Full resolution (JPEG) - On this page / på denna sida - Sidor ...
<< prev. page << föreg. sida << >> nästa sida >> next page >>
Below is the raw OCR text
from the above scanned image.
Do you see an error? Proofread the page now!
Här nedan syns maskintolkade texten från faksimilbilden ovan.
Ser du något fel? Korrekturläs sidan nu!
This page has never been proofread. / Denna sida har aldrig korrekturlästs.
6
I No. 1 fandtes 0.0595 og 0.0602 pCt. CaO.
I „_ 11 — 0.0580 „ 0.0590 „
I 26 — 0.0590 ,. 0.0596 „ „
I ,. 33 0.0585 ’,. 0.0595 „ „
I ,. 34 — 0.0590 0.0593 „
I „ 35 — 0.0585 ,. 0.0588 ,.
I ,. 36 — 0.0577 ,. 0.0579 „
X * -I« — 0.0578 ,. 0.0582 ,.
I 50 — 0.0583 ,. 0.0588 „
Den største Differents mellem Bestemmelserne i et
og samme Vand er altsaa 0.001 pCt.
Til Bestemmelsen af Magnesia inddampedes de to
Filtrater fra Kalkfældingerne i en Plätinskaal til omtrent
150 CC. og bundfældtes derpaa med fosforsurt Natron og
en saa stor Mængde koncentreret Ammoniakopløsning, at
denne kom til at udgjøre omtrent af Vædskens Volum.
Iagttager man ikke den Forsigtighedsregel at tilsætte et
saadant Overskud af Ammoniak, vil man let faa
uoverensstemmende Resultater. Ved at anvende forskjellige
Ammoniakmængder fandt jeg i et og samme Vand følgende Tal
for Magnesia.
Med 9 Dele Vædske og 1 Del kone. Amm. 0.204-1 pCt. MgO.
„ 5 ,. — „1 ,, — • 0.2061 ,. ,.
,. 2 ,. — „ 1 ,. — 0.2140 ,. ,r
I et andet Vand fandtes paa samme Maade:
Med 9 Dele Vædske og 1 Del kone. Amm. 0.2054 pCt. MgO.
„2 „ " — „ 1 „ — 0.2112 ,.
Nødvendigheden af at tilsætte et saadant Overskud af
Ammoniak for at faa en nøiagtig Bestemmelse af Magnesia
efter denne Methode er altfor lidet fremhævet i de
analytiske Lærebøger. Rimeligvis er denne mindre paaagtede
Feilkilde Grunden til, at flere Kemikere bar fundet meget
uoverensstemmende og låve Tal for Magnesia, i Søvandet.
Forchhammer, hvis Resultater med Heusyn til Kalk- og
Magnesiamængden stemmer godt overens med mine
Analyser. inddamper Filtratet fra den fosforsure
Ammoniak-magnesia til Tørhed og faar ved Opløsning af Residuet i
Ammoniak altid en liden Rest af det fosforsure Salt.
En saadan Inddampning er imidlertid ikke nødvendig,
naar man blot før Fældningen koncentrerer Vædsken til
det mindst. mulige Volum og tilsætter det nødvendige
Overskud af Ammoniak. Naar Bundfaldet derpaa efter 24
Timers Henstand frafiltreres, giver Filtratet ved
Inddampning ingen’Rest af fosforsur Ammoniakmagjiesia.
Men uagtet disse Forholdsregler kan Magnesia ikke
bestemmes med samme Sikkerhed i Havvandet, som Kalk
og Svovlsyre. Nedenstaaende Tal viser Differentserne
mellem Kontrolbestemmelserne:
f No. 33 fandtes 0.2160 og 0.2200 pCt.’ MgO.
I ,. 35 — 0.2180 ,. 0.2207 ,.
I ,. 36 — 0.2173 ,..0.2180 ,. „
Største Differents = 0.004 pCt.
In Sample 1 the proportion was 0.0595 and 0.0602 p.ct. CaO.
Iu — 11,. •., ,. 0.0580 ,, 0.0590 ,.
In — 26 ,. „ ,. 0.0590 „ 0.0596 ,. „
In — 33 ,. „ ,. 0.0585 ,, 0.0595 „ „
In —. 34 ,. „ ,. 0.0590 „• 0.0593 „ ,.
In — 35 ,. „ ,. 0.0585 ,. 0.0588 ,. ,.
In — 36 ,. ,. ,. 0.0577 ,. 0.0579 ,.
In — 49 ,. „ ,. 0.0578 ,. 0.0582 ,.
In — 50 ,. ,. ,. 0.0583 ,. 0.0588 ,.
Hence the greatest difference between any two
determinations with the same sample of water is 0.001 per cent.
For determining the magnesia, the two solutions from
which the lime-precipitates had been filtered off were
evaporated in a platinum dish to about 150cc, and then treated,
for precipitation, with phosphate of soda, and with a
concentrated solution of ammonia measuring one-third of the
volume of the fluid. Unless ammonia be added in such
excess, there is a risk of obtaining incongruous results.
On treating different quantities of the fluid with the same
proportion of ammonia. I found the following figures for
magnesia.
With 9 parts of fluid and 1 part of conc.amm. 0.2044 p.ct. MgO.
,, 5 ,. ,. ,. ,. 1 „ ,. — • 0.2061 ,. ,.
,. 2 ,. ,. ,. ,. 1 „ ,. — 0.2140 ,.
In another sample of water. • the percentage thus found
was —
With 9 parts of fluid and 1 part of conc. amm. 0.2054 p.ct. MgO.
„ 2 ,. ,. ,. ,. 1 .. ,. — 0.2112 ,.
The necessity, with this method, of adding ammonia
in so great excess, to ensure a satisfactory determination
of magnesia, lias ’not been sufficiently emphasized in
analytical class-books; and to this comparatively disregarded
source of error, may no doubt be attributed the’’ fact of
divers chemists having obtained for magnesia, in their
analyses of sea-water, results alike incongruous and low.
Forchhammer, whose lime and magnesia determinations
exhibit close agreement with my own, evaporates to dryness
the solution from which the double phosphate of ammonia
and magnesia has been filtered, and can, ou dissolving the
residue in ammonia, invariably detect traces of the
phosphate. Meanwhile, evaporation does not constitute a necessary
phase of the process, provided the fluid, previous to
precipitation. be concentrated to the least possible volume, and
then treated with ammonia in due excess; for, on the
precipitate, after 24 hours, being filtered off. the solution, if
evaporated to dryness, will not leave a trace of double
phosphate of ammonia and magnesia.
But. even assuming every precaution, the magnesia in
sea-water does not admit of being determined with the same
degree of accuracy as do lime and sulphuric acid. The
annexed figures show the extent, to which the
test-deter-minqtions were fouud to differ.
In Sample 33 the proportion was 0.2160 and 0.2200p.ct. MgO.
In — 35 ,. ,. ,. 0.2180 ,. 0.2207 ,. „
In — 36 ,. „ ,. 0.2173 ,. 0.2180 ,. ,.
Greatest difference : 0.004 per cent.
<< prev. page << föreg. sida << >> nästa sida >> next page >>
