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An experimental study of the physical chemistry of anaesthesia in ... online

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complete, or placed on a rotary shaking machine. After the lapse of several days,
during which time the flasks are never opened and are kept shaken up, it is noticed
at what level of concentration the chloroform ceases to be completely dissolved, and
so the solubility is determined

The lower strengths of known value short of saturation and also the saturated
solutions so prepared were kept and used also for the experiments on Vapour pressure
at varying concentration described in the next section of this paper.

The following results were obtained by the application of this method at room
temperatures, approximately (1 j° C), for the percentage by weight dissolved : —

JVater^ o - 8 per cent, dissolved, 0*9 per cent, dissolved, 1 per cent, not dissolved
completely. Estimated solubility, 0*95 per cent.

Saline Solution (0*75 per cent, sodium chloride in water), 0*7 per cent, dissolved,
o*8 per cent, dissolved, 0*9 per cent, not dissolved. Estimated solubility about
0-83 per cent.

Serum % 3 per cent, dissolved, 3-5 per cent, dissolved, 4 per cent, all dissolved
save a few small globules.

Haemoglobin Solution or Blood. — Over 6 per cent, by weight is taken up when
chloroform is shaken with blood or haemoglobin solution of equal strength to the
blood, prepared from blood by centrifugalizing several times with saline and
subsequent laking with distilled water, and no globules of chloroform can be seen on
careful examination with the microscope. But the solution rapidly changes in colour,
and a precipitate is thrown out on standing, as above described, which is quite
insoluble in water or saline, but easily soluble in dilute sodium carbonate solution,
and then gives the spectrum of alkaline haematin.

The blood begins to give this precipitate when about 1*5 per cent, of chloroform
has been added at room temperature, but with a lower concentration, and more rapidly
when heated to body temperature in the incubator. Two per cent, of chloroform
gives a precipitate in the cold, and on heating to 40 C. a red flocculent precipitate
leaving a clear colourless fluid above.

The third method for determining the solubility of chloroform in the fluids
experimented with consists in shaking up thoroughly for several hours with an excess
of chloroform, and then pipetting off and determining the amount of chloroform
in the solution.

The difficulty here is a rapid and accurate method of determining the amount
of chloroform contained in a measured volume of the given saturated solution.

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The procedure finally employed for this purpose was as follows : —
A measured volume (usually 10 c.c.) of the fluid saturated with chloroform is
placed in a flask fitted airtight with a double bored cork, and a stream of hydrogen
is aspirated through the solution, the oxygen present in the flask and connexions is
absorbed by passing through alkaline pyrogallate, and the mixture of hydrogen and
chloroform is then burnt by passing over heated palladium asbestos placed in a very
short combustion tube. All the chlorine in the chloroform is thus burnt to
hydrochloric acid, and the amount of this absorbed in standard alkali is then estimated,
first by back titration against standard acid, and then further checked, either by
volumetric titration with standard silver nitrate solution or by gravimetric
determination as silver chloride.

The SLerum used in these determinations was examined for chloroform emulsion

by the microscope, but no undissolved chloroform in suspension was observed. The

precipitate in serum at atmospheric temperature obtained by this method of shaking

up with excess of chloroform was very dense, so that the serum became quite opaque.

The results obtained by employing this method were as follows : —

Distilled water, dissolved 0*95 per cent., and serum, dissolved 5*08 per cent.

III. — On the Vapour Pressure of Chloroform Dissolved in Varying

Concentration in Water, Saline, Serum, and

Haemoglobin Solutions Respectively

A determination of the vapour pressure of an anaesthetic in solution at varying
concentrations in serum, in haemoglobin, or in blood, is of high practical importance,
since it is upon the relationship of this vapour pressure to the concentration of the
solution that the amount of anaesthetic taken up by the blood circulating through
the lungs depends.

It has hitherto been taken for granted that the Dalton-Henry law can be
applied, and that the amount of anaesthetic taken up is strictly proportional to, and
varies directly with, the percentage of the vapour of the anaesthetic in the inspired air.

This has never, however, to our knowledge, been experimentally tested, and it
seemed to us desirable to attempt such a determination. We have investigated from
this point of view solutions of chloroform in serum, haemoglobin solution (of equal
strength in haemoglobin to the blood from which the haemoglobin was prepared) and
whipped blood, and have contrasted^the pressures obtained with those of solutions in
chloroform, in water, and normal saline at equal concentrations.

The vapour pressures have been measured corresponding to concentrations
ranging from considerably below the anaesthetizing values for chloroform vapour
pressure in air (viz., 8-10 mm.), observed by Paul Bert, up to the saturation points
in most cases.

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The instrument employed for this purpose was a form of 1 differential densimeter/
which, after passing through many modifications, took the form represented in the
accompanying sketch (Fig. i), which is drawn approximately to a scale of one-

The two tubes shown are exactly similar, and are graduated in cubic centimetres
and tenths in the upper portion, and in centimetres in the lower and wider portion.

The tubes are connected as shown by means of thick-walled rubber tubing and
a glass Y-piece to a stout glass mercury receiver capable of holding more than enough
mercury to fill both tubes and their connexions.

Fig. i. — Diagram of Differential Densimeter.

The tubes are held in a vertical position by clamps attached to the strong vertical
iron bar of a massive retort stand, and each tube is capable of being-j moved jin^its
clamp vertically up and down for purposes of adjusting the mercury levels.

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In order to keep a constant temperature (in the case of the experiments carried
out at, body temperature) the upper portion of each tube, from about the middle of
the wide part to the level of the stopper at the top, was encased in a hot-water jacket
of the form shown in detail in Fig. 2, and omitted for clearness from Fig. 1.

It was found convenient to use for the outer glass tubing of this jacket the
largest size of a common variety of paraffin-lamp chimney, which measured 8 cm. in
diameter above, bulged out as shown to 9 cm., and then narrowed below to 6 cm.

The wider top and bulge were found very useful in facilitating the introduction
and vigorous movements of the bar electro-magnet used as a stirrer in connexion with
the iron stud (seen in Figs. 1 and 2, at the top of the mercury), which was dropped
into each tube, and, during an experiment, agitated up and down so as to thoroughly
mix the fluid under experiment, and so bring it into equilibrium more rapidly with the
vapour in the space above it.

The hot- water jacket was made watertight below by means of an india-rubber
cork, as shown (Fig. 2) ; the rubber cork was also bored in each case for two narrow
glass tubes, for the purpose of carrying water to and from the jacket. The two tubes
carrying the water in stopped about 2 cm. above the upper surface of the rubber cork
in each tube, so as to prevent blocking by mercury accidentally run over from the top
when the inner tubes were being cleared out at the termination of a measurement.
The outer ends of these two tubes were attached by means of narrow rubber tubings
and a glass Y-tube to a bath of hot water, placed at a higher level, and a screw-down
clip on each rubber tube regulated the flow until a thermometer placed in the
corresponding hot-water jacket showed the desired temperature. A constant level of
water was kept up automatically in the warm supply bath, and its temperature was
regulated so as to lie 2-3 above that of the jackets. The two outflow tubes passed
up, as shown, inside the jacket to the level of the ground in glass stopper, and their
outside ends were connected by means of rubber tubing to the waste pipe.

In Fig. 1 the upper portion of the left-hand tube is shown in section, and that
of the right-hand one in outline. The ground in stoppers shown were found, when
sealed with mercury, to be much more effectual against minute leakages, which
entirely vitiate the results, than any form of tap, and they are also much more con-
venient for introducing the solutions to be experimented upon, and for cleaning out
the apparatus. Further, since they do not require to be operated between the com-
mencement and termination of each determination of vapour pressure, they are
better adapted to their particular purpose than a tap. In the course of our experi-
ments, we also found it necessary to be able to dilute a solution with more of its
solvent without allowing it to come in contact with any appreciable volume of air,
and for this purpose found the stopper arrangement most convenient.

The side tube shown at the lower end of each main tube was designed to trap
air which was found to slowly leak in through the rubber pressure tubing, when a

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vacuum was established by lowering the mercury receiver, and for a long time was a
source of annoyance. We subsequently learned that the device had been first intro-
duced by Lord Rayleigh. At the end of an experiment, any air which has
collected is discharged by raising the mercury-holder, opening the screw-clip shown,
and allowing enough mercury also to pass through to form a seal in the rubber
tubing above the clip.

Inflc co

JSr ^ . .

Fig. 2. — Section taken through the upper portion of one tube of the Differential Densimeter and the hot-
water jacket, showing the inflow and outflow tubes. Scale, one-third.

The mercury-holder was suspended by means of a ring and loop of wire
attached around the bulb from a vertical rod, swivel, and hook, possessing a slow
screw movement in a block attached to a horizontal rod fixed in a clamp, which could
be moved up and down on a heavy retort stand. For large movements, the clamp
was slid up or down the retort standard, and for fine movements the screw was
raised or lowered in its block.

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In using the apparatus, the two vertical tubes are first placed at the same level,
the mercury-holder is filled with mercury, and, with the two glass stoppers out, the
whole apparatus is filled with mercury, the two stoppers are next inserted, enough
mercury being left above them to form a seal, and the mercury-holder is then
lowered until the two vertical tubes become evacuated. The receiver is then raised
again to the level of the stoppers, and any bubble of air found is discharged.

The apparatus is now ready for an experiment, and, with stoppers out, the levels
of mercury are adjusted until there is an equal volume left above the mercury on
each side. A given volume of the solvent (say 5 c.c.) is now introduced on the one
side (say left), and an equal volume of the solution of chloroform in the same solvent
on the other side. In each case, immediately after the fluid has been introduced, the
stopper is inserted, care being taken to prevent any air being included, either as a
bubble at the mercury surface, or between the surface of the introduced fluid and the
stopper* To achieve the latter end, we have almost always introduced above the
mercury 2 or 3 cc, more than the required quantity, so that it stood in the neck and
slightly above, and then, by easing the stopper and gently adjusting the level of the
mercury-holder, have brought the level of the mercury in the tube to the desired
volume mark. After the solvent on the one side and the solution of chloroform of
the desired strength on the other side have been successfully introduced in equal
volume, and without any bubble of air, the mercury-holder is lowered until a space
containing vapour has appeared on each side. The level of the mercury will be found
to be lower on the chloroform side, and it is obvious, the instrument being inde-
pendent of variations in atmospheric pressure, and the only different factor being the
added chloroform on the one side, 1 that the difference in pressure will give the vapour
pressure directly for that strength of chloroform solution at that particular temperature.

There is hence no need to determine pressure due to dissolved gases on the two
sides, 1 or pressure of aqueous vapour, since these balance, and the quickness with
which readings can be directly obtained makes it possible to carry out a long series
of determinations at varying strengths, without the proteid solutions having time to
undergo bacterial change.

Certain precautions have to be taken, however, and corrections made which may
here be mentioned : —

1. Before taking a reading it is essential to move the tubes vertically and adjust
the levels until the volumes of the vapour spaces above the upper aqueous solution
meniscus in each case are exactly equal, otherwise inequality in pressure of gases
pumped off" on the two sides gives rise to an error, which is greater the smaller the
vapour space.

1. There will be a imall difference in the pressure of water vapour on the two sides, due to there being a stronger
solution on the chloroform side, but this is in all cases too minute compared to the pressure of the chloroform vapour to make
■ny appreciable error.

2. Slight differences in dissolved gases gave a disturbance with very dilute chloroform solutions, and this was later
obviated by pumping the gases off (vide infra).

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2. Before taking a reading, there must be certainty that each fluid is in
equilibrium with its vapour space. This is shown by absence of variation when the
apparatus is left at rest.

For rapid and accurate working the mechanical stirring by means of the studs and
magnet is indispensable, for even after the lapse of an hour when at rest the solution
has not completely discharged its proper amount of chloroform into the vapour space.
When once the control, containing, of course, no chloroform, has been thoroughly
stirred it remains constant, and need not be changed at the end of each determination,
but can be used throughout an entire experiment.

By vigorous stirring, equilibrium can be attained in five to ten minutes, and the
level does not afterwards change, no matter how long stirring, and observation be kept
up. This important experimental observation we have taken occasion to verify
several times during our experiments.

3. For very accurate working, especially with the dilute solutions and low
pressures, it is necessary in the case of serum and haemoglobin to pump off" the
dissolved gases by means of a Tflpler pump, otherwise these come off" unequally from
solvent and solution and disturb the results at the low pressures. The chloroform
solutions are then made up from the pumped-out solvent, which also must be used for
control and for making the dilutions.

4. The temperature must be the same in the jackets surrounding each tube at
the time when each reading is taken, and in a series of determinations at varying
strength and a constant temperature, that temperature must be closely maintained
throughout. The temperature error is a maximum when the solutions are near
saturation, for then the variation in vapour pressure per degree is very large ;
fortunately here the differences in level under observation are also very large, which
diminishes the percentage error arising from small deviations in temperature.

At concentrations away from saturation, the variations arising from small
differences in temperature approximately obey the gas law, and under the conditions
of our experiments become quite negligible.

5. A correction must be made in all cases upon the concentration of the solution
introduced into the tube for the amount of chloroform pumped off from the solution
into the vapour space. This correction is, of course, larger in the case of the more
concentrated solutions with high vapour pressures.

This has been done in the experiments of which records are given below, and
accounts for the concentrations not being exact percentages or small fractions of exact

The amount of chloroform in the vapour space is readily calculated from the
product of the observed vapour pressure and the volume of the vapour space, and this
amount deducted from the quantity contained in the chloroform solution when it was
introduced, gives the necessary datum for calculating the concentration in chloroform
of the solution corresponding to the observed vapour pressure in the vapour space.

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The ratio of the vapour concentration in the fluid to the concentration in the
vapour space gives the coefficient of distribution {coefficient de partage, Theilungs-
coefficieni) ; this should remain' constant if the absorption of the chloroform vapour
by the liquid were normal and strictly proportional to the vapour pressure, and if it
varies it points to a physical or chemical aggregation or compound between the
chloroform and the fluid or its constituents (vide infra).

Method of Reading. — The readings were taken with a cathetometer, 1 placed about
four feet from the tubes, both for greater accuracy in reading than direct measure-
ment would give and to avoid changes in temperature.

Two Methods of Experimentation. — : Wc have employed two different methods of
experimentation in investigating the variation in vapour pressure with varying con-
centration. It is obvious that the concentration of a measured volume of a strong
solution introduced into the densimeter may be diminished by pumping off" more
and more chloroform, by increasing the volume of the vapour space above the

A series of readings of differences in pressure may thus be obtained in which
the vapour space on the two sides is kept equal and of known and increasing value
throughout the series. This method we have called the method of variable vapour

On the other hand, a series of solutions of known and steadily diminishing or
increasing concentration may be introduced into the densimeter and measured one
after another as to their vapour pressure, in each case with a known fixed volume of
vapour space. As stated above, the concentration at which each vapour pressure in
the series is measured is then accurately known. This method we have called the
method of constant vapour space.

Method of Variable Vapour Space. — In this method, unless the volume of the
tubes of the densimeter is very large, the volume of solution and solvent respectively
introduced must be very small. We have usually 'taken £ c.c. on each side, either
of a saturated solution or of a very strong solution of known strength, and, by altering
the levels of the tubes and mercury receiver, have taken a long series of readings, in
each case with equal volume of vapour space on each side, at every increasing volume
of vapour space, until the difference in pressure became of small value, and the
product of volume and vapour pressure became approximately constant, showing that
practically all the chloroform had been pumped off" from the solution. This constant
product then gave the necessary datum for calculating the concentration of the
original solution introduced into the densimeter, the product of the vapour pressure
and volume at each stage gave the datum for calculating the quantity of chloroform
pumped off" from the solution, and, therefore, for deducing the corresponding

1. The instrument used was made by Pye and Co., of Cambridge, and, by means of a vernier and divided screw-head,
read to .J* mm. We have frequently observed that we were able to take readings within five divisions, that is ^ mm., which
is far within the accuracy of other portions of out determinations.

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concentration of solution. Or, also, by plotting vapour pressures as abscissae, and the
product of vapour pressure and volume of vapour space as ordinates, the ratio of
vapour pressure and amount of chloroform absorbed at each stage could be shown.

The method of variable vapour space has, however, two working disadvantages
which caused us in the end to abandon it and replace it by the method of constant
vapour space. The first objection is that the amount of solution taken is small, hence
it is difficult to measure it with accuracy, and to make it equal on the two sides.

The second objection is that at the low concentrations the increase of volume for
a small fall in pressure is very large, and hence the determinations become inaccurate,
a small error in pressure-reading making a large deviation. The values for high
pressures are also inaccurate., but for a different reason ; these readings are taken with
small volumes of vapour space, and unless the vapour spaces are accurately equal on
the two sides, there is a large disturbance due to inequality in pressure of the previously
dissolved gases pumped off on the two sides.

The results, however, in the intermediate pressures are accurate and are given
below, as they confirm those given by the other method.

Method of Constant Vapour Space. — In using this method we have always introduced
a volume of 5 c.c. of the solvent on one side, and 5 c.c. of a solution of known
strength on the other, and have invariably adjusted the levels so that at the temperatures
of observation there was a vapour space of exactly 5 c.c. on each side.

In some cases we have started with a saturated solution of chloroform, and have
then made dilutions of different percentages of that solution in the manner described
below. In the later experiments we found it, however, more expedient, on account
of knowing the exact concentration directly, to prepare a solution of known strength,
say one per cent., and for the more dilute solutions to use various percentage dilutions
of this stock solution. The more concentrated solutions were obtained by making up
solutions varying by one per cent, in strength, and one-half per cent, differences were
got by mixing these with each other in equal proportions. For percentages less than
o*i per cent., aci per cent, solution was first prepared by making a ten-fold dilution
of the one per cent, solution, and this o*i per cent, solution was then diluted similarly
to the one per cent, solution.

Precautions in Preparing, Preserving, in Unaltered Strength, and
Diluting by Known Amounts, of the Solutions Used

In working with the solutions it is indispensable that due precautions be taken
against loss by escape of chloroform into the air during the various manipulations.

If a portion, for example, of a stock solution of known strength be pipetted off
for use in the densimeter, and then the stock bottle be merely stoppered, the air
space over the portion of solution left in the bottle rapidly becomes charged with
chloroform vapour at the expense of the stock solution, and a second sample taken

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later from the same bottle will be found to be weak and give a wrong result. The
way to guard against this is to fill the bottle with mercury up to the neck immediately
after drawing off, and at once stopper up. Similar devices were employed in all
dilutions to complete the process out of contact with air.

The stock solutions were made by direct weighing, by dropping from a fine
pipette into graduated glass -stoppered flasks of 25, 50, or 100 c.c. capacity, according
to the amount required, immediately filling up to the mark with the particular solvent,
and setting at once upon a slowly rotating disc, driven at such a rate that the chloro-
form globules have just time to fall through the solution each time the flask is
inverted. In this way a rapid solution is effected, so saving much time. Further^

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Online LibraryBenjamin MooreAn experimental study of the physical chemistry of anaesthesia in ... → online text (page 3 of 5)