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56
Forat undgaa de her omtalte Feil. benyttedes til
Bestemmelse af Saltmængden i Søvandet følgende
Fremgangsmaade.
I en med tætsluttende Laag forsynet tyk. veiet
Por-cellaindigel indveiedes 30 til 4U Gr. Søvand, som
afdunstedes paa Vandbad. Efterat Saltene vare nogenlunde vel
tørrede, ophededes Digelen med Laaget paa ca.jo Minutter
over en Bunsens Lampe, afkjøledes og veiedes paany.
Derefter bestemtes paa den forud beskrevne Maade den ved
Decomposition af Chlormagnesium dannede frie Magnesia,
hvorved de til Beregning af den samlede Saltgehalt
fornødne Data erholdtes.
I en tidligere Afhandling1 har jeg paavist. at
Car-bonaterne i Søvandet ved Kogning omsætter sig til kulsur
Magnesia, som ved Inddampning eller under enhver
Omstændighed ved Glødning efterlader Magnesia, og man
skulde altsaa strengt taget for den saaledes dannede Del
af den frie Magnesia beregne en anden Correction end for
den ved Decomposition af Chlormagnesium dannede
Hoved-mængde. Den Feil. man begaar, ved at undlade dette, er
imidlertid baade meget nær constant og desuden saa liden,
at den uden videre kan negligeres, idet den kun bidrager
til at formindske den samlede Saltgehalt med omkring
0.0015 °/o. Det er saaledes fuldstændig tilstrækkeligt til
den ved Veiningen fundne Mængde tørret Salt at addere
1.375 Gange den ved Titreringen bestemte Mængde fri
Magnesia, for af det saaledes fremkomne Tal at beregne
Saltgehalten i Frocenter.
Mod denne Methode kan der dog reises Indvendinger,
idet det kunde befrygtes, at mindre Qvantiteter
Chlornatrium. .Chlormagnesium eller Chlorkalium under Glødningen
kunde forflygtiges, eller at en Del af den svovlsure
Magnesia ved den høie Temperatur kunde decomponeres og
give Anledning til Tab af Svovlsyre. Man kan imidlertid
let forvisse sig om. at dette ved Anvendelse af en tyk
Porcel-laindigel med tætsluttende Laag ikke bevirker nogen Feil
af mærkbar Indflydelse. Saaledes fandt jeg, at 1.2 Gr. af
en passende Blanding af Chlorkalium og Chlornatrium ved
1 Yi Times . stærkest mulig Glødning over en Bunsens Lampe
i den sam ine Digel, som jeg benyttede -til mine
Saltbestem-melser, kun tabte 2 Mgr. i Vægt. det vil sige, Blandingen
tabte ved Glødningen ikke fuldt 0.14 Mgr. pr. 5 Minutter.
Ligeledes paavistes ved Bestemmelse af Svovlsyre og
Magnesia saavel ! det benyttede Søvand som i det glødede
Residuum, at man selv ved en meget længere fortsat
Glod-liing end den. der udfordres forat skaffe fuldstændig
vandfrit Salt, ikke risikerer nogen skadelig Feil foranlediget
ved Forflygtigelse af Chlormagnesium eller Decomposition
af svovlsur Magnesia.
In order to" guard against the above-mentioned errors,
the following mode of operation was adopted for
determining the amount of salt in sea-water.
From 30"r to 40,jr of sea-water were introduced into
a thick porcelain crucible of known weight, furnished with
a tight-fittting cover, and evaporated on a water-bath. So
soon as the salt was sufficiently dry, the crucible, with the
cover on. was heated for about 5 minutes over one of
Bunsen’s gas-burners, then cooled and weighed with its
contents. The free magnesia liberated by the
decomposition of the chloride of magnesium was now determined in
the manner previously described, and the last factor
necessary for computing the total amount of salt accordingly
found.
In a former paper1 I drew attention to the fact, that
the carbonates present in sea-water are transformed during
the process of boiling into carbonate of magnesia, which
after evaporation, or. at least, on the salt being thoroughly
heated, leaves a residue of magnesia; and hence the
proportion of free magnesia thus formed would, strictly
speaking. seem to involve the need of a correction different from
that adopted for the principal amount liberated by the
decomposition of the chloride of magnesium. But the error
which arises from applying the same correction to both is.
on the one hand, very nearly constant, and, on the other,
so small as to admit of being safely ignored, seeing that
it reduces the total amount of salt only about 0.0015 per
cent. It is. therefore, amply sufficient, if to the amount of
dried salt fouud by weighing be added 1.375 times the
amount of the free magnesia determined by titration: the
figure thus obtained will serve to compute the percentage
of salt.
This method certainly is so’ far open to objection,
that small quantities of chloride of sodium, chloride of
magnesium, or chloride of potassium may be volatilized
during the process of heating, or some portion of the
sulphate of magnesia be decomposed!at the high temperature,
and thus occasion a loss of sulphuric acid. The error,
however, arising from this source will not exert any
appreciable influence on the result, provided the crucible
used for the operation be of thick porcelain, and have a
tight-fitting cover. Thus, for instance. I found that -1.2"’’
of a proportionate mixture of chloride of potassium and
chloride of sodium, on being heated for the space of an
hour and a quarter over one of Bunsen’s gas-burners in
the crucible I had used for my salt-determinations, lost
only 2 m"r in weight, or 0.14’""’’ every 5 minutes.
More-. over, it was manifest on determining the sulphuric acid
and magnesia both in the water itself and in the heated
residue, that, even. in the event of the heating-process being
much more protracted than is necessary to obtain salt free
from the smallest trace of water, no serious error can
result from the volatilization of chloride of magnesium or
the decomposition of sulphate of magnesia.
1 uOra Kulsyren i Søvandet" Side 40 øverst.
2 "On the Carbonic Acid in Sea-Water." p. 40.
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