethods [4, 18, 20, 27, 28],
In th is p a p e r an a tte m p t w as m ade to use g ra p h ite -sp a rk atom ic
em ission spectroscopy (AES) to assay tra c e am ounts of m e rc u ry in
w a te r. P re lim in a ry stu d ies and e x p e rim e n ts w ith isolation and p a rtia l
electro ly tic sep aratio n of m e rc u ry on th e g ra p h ite electro d e suggested
th a t th e com bination of th is m eth o d w ith th a t of A ES should con­
sid e ra b ly im prove th e low er detectio n lim it of m e rc u ry as com pared
w ith th e ele c tro g rav im etric m eth o d [12, 14, 19, 24]. This p a p e r in clu Oceanology No 13(1980)
des th e d e te rm in a tio n of th e conditions of electrolysis, a stu d y of th e
shape of electro d es on th e sta b ility of th e plasm oid, th e co nstruction
of a calib ratio n cu rv e and provides resp ectiv e a n a ly tic a l relationship.
T he m ea su rem e n ts w ere c a rrie d o u t w ith sam ples of p o llu ted d istilled
w a te r and artificia l B altic Sea w ater.
2. EX PER IM EN T A L
In th e m ethod of m e rc u ry assay in w ater, th e in te n sity of th e
253.65-nm m e rc u ry line w as m easured, w hich w as w ell excitab le in
th e sp a rk plasm oid [20, 29], M e rc u ry fro m a w a te r sam ple to be a n a ­
lysed w as deposited ele c tro ly tic ally onto th e specially pro filed conical
p a rt of a sp e ctral g rap h ite electrode. T he electro d e w as th e n placed
in th e sp ark gap of th e sp a rk g e n e ra to r. T he im age of th e sp a rk p las­
m oid w as p ro je c te d b y m eans of a q u a rtz lens into th e in le t slit of th e
sp ectro g rap h and th e m e rc u ry c o n te n t of th e sam ple w as d eterm in ed
on th e basis of th e b lackening of th e sp ectro g rap h ic plate.
T he follow ing devices w ere used in th e m easurem ents:
(i)
an electro ly zer, schem atic diagram of w hich is show n in Fig. 1.
II consisted of a cy lin d rical glass vessel (60 m m i.d. X 95 m m in
height). T he u p p e r lid of th e ele c tro ly ze r w as m ade from a 20-m m
Fig. 1. Construction of the electrolyzer: 1
cylindrical vessel; 2 — bakelilte lid; 3 — a set
of electrodes providing anode; 4 — cathode; 5
— ring supplying voltage to anodes; 6 — ca ­
thode stiffening ring; 7 — connecting leads
_
Rys. 1. Budowa eletotrolizera: 1 — n aczy­
n ie cylindryczne, 2 — bakelitow a pokrywa, 3
— zespół elektrod stanow iący anodę, 6 — pier­
ścień zaciskow y katody, 7 — przew ody doprowadzaijące
b a k e lite p late. G ra p h ite electro d es connected to each o th er w ith
a m etallic rin g su p plying voltage, w e re m o u n ted in th e lid on th e p e­
rip h e ry of th e vessel 3 m m fro m th e w all. T he g rap h ite cathode was
fixed on th e sy m m e try axis of th e electro ly zer. T he w hole system
w as connected to a stabilized d.c. feed er (stabilization accuracy of 0.1
p e r cent) w ith co ntinuous co n tro l of th e o u tp u t voltage;
(ii)
sp ectro g rap h ic system (Fig. 2) including a Q-24 (C arl Zeiss,
Jen a) sp ectrograph, h ig h -v o ltag e sp a rk g e n e ra to r HFO-1 (C arl Zeiss,
Jena) and a FS 11 sp a rk gap (C arl Zeiss, Jena);
Fig. 2. Block schem e of the spectrographic system : 1 — spark generator; 2 —
spark gap; 3 — electrodes; 4 — spherical mirror; 5 — quartz convergent lens; 6
— shutter; 7 — in let slit; 8 — spectrograph
Rys. 2. Schem at blokow y układu spektrograf Łcznego: 1 — generator iskry, 2
— iskier.nik, 3 — elektrody, 4 — zw ierciadło sferyczne, 5 — kwarcow a soczewka
skupiająca, 6 — m igawka, 7 — szczelina w ejściow a spektrografu, S — spektrograf
(iii) a se t-u p fo r rea d in g and in te rp re ta tio n of sp e ctra in clu d in g
an S P-2 sp e ctru m p ro je c to r (C arl Zeiss in Jena), a G il m icrophotom e te r (C arl Zeiss, Jen a) and an A bbe co m p arato r (C arl Zeiss);
(iv) WU-1 sp ectrographic p la te s (GRWO, W olfen) developed in an
A-71 (ORWO) developer a t 293 K up to a co n tra st coefficient of y = 0 .9 3 .
T he conditions of electro ly sis w ere d e te rm in e d based on [12, 19,
24]. On th e assum ption th a t th e .deposition tim e of m e rc u ry fro m th e
solution should not exceed 1 h o u r and th a t th e efficiency of th e ele­
c tro ly z e r a t a m e rc u ry c o n cen tratio n of 1 0 ' 8 g dm - 3 should not be low er
th a n 25 p e r cent, th e follow ing conditions of th e electro ly sis w e re es­
tablished: du ratio n , 0 .5 < t ^ l h; te m p e ra tu re of th e solution 293 K; cu­
r re n t in te n sity in th e e le c tro ly te 1 = 0.6 A; electro d e voltage, 3 ^ U ^ 9 V.
U n d e r these conditions, fro m a 0.2-dm 3 sam ple of m e rc u ry c o n c en tra ­
tion of 10' 8 g d m '3, a m in im u m am ount of 0.5 ng of m e rc u ry deposi­
te d on th e electrode, th e am o u n t being su fficien t to obtain a signal
stro n g enough to assess th e d etection lim it of th e m ethod [20, 27].
S p a tia i d istrib u tio n of th e plasm oid of th e sp a rk discharge depends
larg e ly on th e d istrib u tio n of sp a rk b realcd o w n channels [10, 11]. E le­
ctrodes w ith specially p ro filed tips proved p a rtic u la rly u sefu l in th e sta ­
bilization of th e sp atial co n fig u ratio n of th e plasm oid. W ith a com bi­
nation of electrodes in w hich th e tip of th e low er one w as p ro filed
in th e shape of a cylindrical ridge (concentric w ith th e electrode), 2 m m
high, and th e u p p e r one conical w ith an apex angle of 120° (Fig. 3),
Fig. 3. The optim um position of electrodes in the spark gap:
1 — upper electrode w ith conically-profiled tip; 2 — lower ele­
ctrode w ith cylinder-shaped tip
Rys. 3. Optym alny układ elektrod w isfciernikiu: 1 — ele
ktroda (g6rna) z końców ką w yprofilow aną w kształcie stożka,
2 — elektroda (dolna) z końców ką w yprofilow aną w form ie w a l­
ca
o plasm oid w ith h ig h ly stable sp atial d istrib u tio n w as obtained, e m b ra ­
cing th e m axim um area of th e u p p e r electrode.
T he m e rc u ry c o n te n t in w a te r sam ple w as estim ated from re la tiv e
tra n sm itta n c e , T, corresponding to th e black en in g of th e spectrographic
p la te w ith th e 256.65-nm line of m erc u ry . T he CII 229.69-nm carbon line
Fig. 4. Calibration curve ox the system : B — for artificial B altic Sea w ater;
D — for distilled w ater
Rys. 4. Krzywa cechow ania układu: B — dla zastępczej w ody bałtyckiej, D
— dla w ody destylow anej
w as used as an e x te rn a l refe re n c e . T he re la tiv e tran sm issio n w as cal­
c u la ted from th e follow ing expression:
^253
T = --------'I'229
(1)
w h e re T - 5 3 is th e tran sm issio n of th e 253.65-nm a n a ly tic a l line of m e r­
c u ry and X2 2 9 th e tran sm issio n of th e CII 229.69-nm line. T he calib ratio n
curve, T
(Fig. 4), w as co n stru cted for distilled w a te r (D) and for
artificia l B altic Sea w a te r (B). T he la tte r w as p re p a re d according to [25],
T he calib ratio n cu rv e w as described by th e S cheibe-L om akin equation
[10, 28, 30] w ritte n in th e form :
=T(C)
T = a (10-3) b
(2)
w h ere a and b are p a ra m ete rs, and Cj = 1 0 g din "3 th e m e rc u ry con­
c e n tra tio n in solution. F o r th e tw o kinds of w a te r, aD — 0.3656, b D =
= — 0.058 and a B — 0.3657, b n = — 0.107 w e re obtained. L im itin g con­
c e n tra tio n corresponding to th e detectio n lim it c w , and th e d e te rm in a ­
tion lim it, c a , w as calcu lated fro m th e follow ing relatio n sh ip s [9, 21]:
T =
iv
/ — t
\/ n
w
1
and
To = 0.92 s,
(4)
w h ere s is th e sta n d a rd deviation and t w is th e c ritical v a ria b le of th e
S tu d e n t d istrib u tio n . T he lim itin g v a lu e s calcu lated fro m eqns. (2) — (4)
fo r both konds of w a te r a re listed in Table.
distilled water; B — arD etection lim its of m ercury itj w ater (D
tificial B altic Sea water)
woda deGraniczne w artości w ykryw alności rtęci w w odzie (D
stylow ana, B — zastępcza bałtycka woda morska)
o S a n a Ww ody|
s
D
B
|
Tw
| Cw' g/dm‘
T°
[ C" ^
0.986
0.945
8X10-8
0.908
2X10-7
0.984
0.943
1X10-4
0.905
3X10-*
3. D ISC U SSIO N OF RESU LTS
B oth th e detection and d e te rm in a tio n lim its obtained in th is w ork are
lik ley to be co n sid erab ly elev ated . O ptim ization of th e a n a ly tic a l p ro ­
ced u re w ould m ost p ro b ab ly sh ift th ese lim its by 1— 2 orders of m agni­
tu d e.
A t th e p re se n t sta te of investigations th e se n sitiv ity of th e m ethod,
d escribed by th e p a ra m e te r b in eq. (2) is a m a tte r for discussion. The
p a ra m e te r depends la rg e ly on th e kin etics of d esorption of m e rc u ry
fro m th e surface of th e electro d e and on th e k inetics of its volatility
outside th e region of th e plasm oid. V ariations of th e m e rc u ry concen­
tra tio n w ith in th e plasm oid, dC’, caused by its lib e ratio n from th e ele­
ctrode (w ith consideration of chem ical reactio n s occurring in th e e le ­
ctrode) and those due to th e rem o v al of m e rc u ry outside th e p las­
m oid region are described by th e eq u ation [1, 23]:
rJfyr
----= C" [/^exp [-h-^tn] — Avexp [-k2tn)]
(5)
w h ere k l and k 2 are th e coefficients of desorption and elim ination, re
spectively, and n is th e p a ra m e te r describing th e k inetics of chem ical
reactio n s occurring on th e electrode. A solution of eq. (5) can be w ritte n
in th e form :
C' = C ~ BC" = A C b’
(6)
Eqn. (6) show s th a t n
1 —> b'
1 and vice versa, n < 1 —> b'
1
A ccordingly, th e kin etics of reactio n s accom panying th e desorption of
m e rc u ry fro m th e electrode and its volatilizatio n outside th e region of
th e plasm oid has a decisive effect on th e course of th e calib ratio n graph.
4. CO NCLUSIONS
T he re su lts of th ese investigations show th a t th e m ethod em ployed e n a ­
bles th e d e trm in a tio n lim it to be sh ifted b y approx. 4. o rd ers of m agni­
tu d e in rela tio n to th e ele c tro g rav im etric m ethod. In re la tio n to th e con­
v e n tio n a l AES (w ith th e rm a l ex citation sources) th e d e te rm in a tio n lim it
is at least one o rd er of m ag n itu d e b e tte r. T h ere are also som e p o ten tial
possibilities enabling th e im p ro v em en t of th e d e te rm in a tio n and d etec­
tion lim its b y a t least one o rd er of m agnitude. T he re la tiv e ly low tim e
consum ption, th e possibility of using sim ple an aly tical in stru m e n ts and
th e ca rry in g out of th e analyses by technicians afford fu rth e r a d v a n ta ­
ges of th e m ethod.
H e n ry k Z bigniew W REM BEL, Zbigniew FR Ą C K O W IA K
K rzysztof KIDO
W yższa Szkoła Pedagogiczna w Słupsku
OZN A CZA N IE ŚLADÓW R T ĘC I W W O D ZIE PR Z Y UŻYCIU
S P E K T R O S K O P II ISK R Y G R A FITO W E J
Streszczenie
W pracy przedstawiono w yniki w stępnych badań nad m ożliwością oznaczania śla­
dów rtęci m etodą em isyjnej spektroskopii atom owej z użyciem iskry grafitow ej
i elektrolitycznego, w stępnego w ydzielania rtęci z roztworu na elektrodzie. Bada­
niami C 'bjęto próbki w ody destylow anej i zastępczej w ody m orskiej skażone rtęcią.
Osiągnięta granica oznaczalności rtęci w tych odm ianach w ody św iadczy o m ożli­
w ości użycia tej m etody do oznaczania m ikrośladow ych zawartości rtęci w w odzie.
W porównaniu z metodą elektrograw im etryczną uzyskano przesunięcie granicy
oznaczalności rtęci w w odzie o ok. 4 rzędy w ielkości, natom iast w porównaniu z k la­
syczną analizą em isyjną (przy zastosow aniu term icznych źródeł wzbudzenia) osiąg­
n ięto poprawę granicy oznaczalności o jeden rząd w ielkości.
Stosunkow o m ała czasocnionność analizy w ykonyw anej w tej m etodzie, m ożli­
wość użycia dość prostej aparatury analitycznej, jak rów nież m ożliw ość w ykonyw a­
nia analiz przez średni personel techniczny — stanow ią dodatkowe zalety opra­
cowanej m etody.
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