MODIFICATIONS OF THE SAND AUXILIARY ELECTRODE.

BY THOMAS BROOKS SMITH.

(Received 13th December, 1927, alzd read before the Society, 25th la?zuary, 1928.)

The value of the Sand electrode is generally recognised for accurately controlling the potential drop at the cathode during separations of metals, the oxidation potentials of which are very close to one another or to that of hydrogen. Where such electro-analytical work is only carried out inter- mittently in a laboratory equipped primarily for physico-chemical work the original arrangement leaves but little to be desired. If, however, equipment is to be obtained specially for work of this nature, it is obviously worth considering whether one instrument may be made to serve both as volt- meter and null point detector, but a difficulty is encountered in connection with the high resistance of the usual type of half-cell which would necessi- tate the use of an extremely sensitive and therefore costly voltmeter. Means have been found of reducing the resistance of the auxiliary electrode circuit without introducing sensible errors on account of concentration polarisation in the standard half-cell. The dual use of the voltmeter which thus becomes possible has the double advantage of considerably reducing expense and providing an exceptionally convenient means of observing the null-point.

A rough determination of the resistance of the various parts of an elec- trode of the design shown in Fig. I, fitted with a Sand pattern tap a (Fig. ~a), was carried out in the usual way with the aid of a Wheatstone bridge using alternating current. The results obtained were as follows :-

TABLE I.

Resistance in Ohms. I When whole of

Apparatus is at 18" C.

I Tube between A and B containing Hg,SO,

Tap (resistance very variable). Tube between E3 and C containing N Na2S0,. Tube between C and E containing N Na,SO,.

in zN H2S0,. 700

12,000 3,000 6,000

Total I 21,700 ,

When end of Connect- ing Tube is Immersed

in a Bath at go" C.

I 12,000

2,000

14,000

'H. J. S. Sand, Y.C .S . , 1go7,91, 373. This type of tap is used without a lubricant. Small quantities of liquids from the

The adjacent tubes are retained by capillary forces between the tap plug and its socket. film thus formed affords the only electrical connection between AB and BC.

216

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T. B. SMITH

Early Modifications. Paffern shown in Pig. I.-The reasons which make the use of dia-

phragms unsuitable for very accurate potential measurements are of small weight when an order of accuracy of only 0.01 volt is concerned, and in the first modification attempted the special tap (Fig. ~ a ) was replaced by an ordinary three-way tap containing a filter paper diaphragm (Figs. 18 and 2).

This consisted of two thicknesses of Whatman's No. 50 filter paper, cut slightly larger than the hole in the tap and rammed tightly into it. If this type of apparatus were to be adopted for routine work it would doubtless be desirable to find some more permanent way of fixing the diaphragm, but the method adopted was found to be adequate for the purposes of preliminary experiments, the apparatus remaining in working order for at least two or three months. The resistance of the diaphragm is only of the order of 5 0 to IOO ohms and the total resistance of the electrode when the capillary tube is immersed in a bath at goo is reduced to about one-third of that of the original pattern.

The modified electrode was tested by carrying out a series of bismuth estimations during the course of which the sensitivity of the means adopted

0 5 10 cm. - E

X FIG. I. FIG. 2.

for the null point detection was determined when the fairly sensitive volt- meter referred to as No. I. was employed. The characteristics of the in- strument are given below, Table 11. The circuit arrangements are shown 3n Fig. 4 from which it will be seen that when the switch S is to the right the instrument serves for the detection of the null point, and with the switch t o the left for the measurement of the potentiometer voltage. I t will be seen from the Table that a deviation from the null point of one scale divi- sion (about 0.75 mm.) is equivalent to a difference of potentiometer voltage of 0.15. Since the minimum deviation which can be detected with ease is about one-fifth of a division, such an arrangement would enable the potential to be controlled to within 0.03 volt. With great care such an arrangement might possibly be used but is hardly to be considered satisfactory.

P a f f e m shown in 233 2.-Except near the extreme end the connecting tube is of wide bore. The form of the portion between T and 2 is similar to that of the corresponding part in the Fig. 3 pattern and will be discussed later. The connecting tube to the left of the tap is made as short and wide as practicable, thus avoiding the quite unnecessary resistance caused by the tube AB in the original pattern. The possible risk of the electrolyte con- centration being affected by diffusion of the connecting liquid from B was

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2 I 8 MODIFICATIONS OF THE SAND AUXILIARY ELECTRODE

shown to be negligible, and since a lubricated tap may be employed, diffu- sion can be stopped altogether during periods when the electrode is not in use. Furthermore, the use as a connecting liquid of sulphuric acid of the same concentration as that employed in the preparation of the half-cell electrolyte is usually permissible; its employment not only reduces the re- sistance still further but also enables this type of electrode to be used for some months without refilling with electrolyte.

If this pattern is used in conjunction with voltmeter I a change of 0.01 volt in the potential drop at the cathode causes a null point deflection of a quarter of a scale division when Nsodium sulphate is used as connecting

FIG. 3. The joining of the small bore tube J and

the bulb Q requires careful workmanship if the downward passage of liquid from J to R is to leave the electrolyte in Q undisturbed. This, however, is only of importance when the connecting liquid is less dense than the electrolyte, and if the auxiliary cell is to be used under these conditions it is advisable to test it in the following way. The entire ap- paratus except the thistle funnel G is filled with electrolyte, and G is then filled with connecting liquid which should be coloured with a suitable dye ; the tap H is then opened and the flow of this liquid is observed in the neighbourhood of J. Generally, however, liquids of high specific gravity are to be preferred for use in the connecting tube, and under these conditions the junction is arranged near S. The form and dimensions of the gap between J and R are not then critical, and in any case the resistance of the gap is comparatively insignificant.

After the introduction of a little mercur- ous sulphate paste into the pocket in the amalgamated platinum foil the latter is placed in the tube MP. Electrolyte is then intro- duced by applying suction at K whilst the end Z is immersed in the liquid, the tap H being closed and L open. In cases where

the connecting liquid is less dense than the electrolyte the excess of the latter is allowed to run out of the tube RTZ by opening the tap H. Connecting liquid may then be drawn up into G through Z by applying suction at F. When, as is usually the case, the liquid is denser than the electrolyte the end 2 should be immersed in the former immediately after the electrolyte has been sucked up, but without first removing it from RTZ as in the former case. Suction is then applied at F and the tap H cautiously opened until the con- necting liquid is drawn up to a point about 2 cms. above the bend T, after which the tap is again closed. This operation may be carried out most conveniently when the tube TU is horizontal. To enable the junction of the two liquids to be seen readily it is advan- tageous to colour the connecting liquid with a suitable dye. Brilliant cresyl blue was used in most of the experiments described.

liquid. Such an arrangement is quite sensitive enough for practical pur- poses. If dilute sulphuric acid is employed as connecting liquid, the deflec- tion corresponding to a potential change of 0.01 volt can readily be seen when the null point instrument is viewed from a point several feet away.

The Pattern finally Adopted.

In the course of experiments to ascertain whether a less sensitive control instrument could be employed, it seemed desirable to make a further sub- stantial reduction in the resistance of the auxiliary electrode circuit. Since it was not found practicable to achieve this by further modification of the

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T. B. SMITH 2 1 9

design attention was next turned to the possibility of utilising connecting liquids of high conductivity such as concentrated ammonium sulphate solu- tion or maximum conductivity sulphuric acid. The pattern last described was not regarded as suitable for connecting liquids of higher S.G. than the half-cell electrolyte, and! the pattern shown in Fig 3 was designed to permit of the use of connecting liquids of any desired density. Since no use is made of a diaphragm the relative permanence of the former pattern is sacrificed, but in practically all other respects the newer pattern was found to be preferable.

The electrode 0 (Fig. 3) consists of platinum foil amalgamated electro- lytically before introduction into the apparatus. The end of the foil is turned up to form a pocket into which is placed a small quantity of mercur- ous sulphate paste. If a connecting liquid were used of specific gravity less than that of the electrolyte, the junction of the two liquids should occur in the annular space between the tube R and the bulb Q. As a rule, how- ever, connecting liquids are used which are denser than the electrolyte, and in such cases the junction should occur in the neighbourhood of S.

I+$?uence of Convection Cwrents.-The connecting tube is almost invari- ably immersed in a hot liquid and this sets up convection currents in the tube UV (Fig. 3). I t is obviously undesirable that these currents should reach the junction between the connecting liquid and the electrolyte and to obviate such a difficulty, the tube T U (Figs. 2 and 3) is inclined as shown in the diagrams. I t also follows from consideration of this convec- tion current, that the sinuous tube between V and 2 should be long enough to prevent any appreciable amount of the metal to be estimated from reach- ing the point V. If attention is paid to this, it is advantageous to use a fairly wide tube between U and V since the heating up of the contained liquid causes the expulsion of a considerable amount of the connecting liquid, and this helps to render superfluous the washing out of the con- necting tube during the course of an estimation.

The following data show the extent and persistence of this expul- sion effect :-

Time . . I 2 4 8 15 mins. Vol. expelled . 0.07 0.85 0.95 0-115 0'135 c.cs.

I t will be seen that even after the first minute the rate of expulsion is con- siderable in comparison with the volume of the last centimetre length of the capillary (Y to 2, Fig. 3), which is 0.025 C.C.

Dzfksion E'ects.-Whilst the design of the tube between V and 2 was under consideration diffusion effects were studied either by placing the connecting tube filled with a suitable indicator into a bath of acid or alkali, or alternatively by filling the tube with water and placing in a bath of potassium permanganate. No attempts were made to measure exact rates of diffusion since the phenomenon in question is complicated by other effects such as the irregular shape of the tube and the influence of stirring. All that was attempted was to design a connecting tube having as low a resistance as possible, but in which diffusion effects would be negligible even for such substances as acids and uni-univalent salts which have higher diffusion coefficients than the salts of the bi- and ter-valent metals involved in most estimations.

As a result of the diffusion experiments it was concluded that the common practice of washing out the connecting tube during the course of an estimation is quite unnecessary. The upper limit of the errors that may

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2 2 0 MODIFICATIONS OF T H E SAND AUXILIARY ELECTRODE

be introduced by omitting this precaution were roughly assessed in the cases of the following representative estimations :-

(I) Bismuth-If 0.3 gram of the metal is to be deposited from IOO c.cs. of solution,. and the current at the outset is 3.4 amperes, then about 7 5 per cent. of the bismuth is deposited by the end of the second minute, and go per cent. by the end of the third. In 3 minutes the relatively fast moving hydrochloric acid does not penetrate more than two or three millimetres into the capillary, and the volume of this portion of the tube is about 0.005 c.cs. Even if the bismuth concentration throughout this volume were the same as that of the bath before the commenceinent of the deposi- tion the amount lost would amount only to O - O O O , O I ~ gram, an amount not weighable on an ordinary analytical balance. In reality the amount lost during the first three minutes would be considerably less than this, and the further loss during the remainder of the experiment would be still more insignificant.

(2) Nickel in the Presence of Zinc.-In this case the time required for the deposition is considerably longer, and there is not the same tendency for the greater part of the metal to be deposited during a small fraction of the total period. Against these factors which tend to increase the loss by diffusion must be placed the fact that the volume of solution employed (250 to 300 c.cs.) is considerably larger than those used in the bismuth estima- tion. During the 15 minutes required for the practically complete deposition of nickel, diffusion effects are only perceptible to a depth of 8 millimetres in the capillary tube, and the volume concerned is 0.02 c.cs. Even if this represented loss of nickel solution of the original concentration, the resulting error would only be about 0.007 per cent. and the weight of nickel lost O-OOO,OI gram.

i%asi&ility of Empdoying Connecting Liquids of fl&h Conducfivify.-The following possible sources of error were considered : (i) alteration of the deposition bath ; (ii) alteration of the half-cell electrolyte ; (iii) concentra- tion polarisation of the half-cell owing to the relatively heavy currents which may pass through it ; (iv) variation in diffusion potentials at the liquid junctions. Concerning (i) it has been shown that washing out of the con- necting tube is unnecessary and that the total amount of connecting liquid otherwise expelled when the connecting tube is immersed in a bath at goo is of the order of 0.16 c.cs. in half an hour. Even when maximum con- ductivity sulphuric acid (circa 7.5 iV) is employed, this small amount would not as a rule cause any undesirable change of hydrogen ion concentration even in an alkaline electrolyte. In this connection it should be remembered that in all cases where acidity is a critical factor buffer solutions are invari- ably employed as electrolytes. Doubtless, however, it would be safer in such cases to employ either weaker sulphuric acid or a solution of a neutral salt such as ammonium sulphate.

To test whether the use of maximum conductivity sulphuric acid in the connecting tube could lead to appreciable alteration in the half-cell electrolyte, the electrode vessel was filled with sulphuric acid of?, value 0.9 coloured with brilliant cresyl blue, and the maximum conductivity acid was also coloured with this indicator. The junction between the two liquids occurred at the point S (Fig. 3). The connecting tube was placed in a bath at goo for half an hour and then allowed to remain at the laboratory temperature for g hours. At the end of this period the junction was still quite sharp and no perceptible change of colour had taken place in the electrode vessel. This test showed that if any change at all had taken place it was less than 0 - 2 in thepH value. Even if the change had

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T. B. SMITH 221

been as large as this the effect on a hydrogen or quinhydrone electrode, in which the potential is directly dependent on p , value, would amount only to about 0-01 volt. In the case of a Hg-Hg,SO, half-cell any change in the sulphuric acid concentration is only of importance in so far as it affects the solubility of mercurous sulphate, and this influence is inappreci- able.

I t may be concluded that with the aid of connecting liquids of high conductance the resistance of the auxiliary electrode circuit can be reduced sufficiently to permit of the use of an instrument of moderate sensitivity, such as the voltmeter referred to as No. I1 (Table II.), provided that the potential of the auxiliary cell remains constant under the working con- ditions.

Polarisation Tests on the Auxiliary Cell. The half-cell to be tested was used in conjunction with a Bi-Bi +++

half-cell formed by interrupting a bismuth estimation a minute or two after switching on the current. Various initial demands were made from such a cell by a suitable adjustment of the potentiometer. The variation of current with time was noted in each of these cases and typical results are shown by the curves in Fig. 7.

Curve (a) relates to an auxiliary cell containing mercury in contact with a saturated solution of mercurous sulphate in zNsulphuric acid, but in which no solid mercurous sulphate was present. I t will be seen that such a cell is incapable of supplying the current necessary to give an appreciable deflection on voltmeter II., though it would be adequate for use with voltmeter I.

Curve (b) refers to a half-cell similar to that shown in Fig. 3 except that the solid mercurous sulphate was placed in the bottom of the bulb Q instead of in a pocket in 0. It will be seen that this arrangement is only slightly better than that in which no solid is present, and decidedly inferior to the Fig. 3 pattern to which the remaining curves refer.

Curve (c) shows that the recommended form of auxiliary cell is capable of functioning under a load of 0.3 milli-amperes for at least a quarter of an hour without showing signs of polarisation. This current gives a scale deflection of two divisions on voltmeter 11. In practice there is never any need for the deviation from the true null point to exceed one scale division, and without much difficulty the variation could be kept within still smaller limits. A deviation of two divisions would be quite exceptional. Hence the electrode shown in Fig. 3 is quite capable of being used in conjunction with voltmeter 11.

Curves (d) and (e) show that polarisation effects might be encountered if a voltmeter were employed, the sensitivity of which is much less than that of pattern 11.

The adequacy of the auxiliary electrode shown in Fig. 3 for use in conjunction with voltmeter 11. was further tested by frequently checking the potential of the half-cell immediately before and after use. The maximum variation observed was 0.0026 volt. This shows that the combined effects of polarisation, variation of diffusion potentials, and minor variations of temperature, are practically negligible.

Results. Use as a Quinhydrone EZectrode.-The Fig. 3 pattern serves excellently

as a quinhydrone electrode if a strip of plain platinum be substituted for

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2 2 2 MODIFICATIONS OF THE SAND AUXILIARY ELECTRODE

Electrode Shown

in

Fig. 2 Fig. 3

? *

,, ,,

TABLE 11.-COMPARISON OF VARIOUS PATTERNS OF AUXILIARY ELECTRODES,

Containing H g in contact with a saturated solution of Hg,SO, in 2NH,SO,. The figures in the last two columns were obtained during the course of a bismuth

estimation.

Nature of Half-Cell.

Hg-Hg,SO, in 2N H,SO, 7 9 I , $ 9 ,, 1 , 9 , 9 , 7 ,

Quinhydrone 0.1 N H,SO,

Hg-Hg,SO, in 2N H,SO,

Pattern.

Sand.

Similar to Sand but with tap shown in Fig. ~ a .

Fig. 2.

Fig. 3.

Liquid in Connecting

Tube.

N Na,SO, 2N H,SO, N Na,SO,

N Na,SO, 2 N H2SG, N N+SO, 2 N H,SO,

8 N Am,S04 7'sN H2S04

Resistance.

Whole of Apparatus at 18" C.

ohms. 22,000

10,000

4,000 550

4,000 550 270 850

Connecting Tube Immersed in

Bath at go" C.

Difference of Potentiometer. Voltage I Division.

Deviation from Null-point.

Voltmeter I.

volts. 0'4

0.15 -

0.03 5

0.03 5

0.006

0'01

0'01

0'01

DATA CONCERNING THE VOLTMETERS EMPLOYED.

Sensitivity in ohms per volt Current in milli-amps. for

full scale deflection No. of scale divisions Distance between scale

divisions near zero end of scale

Ranges

Voltmeter resistors (V.R. in

Resistance of moving coil Figs. 4 and 5)

only

Voltmeter I.

One 75 and one 50 division scale 0.75 and I mm.

Multi-range inst. for use with external multipliers. The 2.5 volt range was used for the experiments here described

1,200 ohms for 2.5 volt range

50 ohms

Voltmeter 11.

volts. - - -

0'2

0'2 0.05

0.05 0.03 0.06

Voltmeter 11.

134 7'5

50 0.9 mm.

I and 2.5 volt.

91 and 292 ohms.

43 ohms.

TABLE 111.

SATISFACTORY RESULTS WERE OBTAINED FOR THE FOLLOWI+NG ESTIMATIONS.~

Estimation.

Bi 9 1

9 ,

9 ,

Ni in pres. of Zn

Volt- meter.

I. I. 11.

I.

11.

Connecting Liquid.

N Na,SO,

Max. cond. H,SO, (S.G. 1.223)

0.9 N Na,SO,, 0'1 N H,SO,

8 N Am,SO,

7 9 9 9

3 A rotating anode and Sand pattern cathode were used in each case. For details of procedure see Sand, Y.C.S., 1go7,91, 373, concerning the bismuth estimation, and A. Fischer, Chem. Ztg., 1908, 32, 185, concerning the separation of nickel and zinc.

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T. B. SMITH 223

the amalgamated piece. Such a half-cell works admirably in conjunction with Voltmeter I. Difficulties are encountered in trying to adopt the quinhydrone for use with a less sensitive voltmeter and these are at present under investigation. The use of o*rN H,SO, together with o*gNNa,S04 is recommended as a connecting liquid ; high concentrations of HzS04 are not available since they lead to the decomposition of the quinhydrone. The potential of the quinhydrone,electrode in O - I N H2S0, is 0.032 volts less than that of the Hg-Hg2S0, in zN H2S04 electrode generally recom- mended for electro-analytical work.

The Electrical Apparatus used. The layout of the electrical apparatus is shown in Figs. 4 and 5. A

considerable use is made of the better class of radio apparatus the compact design of which greatly facilitates the enclosure of the parts most vulner- able to attack by the ‘‘ atmosphere ” of a chemical laboratory.

The usual type of single range voltmeter contains within the case a fixed resistance in series with the moving coil, and it is necessary to insert between them a tapping leading to a third terminal in order to adapt such an instrument for use in the circuits shown. Most multi-range instruments de- signed for use with external volt multipliers do not re- quire any alteration. Some multi-range v o 1 t m e t e r s fitted with enclosed resistors have terminals which in- clude only the moving coil in the circuit, but other patterns necessitate the ad- dition of an extra tapping.

On reference to Fig. 4 it will be seen that when V is used as a voltmeter to

FIG. 4. FIG. 5.

measure the potential between D and F a small current necessarily flows through VR, and this may be appreciable compared with the current flowing in AC when a high resistance radio pattern potentiometer is employed. Hence VR should remain in parallel with AC even when V is being employed to determine the null point. Theoretically the removal of V from the circuit DVRF will affect the potential between A and C, but with resistances of the order shown the change will be less than 0.01 volt.

This will no longer be true if a less sensitive voltmeter is used for which VR may perhaps be of the order of IOO to 300 ohms. In such a case the potentiometer should have a much lower resistance. Most radio pattern rheostats of about 30 ohms resistance are fitted only with two terminals, but it is an easy matter to add a connection to the free end of the resistor and though the potentiometer so formed is only capable of altering the voltage by a series of steps nevertheless these steps are small enough to admit of the adjustment of the potential to within 0.01 to 0.02 volt. The manipulation of the apparatus will, however, be greatly facilitated if two rheostats are placed in series, one having a resistance of about a quarter of that of the other, and serving as a fine adjustment. During the course of an estimation it is usual to allow the cathode potential to rise by

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2 24 MODIFICATIONS OF THE SAND AUXILIARY ELECTRODE

about 0.2 to 0.3 volt, and such a change will be brought about by rotating the knob of the fine adjustment through about 180~.

A second modification which becomes necessary when the resistance of VR is relatively small arises from the fact that the resistance of V is no longer negligible in comparison with that of VR. In these circumstances a switch S2 should be introduced which throws into the circuit a resistance R2 equal to that of V when the latter is removed from the circuit by means of switch S,. In practice, both S , and S, are incorporated in one double

pole double throw switch preferably of the panel mounting tY Pe.

If at the commencement of an estimation the voltmeter is switched into circuit as a null point detector before the main current is roughly adjusted, the pointer will be driven hard against the left-hand stop which should therefore be

FIG. 6. of a resilient nature. If a Gery sensitive instrument is employed it is advantageous for the stop to short circuit the moving coil.

It is obviously advantageous to enclose all switch and rheostat contacts, and in the actual apparatus described the control-box included not only the switch just mentioned and the potentiometer resistors, but also the main circuit switch and rheo~tat .~ The latter must be capable of carrying from 3 to 4 amperes at the commencement of an estimation and the ordinary radio patterns are too light to serve for this purpose. On the other hand a 15 or 2 0 ohms resistance of the ordinary laboratory type entirely wound with wire capable of carying 3 to 4 amperes is unnecessarily cumbrous, since heavy duty wire is only essential up to a value of 3 or 4 ohms.

mla 1.8

1'2

0.6

0 0 6 9 I2 I5

Time in minutes; FIG. 7.

To combine compactness with adequate current carrying capacity it is desirable either to use a rheostat consisting of sections wound with wire of several different thicknes~es,~ or alternatively, to arrange that two or

4When the main circuit rheostat is enclosed in a box it is desirable that the latter should be provided with a door or Aap which can be opened during estimation t o ventilate the rheostat and closed when the apparatus is not in use. I t is advantageous to include in the box a small vessel containing soda lime to absorb acid fumes.

J A very satisfactory form of rheostat was made by adding two coils C and D (Fig. 6 ) to a commercial radio pattern dual rheostat consisting of a rotating copper arm moving over two coils A and B. A longer contact arm must of course be fitted, and a portion of the wire on the existing coil B must be removed to make room for the stop I;.

3

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T. B. SMITH

more resistors may be put in parallel for low resistances or in series for high.

Summary. (i) By the use of a porous diaphragm in place of the special tap of the

Sand electrode, and by modifying the design of the tube, it has been found possible to reduce the working resistance of the auxiliary electrode to about one-tenth of the usual value. This enables the potentiometer voltmeter to function also as a null point detector without causing any appreciable error on account of concentration polarisation of the half-cell.

(ii) A second modification is described in which the use of either taps or diaphragms in the electrical circuit is avoided. I t has about the same resistance as the first pattern but permits of the use of connecting liquids of higher conductivity than usual. This brings the null point detection within the range of a voltmeter of relatively low sensitivity. For most purposes this modification is to be preferred, but the diaphragm pattern has the advantage that it may be used for a considerable time without refilling with electrolyte.

(iii) The employment of a voltmeter for the detection of the null point is more convenient than the use of a capillary electrometer and the cost of the latter is saved.

(iv) The second modification can conveniently be employed as a quinhydrone electrode, the use of which has been shown to be effective in, the control of bismuth estimations.

(v) Arrangements are described for utilising, and, where necessary, modifying panel-mounting radio apparatus so as to permit of the enclosure of the parts most vulnerable to attack by acid fumes.

6 Such an arrangement is commercially obtainable, the two resistors being operated by a single control knob. This requires less alteration to adapt it to the present purposes, but it is unfortunate that the current carrying capacity is barely sufficient. The coils in parallel will certainly not take 3 amperes for any appreciable length of time, but it is nevertheless possible to utilise this rheostat for an estimation such as that of bismuth in which the current of the order of 3 amperes only persists for about a minute, rapidly falling in the course of a few minutes to less than an ampere. Obviously it would also serve for the estimation of nickel in the presence of zinc, since in this instance the current is never required to exceed I ampere.

T A e University, Shefie Zd.

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