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39
Apparat i omkring 2 Timer, idet den undvigende Kulsyre
som for opfangedes og bestemtes ved ■ titreret Baryt vand.
I den anvendte Mængde Sodaoplosning var indeholdt 63.3
Mgr. Soda svarende til 26:3 Mgr. Kulsyre, medens der
ved to Experimenter paa denne Maade fandtes uddrevet
den ene Gang 26.8 og den anden 27.7 Mgr. Ved
Forsøgets Gjentagelse med en lidt større Qvantitet paany
ind-veiet Soda fandtes uddrevet 34.1 Mgr. Kulsyre’ istedetfor
beregnet 33.2. Under Kogningen var Opløsningen i
Begyndelsen fuldstændig klar. hvorpaa der efter circa ’
Times Forløb begyndte at fremkomme et Bundfald af
Magnesia. hvori der trods al anvendt Moie ikke lykkedes mig
at paavise det mindste Spor af Kulsyre. Den basisk
kulsure Magnesia er nemlig ifølge H. Rose og Flere ikke
ubetydelig opløselig saavel i Vånd som i forskjellige Salte,
saaledes at der af smaa Mængder intet Bundfald
fremkommer.’ og det er derfor ikke paafaldende, at Oxydet ved saa
fortyndede Opløsninger, som her ere anvendte, ikke udfældes,
førend det ved Kogningen er befriet for sin Kulsyre.
Koges Søvand .under Concentration, kan man meget.
let komme til at overse deiine Udskillelse af Magnesia,
idet denne først indtræder efter nogéh Tids Forløb,
saaledes at man kan antage Blakningen fremkommen ved
Udfældning åf .Gips i den concentrerede Vædske. Udskillelsen
vil ogsaa, naar Inddampningen foregaar i aabent Kar.
forsinkes betydeligt, idet Kulsyrens Undvigen foregaar meget
langsommere i en kulsyreholdig Atmosphære end i en
kulsyrefri Luftsti-øm, og det kan derfor ikke forundre., at
denne Udfældning af Magnesia af kogende Søvand, saavidt
mig bekjendt; ikke tidligere er observeret, naar undtages, at
l siglio1 i det tørrede. Residum har paavist fri Magnesia.
Dersom man imidlertid koger Søvand uden Concentration
i en kulsyrefri Atmosphære med omvendt Kjøler, saaledes
som det let kan ske ved Classens Apparat, fremtræder
Phænomenet meget tydeligt paa samme Maade som ved
Kogning af en Blanding af Soda og Bittersalt, idet der
efter circa Times Kogning begynder’ at udskille’ sig et
Bundfald bestaaende af ren kulsyre- og kalkfri Magnesia
ved Siden af et lidet Spor af- Kiselsyre hidrørende fra det
benyttede Glaskar. Om man samtidig opfanger og
bestemmer den undvigende Kulsyre, viser det sig, at den
Villighed. hvormed Kulsyren undviger, ikke saameget afhænger
af Concentrationsgradén som af Varigheden af- det
Tidsrum, hvori Kogningen fortsættes, da ogsaa paa denne Maade
circa 2 Timers Kogning strækker til for at skafte en
fuldstændig kulsyrefri Vædske.
Det vil forhaabentlig hermed kunne ansees
fuldstændig godtgjort, at Søvandets Evne til vecl Kogning at decom-
1 Journ. für pract. Chem. 46 — 10(3.
. paratus for.2 hours exposed to a current of air free from
carbonic acid, the carbonic acid driven oft’ being collected
and determined as before by titrated baryta water. The
portion of the solution of carbonate of soda employed
contained 63.3m’Jr of that substance, representing 26.3"T
carbonic acid; and two experiments performed by this
method gave respectively 26.8mf’’ and 27.7 ™fr as the amount
of the latter driven oft". On repeating the experiment
with a somewhat larger quantity of carbonate of soda,
carefully weighed. 34.1 "f" were found to have been
liberated. whereas the exact proportion would have been 33.2",fr.
During the process of boiling, the fluid at first continued
perfectly clear; but after the .lapse of about half an hour
magnesia began to be precipitated, in- which with the
most delicate tests I failed to detect the smallest trace
of carbonic acid. According to H. Rose and other authors
basic carbonate of magnesia is to a considerable extent
soluble alike in water and in solutions of divers salts,
so that small quantities give no precipitate; and hence
it is not surprising that with a solution so diluted as that
made use of for these experiments, the oxide should not
have been- precipitated before it had given off all its .
carbonic acid.
If. sea-water be- boiled during concentration, this
precipitation of magnesia may be easily overlooked, since
it does not take place till after the lapse of some time,
and the turbidness of the concentrated fluid might
therefore be ascribed to the deposit of gypsum. When the
water, too, is evaporated in an open dish, the precipitation
will be considerably retarded, since the carbonic acid
escapes much more slowly in- an atmosphere Containing
carbonic acid than in a current of air from which all
carbonic acid has been expelled; a fid this accounts for the
precipitation of magnesia in boiled sea-water, so far as
I am aware, not having been previously observed, except
at least by Usiglio.1 who detected the presence of free
magnesia in the dried residue. When, however, sea-water
is boiled without being concentrated in an atmosphere
.free from carbonic acid. and. with the cooler inverted, which
it easily may be with Classen’s apparatus, the phenomenon
appears with great distinctness, as is the case on boiling
a mixture of soda and EpsOm salts; for after about half
an hour’s boiling a precipitate begins to form, consisting
of pure magnesia, with no admixture of .lime and carbonic
acid and a trace only of ’silicic acid, the latter arising
from the glass vessel employed in the operation. If the
carbonic acid be simultaneously collected and determined,
the readiness with which it escapes is found to depend
not so much on the degree of concentration as on the
duration of the boiling-process, about 2 hours proving
amply sufficient to obtain a fluid free from the smallest
trace of carbonic acid.
From what has been stated, there is. we think,
conclusive proof, that the property possessed by sea-water of
1 Journ. für pract Chem. 4(5. p. 10(5.
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