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55
Correctionernes Bestemmelse udførtes ved Hjælp af
Vandprøverne I og VII. hvis Egenvægter .tidligere ere
bestemte til 1.02691 og 1.02669 ved . For det paa
første Togt anvendte Aræometer erholdtes saaledes gjennem
5 Aflæsninger i I Correctionen —0.00023 og gjennem 12
Aflæsninger i VII ligeledes —0.00023. Paa samme Maade
bestemtes det andet Aræometers Correctioner ved 5
Aflæsninger i I til —0.00037 og ved 8 Aflæsninger i VII til
—-0.00038. Under disse Aflæsninger var Vædsken altid
bragt til 17.°5 eller en meget nærliggende Temperatur,
hvorfra Aflæsningerne efter de forhen gjengivne
Correctioner reduceredes til Normaltemperaturen. Gjennem flere
Rækker Aflæsninger i Vandprøven I ved forskjellige
Temperaturer har jeg tillige forvisset mig om, at den ved
Beregning af Correctionstabellen benyttede Udvidelsescoefflcient
for Aræometrene er passende valgt.
Hermed er givet de fornødne Data til Reduction af
de paa den norske Nordhavsexpedition aflæste Egenvægter,
og jeg gaar dernæst over til Bestemmelsen af Relationerne
mellem Saltgehalten, Chlormængden og Egenvægten.
Til Bestemmelse af Saltmængden har, saavidt jeg ved,
tidligere kun været benyttet den simpleste Methode,
bestaaende i Afdampning af Vandet og Residuets Tørring ved en
passende Temperatur, som af de forskjellige Chemikere er
bleven valgt noget forskjelligt fra 150°—180°. Denne
Methode har jeg imidlertid af flere Grunde fundet lidet
tilfredsstillende, hvad man ogsaa paa Forhaand maatte vente.
Efter Graham1 og andre taber nemlig clen svovlsure
Magnesia, om hvis Tilstedeværelse i Søvandet der vel ikke kan
reises Tvivl, først ved en Temperatur af over 200" sit
sidste Molekyl Vand, medens man paa den anden. Side
allerede ved en Temperatur af betydeligt under 200" maa
befrygte en delvis Decomposition af den i Saltene
tilstedeværende Chlor magnesium. Efter de Forsøg, som jeg
anstillede, viste det sig, at Saltene selv efterat være tørrede
ca. 20 Timer i Luftbad ved en Temperatur fra 170°—
180° endnu indeholdt ikke ubetydelige Mængder Vand
(omkring 15 Mgr. pr. Gr. Salt), medens de tørrede ved lidt
lavere Temperatur indeholdt noget mere. Samtidig
undersøgtes ogsaa Saltene paa fri Magnesia, hvorved jeg i Strid
med ældre Angivelser fandt, at de bestandig selv ved
Tørring ved 160° til 170° indeholdt uventet store Qvantiteter,
saaat der for hvert Gr. tørret Salt fandtes en
Magnesia-mængde tilstrækkelig til at neutralisere over 20 Mgr. HC1 (ved
Tørring ved 180° fandt jeg endog en enkelt Gang 40 Mgr.).
Bestemmelsen af den frie Magnesia foretoges ved Saltenes
Opløsning i en afmaalt Mængde titreret Svovlsyre og
derpaa følgende Retitration med fortyndet Natronlud af
bekjendt Styrke. Ved Anvendelse af Rosolsyre som Index
erholdtes her en meget skarp Endereaction.
The determination of the corrections was performed
with the water of samples I and VII, whose specific
gravity at had been found to be respectively 1.02691 and
1.02669. For the areometer used on the first voyage, 5
readings with the water of sample I gave the correction
—0.00023, and 12 readings with the water of sample VII
likewise —0.00023; in the same manner, the corrections
for the other areometer were determined, by 5 readings
with the water of sample I. to be —0.00037, and, by 8
readings with the water of sample VII. to be —0.00038. For
these readings, the fluid was always brought to 17."5, or
as near that temperature as possible, the readings having
in the latter case to be reduced, by means of the
corrections given above, to the normal temperature. Several
series of readings with the water of sample I, at different
temperatures, convinced me that the coefficient of
expansion for the areometer which I had computed for
preparing the Table of Corrections was practically correct.
Having now specified the data necessary for reducing
the specific gravities read on the Norwegian North-Atlantic
Expedition. I shall proceed to determine the relation
between the specific gravity of sea-water and the amount
of salt and chlorine it contains.
For determining the amount of salt, the only method
formerly resorted to was, so far as I am aware, the
simplest, viz. that of evaporating the water and then drying
the residue at a proper temperature, which has been
variously fixed by different chemists at from 150° to 180°.
This method, however, has proved in several respects
defective, as was indeed to be expected. According to
Graham1 and others, sulphate of magnesia, the presence of
which in sea-water can hardly admit of doubt, does not
part with its last molecule of water till exposed to a
temperature of more than 200° whereas, on the other hand, it
is highly probable that partial decomposition of the chloride
of magnesium contained in the salt takes place
considerably below 200°. Even after the salts had been dried for
about 20 hours in an air-bath at a temperature of 170°—
180°, they were still found to contain, according to my
experiments, a considerable quantity of water (about lhmijr
salt per gramme); dried at a lower temperature, the
amount was somewhat greater. I also tested the salts for
free magnesia, and found, in direct opposition to earlier
statements, that, even when dried at 160"—170°, they
invariably contained a very large amount, the quantity of
magnesia to every gramme of dried salt being sufficient to
neutralize more than 20mf HC1 (once, when" dried at 180 .
even 40",fr). For determining the free magnesia, the salts
were dissolved in a given quantity of titrated sulphuric
acid, and the fluid then retitratecl with dilute soda-lye of
known strength. With rosolic acid as the index, the final
reaction was very decided.
1 Phil. Mag. J. (i — 422.
1 Phil. Mag. J. (», p. 422.
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