AN ELECTROLYTIC METHOD FOR THE DETERMINA- TION OF SODIUM PLUS POTASSIUM.

BY JAMES L. STODDARD.

(From the Chemical Laboratory of the Massachusetts General Hospital, Boston.)

(Received for publication, July 11, 1927.)

The method described in this paper is designed for the deter- mination of the sum of sodium and potassium, and thus of sodium by difference, if potassium is determined.

The material is first ashed and dissolved. Magnesium and calcium are then precipitated. Part of the remaining solution is electrolyzed in a simple apparatus, consisting mainly of one test-tube fused inside another, with mercury in each tube, and a passage through the wall of the inner tube (Fig. 1). Under an E.M.F. of 110 volts the negative ions migrate to the mercury in the outer tube. The positive ions migrate to the mercury in the inner tube, where they form an amalgam, which is then re- moved and titrated.

Results check within 2 per cent when the quantity of sodium plus potassium is equal to at least one-fourth that contained in 1 cc. of plasma.

The method was designed principally for use with the new and accurate method for the determination of potassium of C. H. Fiske. Unfortunately up to the moment of writing only a pre- liminary note on this method has been published.

Development of the Method.-It was found early in the investi- gation that a voltage of 110 is necessary to give results in a reason- ably short time. Even with this voltage, however, it is necessary to reduce the maximum distance of travel of ions as much as possible. A mechanism by which the amalgam formed could be shut off from the remainder of the solution and then titrated seemed to involve the use of a stop-cock or pinch-cock; but such devices increased the dist,ance to be traveled to an impractical

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678 A Method for Sodium Plus Potassium

figure. The concentric apparatus of Fig. 1 was then invented. Mercury in’ the inner tube acts as cathode, mercury in the outer tube as anode. The solution to be analyzed floats on the mercury in both compartments and connection is had through the holes in the inner tube. Connection is broken by tilting the apparatus so that the level of liquid drops below the connecting holes. Experiments showed that in the upper four-fifths, at least, of the

FIG. 1. FIG. 2.

FIG. 1. A represents the inner tube, B the outer tube, and w one of the platinum wires.

FIG. 2. 2’ indicates the electrical apparatus, L the incandescent lamp.

liquid there is nothing titratable at the end of the amalgam forma- tion, so that the very slight mixing possible by uneven manipula- tion of the apparatus would cause no error. The mercury and liquid in the inner tube are then run out into another tube and titrated. (Details are given later.)

Description of Apparatus.-A small Pyrex test-tube, of about 12 mm. internal diameter and 13 cm. length, with fairly thin wall, is fused at the bottom inside a short Pyrex test-tube, internal diameter about 20 mm., length 6.5 cm. There should be about

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J. L. Stoddard

a 3 mm. space between the tubes (Fig. 1). The inner tube has two holes in one side, about 19 mm. from the bottom and about 3 to 4 mm. in diameter, placed horizontally. One platinum wire is fused into the bottom of the inner tube where the latter is fused to the outer tube, and another is fused into the outer tube about 0.9 cm. from the bottom.

Directions for MaIcing the Apparatus.-Cut off the lower 6.5 cm. of the 20 mm. internal diameter Pyrex tube as follows: With a new or very sharp file make a circular mark deeply at the place to be cut, and completely around the tube; then heat one point on this line with a very small fine flame of the blast lamp, and immediately touch the heated place with a drop of water: then pull apart the two sections of the tube. Fuse the platinum wire into the side of the tube, and fasten it in firmly by melting a fine sliver of Pyrex glass about it as it emerges from the tube. Then, with a pair of snaps or forceps, grasp the inner end of the wire so as to break off some of the glass and allow the possibility of electrical contact. Blow the two holes in the smaller tube, place it in the larger one, and fuse it in at the bottom. Insert a plati- num wire into the smaller tube through the fused place at the bottom, and fasten it as described above. Heat the whole apparatus in a smoky flame until well covered with soot, then wrap in a cloth until cool. Break off the glass on the inner part of the platinum wire in the inner tube as described, or by carefully pushing the sharp edge of a triangular file down along the side of it.

Description of Electrica. Con.nections.-Each tube is connected in series with a 25 watt 250 volt lamp on a 110 volt direct current (see Fig. 2). In case such a circuit is not at hand, radio B batteries to make this voltage would undoubtedly work equally well. The amount of current used is very small. The platinum wire of the tube is conveniently connected with the circuit wires with fine covered copper wire, such as No. 26 gauge. Since the usual covering is too thin to insulate perfectly against 110 volts,, it is important not to allow the wires to remain in contact with each other. The outer wire of each tube is connected to the positive pole, and the inner wire with one terminal of a lamp, the other terminal of which is connected to the negative pole. A large number of tubes can be connected to one electrical outlet.

The clamps holding the tubes are conveniently attached to a horizontal rod-an iron stand on its side, for instance.

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680 A Method for Sodium Plus Potassium

Directions for the Method.-Ash the material by the wet method, as follows (these details are taken from Fiske’s total base method) : To 1 cc. of plasma or urine (or what other amount may be ad- visable, as discussed later) in a non-protein nitrogen tube (a Pyrex test-tube about 20 by 200 mm.) add one or two pebbles, 1 cc. of 4 N sulfuric acid, and 0.5 cc. of nitric acid. Boil until dense white fumes fill the tube, then heat for 3 or 4 minutes more. If the solution is n6t clear, let it cool a little, add 0.5 cc. more of nitric acid, and boil again. Usually urine becomes clear after the first heating and plasma after the second, but other material may require much more ashing.

Let cool, add about 2 cc. of distilled water, mix, and transfer to a platinum dish. Rinse four times down the sides of the tube with about 1 or 2 cc. of water each time. Evaporate the water on the steam bath. Place the platinum dish in a Pyrex beaker on a tin or thin iron plate which is placed on wire gauze over a Meeker burner. Heat very slowly, gradually raising the flame until the plate shows a dull red heat, and keep this heat until fumes cease to come out of the crucible. Then place the crucible on a triangle and heat all parts to a dull red heat for a moment with a micro burner. Four or five platinum crucibles can be heated at once on a tin plate in this manner.

Let cool. Add exactly 2 cc. of approximately 0.012 HCl (dilute the concentrated HCl 1: 1000). Warm slightly over a small flame of a micro burner, then stir with a rubber-tipped glass rod, rubbing all the interi0r.l Add exactly 1 cc. of precipitating solution (1.0 per cent of very pure (NH&HP04 in 10 per cent of concen- trated NHJOH C.P. (sp. gr. 0.9)). Mix thoroughly. Pour back and forth twice into a Pyrex centrifuge tube with the aid of the glass rod, mixing again in the crucible each time, and rinsing the inner surface, rubbing it with the policeman. Let stand in the centrifuge tube, tightly stoppered, for 3 to 4 hours. Centrifuge for 15 minutes.

Remove 1 or 2 cc., keeping the finger over the end of the pipette as it is introduced into the solution in order to avoid drawing up floating crystals, and wipe off the end of the pipette carefully after removing. Put this measufed amount into a 30 CC. Pyre%

1 There will usually be present an undissolved precipitate of calcium, magnesium, and phosphate, of course.

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J. L. Stoddard

beaker and evaporate on the steam bath. When dry add about 0.6 cc. of approximately 0.15 N HCl (dilute the concentrated 1: 80) if 1 cc. of solution is taken, or 1.2 cc. if 2 cc. are used. Evap- orate again. Put in an air oven at about 115’ for 10 minutes on a piece of asbestos, or a pad of paper, in order not to heat the bottom quickly, so that it may dry thoroughly without spattering. If this step is omitted, the heating done next must be very gradual at, first.

Place on a tin plate on gauze over a Meeker burner and heat gradually, at first with the flame just touching the gauze, until fumes come off, then raise the flame until the tin plate shows a very dull red heat directly over the flame, and keep this heat till fumes of NH&l cease to evolve. Then put the burner directly under each beaker in turn for about 60 seconds. Remove each beaker with forceps and heat ‘the sides to remove any condensed NH&l. Let cool.

Add about 1 cc. of distilled water and approximately 0.15 cc. of the 0.15 N HCl. Heat over a micro burner until it steams, stirring gently and cautiously with a rubber-tipped glass rod. Let it stand a minute, then heat again, with stirring.

From a separatory funnel with fine tip attached run pure, and tested, mercury (test by titration as described later) into the inner tube of one of the pieces of apparatus to a level a little below the holes, then fill the outer tube to the same level. Clamp vertically, by the outer tube. Attach the outer tube wire to the positive electrical connection, and the inner tube wire to the negative.

With the aid of the glass rod, transfer the solution in the beaker to the inner tube, then rinse the inside of the beaker four times with about 0.5 cc. of distilled water each time. Then rinse the inside of the tube with 0.5 cc. Do not get the total volume in the tube over about 4 or 5 cc.

Leave the tube with the current on for 3 to 2 hours. Empty the amalgam which has formed in the inner tube into

a titration tube consisting of a Pyrex test-tube about 6 inches long by 9 inch internal diameter narrowed somewhat near the top. In order to do this without letting any of theaouter liquid or mercury get into the titration tube it is best to tie a narrow strip of cloth about the inner tube about 2 inches from the end.

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682 A Method for Sodium Plus Potassium

Place the titration tube in a beaker, with another beaker of the same height next to it on the right to catch the outer liquids. Loosen the clamp holding the apparatus, then carefully, without tipping it, or moving it suddenly (in order to avoid any movement of liquid or mercury through the holes in the inner tube) take it out of the clamp. Then smoothly and rapidly tip it toward the side opposite the holes to an angle of about 45” so as to bring the surface of the liquid below the holes, thus shutting off connection between the two compartments of the tube. Then place the end of the inner tube over the edge of the titration tube and empty the inner tube into it taking care not to let the mercury run over the further edge of the titration tube. It is well to tip the titration tube further tomward the apparatus at this point. Then drain the inner tube with the aid of a fine glass rod (hooked at the top so that it can hang on the titration tube). Hold the apparatus vertically, rinse down the sides of the inner tube with about 0.5 cc. of distilled water, empty this into the titration tube, then repeat the rinsings twice. The wires should have been connected and the current on up to this point.

Titration.-Add 1 cc. of 0.1 N HCI and 4 cc. of distilled water, bring to boiling over the micro burner, with constant stirring by a circular motion, then clamp at an angle of about 60” and boil for 3 minutes. Take out of the &mp, abb phenol reb, kold in a test-tube holder, and titrate with 0.02 N NaOH from a micro burette with a fine tip, keeping the solution gently boiling.2 The alkali should be standardized by a similar titration, and in a similar volume of solution, and with a preliminary boiling of 3 minutes to remove COZ. The alkali should be kept in a Pyrex or paraffined bottle and guarded with soda-lime. It should be restandardized often. Of course, if 1 cc. of HCI fails to make the solution acid, another cc. should be added, and the solution boiled for another 3 minutes in order to decompose the remaining amalgam.

Calculation.-If 1 cc. of the centrifuged solution is used the

2 The titration is best done by a strong artificial light placed back of the titrator. Place a white envelope or card along one side of which a strip of yellow paper has been pasted, back of the titration tube, with the white part directly behind it. Just before the end of the titration the color of the solution is close to that of the paper, and by contrast the first trace of pink tint shows out definitely. This is the end-point.

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J. L. Stoddard 683

amount of alkali found by titration X 3 X 1 .012 equals the amount in the original material. If 2 cc. of solution are used, the amount

of alkali found X 5 X 1.012 equals that in the original amount of

material taken. To clean the apparatus, rinse once with water, then pour con-

centrated HNOI in the outer tube, and shake until adhering deposits are detached. Rinse three times with water, and three times with distilled water, draining carefully.

If the mercury is pure to begin with, and handled carefully, it can be used again after washing, filtering, and stirring for 3 hour with a mechanical stirrer in a large beaker with 0.1 N HCl, then washing free from acid, drying, and filtering. Each lot should always be tested by placing a few cc. in the titration tube, adding 1 cc. of 0.1 N HCI, 8 cc. of distilled water, boiling for 3 minutes, and titrating. If the above method does not purify it, draw a rapid current of air through it for 24 hours, or run it in a fine stream through a tall column of 8 per cent HNO,. Distillation is ordinarily unnecessary.

Notes on the Method-The amount of sodium plus potassium in the amount of material ashed should be between 0.11 and 0.75 cc. of 1.0 N base. Ordinarily 1 cc. of plasma or whole blood will fall between these limits. In the case of urine, the specific gravity would give indication of any unusual concentration; usually 2 cc. would be sufficient. If a low sodium plus potassium content is suspected, or if an extraordinarily high phosphate coexists with a moderately low sodium plus potassium, a larger amount of material should be ashed.

In case a low sodium plus potassium content, near the above limit, is found, and it is impossible or inconvenient to ash a larger specimen, the titrated solution at the end should be tested for ammonia by Nesslerization. If none is present, the determina- tion is not in error. If ammonia is found, the result is too high. Evaporate another cc. of the centrifuged solution, adding before evaporation exactly 1 cc. of 0.05 N NaCl (made from the fused salt). Proceed according to the method, but subtract the value this method gives from the NaCl solution added; i.e., 0.0494 CC.

Of 1.0 N.

The explanation of the directions just given is as follows. Un-

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684 A Met,hod for Sodium Plus Potassium

less the sodium plus potassium content of the precipitated solu- tion is of the same molar concentration as the phosphate present, not all of the phosphate will be changed later on to NaH2P04, and some ammonium phosphate will remain. It requires a higher degree of heat than is used in this method to drive off ammonia completely from ammonium phosphate, so that not all ammonia would be removed, and an error result. The molar concentra- tion of phosphate from the precipitant is 0.0252. Phosphates in the material, if blood or urine, would not ordinarily bring it to over 0.035, corresponding to an original concentration of sodium plus potassium in the material of 0.105. By adding the sodium chloride solution, all of the phosphate will be enabled to form NaH2P0*.

HCl is added after the first evaporation in order to form NaH2P04 and NH&l, and thus enable the ammonia to be easily vaporized with the minimum amount of base present. HCl is added in small amount to the final solution of the residue, and it is warmed, in order to insure the solution of any sodium meta- phosphate which may have formed during the previous heating of the dry residue.

To ash whole blood, it may be necessary to add a second cc. of sulfuric acid, and to digest it for a long time with repeated additions of small amounts of nitric acid. Charred spots after heating in the platinum dish can be cleared up with a small flame of the micro burner; avoid more than a faint dull red heat for a short time.

It was proved that there is no recognizable difference between using Pyrex beakers and platinum crucibles for the evaporation and heating of the precipitated solution. It is important to avoid having acid fumes or ammonia in the air of the laboratory while the solution is being electrolyzed. It is well to have a slight draft from the worker toward the titration tube while titrating, in order to avoid any possibility of absorbing mercury vapor.

The principal cause of low results seems to be losses by spatter- ing of liquid or solid. Such losses are especially likely if the digested residue in the platinum dish is dried too quickly, or if the later residue dried on the steam bath is heated over the burner without being previously heated in the air oven. In this latter case the dried residue pops at a certain point of the heat,ing, unless it is exceedingly cautiously done.

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J. L. Stoddard 685

Blanks can be run without any special trouble on all of the reagents except the ammonium phosphate. If this is done, enough of a standardized sodium (or potassium) chloride solution must be added to enable all of the phosphate to form NaH2P04, and the solution must be evaporated, etc., and heated as in the method.

Amount. TiUlt3. Per cent of total found.

cc. min.

2 60

2 60

2 60

2 60 1 69 1 75 3 40 3 40 3 90 3 110 3 170 1 78 1 80 2 75 1 60 1 60 1 40 1 40 1 45 1 45

Total.... . .20 I , Average . ..98.80

TABLE I.

99.3 98.9 98.4 98.5 98.1 98.9 99.5 99.0 98.5 98.6 99.4 98.14 98.3 98.8 98.7 99.0 98.5 98.87 99.5 99.15

Analysis of Known Solutions.

Sodium Chloride.-In order to standardize the electrical part of the method a sodium chloride solution was made up in 0.1 N

concentration from Eimer and Amend T.P. analyzed salt, freshly fused in a platinum crucible, and cooled in a hot air oven, then in a desiccator. The amounts shown in Table I were measured directly into the inner tube of the electrical apparatus, and left for the time indicated. The figures include all results in the

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686 A Method for Sodium Plus Potassium

TABLE II.

Ns

0.0641 3.0128

0.1282

0.1282

0.0641

0.100

D. 0128

0.0128

0.0128

0.0125

0.0892

---

Cc. 1.0 N base per cc. solution. lp qa+ K

I.0769 1.0065 I.0079

3.1410 ). 0065 I.0079

D. 1410 I.0021 ). 0025

0.0769 3.0065 I;0079

0.1125 3.0100 I.0125

0.0137 0.0111

-

Mg Ca

I.0738 I. 0748 3.0744 3.0764

f

r .-

1

97.1 98.5 98.0

loo. 5

hrs.

2 2 4 4

3.1354 97.2 4 D. 1369 98.3 96 D. 1390 99.8 4 D. 1376 98.8 4

0.1386 99.5 4 0.1364 97.9 4 0.1391 99.8 4

0.0752 99.0 4 0.0732 96.3 4

0.1091 97.8 4 0.1089 97.8 4 0.1108 99.6 4 0.1117 100.4 4 0.1115 100.3 4 0.1115 100.3 4 0.1089 97.8 96 0.1121 100.8 96 0.1115 100.3 96 0.1099 98.8 96 0.1115 100.3 3 0.1089 97.8 3 0.1087 97.8 2 0.1067 96.0 2 0.1097 98.7 2

0.0983 0.0973

99.2 98.2

3 3

-

ercent of

total ound. COP

e&d.

Time ~recipi- tated.

3olu- tion.

cc.

1 1 1 1

1 1 1 1

1 1 1

1 1

1 2 2 1 1 2 1 1 1 1 1 1 1 1 1

1 1

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J. L. Stoddard 687

TABLE II-Concluded.

Na

0.08 0.02 3.10 0.012

Cc. 1.0 N base per cc. solution. 1

K \Ta + K Mg

-r-

-

Ch

0.010 0.0996 100.8 0.0996 100.8 0.0996 100.8 0.0965 97.7 0.0968 98.0 0.0971 98.3 0.0991 100.3 0.0991 100.3 0.0991 100.3 0.0979 99.1 0.0985 99.7 0.0996 100.8 0.0989 loo. 1 0.0997 100.8 0.0981 99.3 0.0995 100.7

-

?er cent of

total found.

cm- rected.

Time neoipi- tated.

hrs.

5 5 5 2 2 2 2 2 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75

pi-

cc. 1 1 1 1 2 1 2 1 1 2 1 1 2 1 1 1

series arranged in chronological order. On account of the close checking of the results, in spite of variations in time and amount, with a maximum deviation from the average of only 0.7 per cent, it was decided that the 1.2 per cent of undetermined substance was inherent in the method and constant enough to be introduced as a correction. All later results substantiated the correctness of this point of view.

Experiments were then done to find whether additional nega- tive ions would interfere with the determinations. 1 cc. of approximately 0.05 N HCl, of 0.05 N H&Sod, or of 2.5 per cent trichloroacetic acid failed to move the result out of the range in Table I. 1 cc. of 0.1 N HCl alone in the apparatus gave an absolute blank. 1 cc. of 0.3 N HCI, however, lowered the total when 1 cc. of NaCl solution was used, by 1.2 per cent. Addition of water up to 6 cc. to the solution in the tube had no effect.

Potassium Chloride.-A similar series of twelve tests on a 0.1 N KC1 solution (from Baker and Adamson, C.P. analyzed, re- crystallized twice, dried 2 days at 115”, gave an average of

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688 A Method for Sodium Plus Potassium

98.52 per cent with a maximum deviation from the average of 1.3 per cent.

Potassium Phosphate.-A 0.1 N solution from Baker and Adam- son, C.P. analyzed KHzPOd, recrystallized four times, and dried at 115” for 24 hours gave as an average of three tests 98.53 per cent, with a maximum deviation from the average of 0.4 per cent.

Mixtures of Sodium, Potassium, Calcium, and Magnesium.-A calcium chloride solution was made from Eimer and Amend T.P. precipitated CaC03, dried for 2 days at 115” and dissolved in HCI. A MgClz solution was made from MgO, c.P., dissolved in HCl. From these solutions and the sources noted above various proportions of the bases were mixed, and the solutions run through the whole method. The actual amount found is given in the sixth column of Table II, while the “per cent of total found” was figured according to the method; i.e., the established correction of 1.2 per cent was added to the total. In Table II the “time precipitated” column represents the time the solution was allowed to remain in the centrifuge tube before a sample was removed for evaporation on the steam bath; ‘(cc. of solution” means the cc. removed for evaporation.

BIBLIOGRAPHY.

1. Fiske, C. H., A method for the estimation of total base in urine, J. Biol. Chem., 1922, li, 55.

2. Fiske, C. H., and Litarczek, G., A new method for potassium, J. Biol. Chem., 1926, Ixvii, p. xvi.

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James L. StoddardPLUS POTASSIUM

THE DETERMINATION OF SODIUM AN ELECTROLYTIC METHOD FOR

1927, 74:677-688.J. Biol. Chem. 

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