Indian Journal of Engineering & Materials SciencesVol. 5, October 1998, pp. 278-284

Magnetic float densitometer - A modified versionR N Pathak & Indu Saxena

Department of Chemistry, Lucknow University, Lucknow 226 007, India

Received 9 March 1998; accepted 9 July 1998

Four major modifications have been introduced in the magnetic float densitometer of F J Millero'sDesign. Namely, Pt-point in place of blue fused glass at the lower end of the float; light fucusingarrangement to add in view instead of using magnifying glass only; modified electrical circuitry andintroduction of ferrite core inside the cylinder to increase the efficiency of the coils. These modificationshave improved the working of the float considerably. Operation and taking observations have becomesimpler.

There are several methods to measure the densitiesof liquids and salt solutions such as by densitybottle, pyknometer, dilatometer etc. None of thesemethods give very accurate density values ofsolutions, at least, in the very dilute solution range.The reasons being that these methods have threefundamental drawbacks. In the first place it isdifficult to secure a constant and definitetemperature inside a considerable volume ofunstirred liquid. Secondly, because of variations inthe humidity of their surfaces, glass vessels arealways difficult to weigh with the utmost accuracy.Thirdly, the apparatus, with its contents, puts aconsiderable load upon the balance arm and soreduces the sensitivity of balance. All these threeunfavourable effects increase with increasing sizeof the apparatus and hence it is useless to attemptto get the accurate values with a large apparatus.

In 1894 Kohlrausch and Hallwach 1 gavedisplacement method to measure the densities ofliquids and salt solutions. Hallwach's refinementof the displacement method escapes these threeobjections. Unfortunately a fresh difficulty ofpronounced surface tension at liquid gas surfacearose. Kohlrauch and Hallwasch/ reduced thissurface tension effect by depositing platinum blackon the suspension wire. The surface tension effectof the suspension wire was reduced but notcompletely eliminated. But this method gave muchbetter results as compared to other ordinarymethods.

The submerged sinker method of Pisati and

Reggiani' had no suspension wire and so had noproblem of surface tension at liquid gas surface.The submerged sinker method is thus superior tothe displacement method. Its one drawback lies inthe difficulty of varying the buoyancy of the sinkerby small amounts. If this variation in buoyancy isto be secured by the addition or removal of, ..weights, then it is necessary to use weights assmall as hundredth of a milligram which isevidently impractical. This difficulty was over-come by Lamb and Lee4 by using externalelectromagnetic attraction in vertical direction byusing electric current outside the bulb of the sinker,in which a soft iron core was placed. Various otherworkers'" have modified this method by using amagnetic core inside the float; however, inprinciple, the method being the same. Miller09

further improved the method and made it moresensitive and modest in cost. We have furtherimproved Millero's model of densitometer bymaking four changes in his design. Thesemodifications are: blue fused glass has beenreplaced by a sharp platinum point at the bottom ofthe float; light focusing arrangement has beenintroduced to add in view of the platinum pointinstead of ~sing magnifying glass only; a modifiedelectrical circuitry has been incorporated in thedesign and lastly Ferrite core inside the solidcylinder concentric to the coils and cylinder itselfhas been used. These changes have improved theworking and efficiency of the instrumentconsiderably and enabled the observer to use it

PATHAK & SAXENA: MAGNETIC FLOAT DENSITOMETER 279

conveniently. The details of the principle involvedand different components used therein, are beinggiven here.

Magnetic Float DensitometerThis densitometer has been designed to measure

the densities of various salt-solutions in aqueousand few non-aqueous solvent media. Thedescription of the design and improvement madetherein is divided into four categories (i)description of magnetic float densitometer; (ii)circuit diagrams; (iii) working and procedure and(iv) calibration and measurement of densities ofsolutions.

DescriptionThe basic design and different components used

in the densitometer are almost the same as thatgiven earlier by Millero9 except few changes whichare needed for the modification.

The float is made of Pyrex glass. A permanentmagnet is introduced before sealing it. Blue glassused at the lower end of the float by Millero wasnot giving clear visibility of the field so it isreplaced by a fine platinum point. With the aid oflight focusing arrangement, this platinum wirehelps in ascertaining the position of the floet whenit touches the bottom. To make the float fit forrunning in the solutions of various density values,few Pb-shots were added to adjust the weight ofthe float so that a critical density value(W=77.4240g and p = 0.94g/mL for our purpose)was obtained for adjusting the buoyancy force ofthe solution.

The solution container was also made of Pyrexglass. The design of the container is so chosen asto make the float to move freely up and downinside it. The top of the container has a glass coverfitted with a stop cock.

The support and levelling platform are made ofbrass. The support has a hole drilled into it to holdthe solution container. The bottom of the hole hasa cross-window for viewing the platinum point ofthe float, when it touches the bottom of thesolution container. Two windings made ofenameled Cu wire are provided in brass supportunder the cross window. One winding which isnear the cross-window works as pull downsolenoid and other, just little downward, works asmain solenoid. To increase the efficiency of the

coil a ferrite core is also introduced at the center ofthe brass support. The upper part of this ferritecore is surrounded by pull down solenoid andlower, by the main solenoid (Fig. I). The ferritecore concentrates the magnetic lines of force at thecenter and thus provides sufficient magneticattraction on the float. If only coils were used, asdone by Millero, very weak attraction force wasavailable which was not too sufficient to controlthe motion of the float properly.

A telescope is used to view the movement of thefloat when its platinum point touches the bottom ofthe solution container. To assist in observing theplatinum point by telescope, light focusing deviceopposite the telescope is used. A focussed light ismade to fall on the glass window of thedensitometer with its help. This arrangement was.not provided earlier in Millero's model.

Circuit diagram (modified electric circuitry)The whole circuit diagram is divided into three

sections, namely, (i) Selection of number of turnsof main solenoid section, (ii) Operation of the coilssection and (iii) Battery ON/OFF section. Eachsection consists of six pin-push buttons. Thebattery section uses a set of two push buttons whilethe other two sections; each uses a set of threepush buttons.

Selection of number of turns in the main solenoidFor changing the range and sensitivity of the

instrument, the provision of changing the numberof turns in the main solenoid is given. For this, theselection of 100, 200 or 700 turns can be done by

WINDOW FOR VIEWINC,PLATINUM POINT

] PULL DOWNSOLENOID LEADS

PULL OOWN -;~ •• ~iiiiE~~SOLENOID

~t~~NOID -----

FERRITE CORE __ +-_ ••••]

MAIN SOLENOIDLEADS

SUPPORT

PLATFORM

fig. I-Sectional view of support showing positions of pulldown solenoid and main solenoid

280 INDIAN J. ENG. MATER. SCI., OCTOBER 1998

1.5V And 12'1801l.ry ON IOFFS.ctlon

Joint OCMI'Glionofmoln and pull downsol.noid

s.I.ction of no·of l."rIa of moinsol •••oId

•..•.YELLOW

f:s-UIo

GREEN jII..•.

REO~----------~K~~------~ ~~§:

~o-g-BLACK II~------------------------~~------~~~

Fig. 2--Complete electrical circuit of magnetic floatdensitometer showing first stage of operation (not given)

1·5 V and 12 V Joint ap.rat ion ofBal1.ry ON IOFF moin and Pull downS.ction sol.noid

Two I2Y baU..-i"in porll.t

1:"5·O'oGREEN -~--~------~--~~0.

S.1.ct ion of No.

of turns of mainsol.noid

1-

YELLDW

'-- ~K~&~U~E~----~- ~~i;;:

aLACK !!. i'----------------------------------~ Q. ~

REO

Fig. 3--Complete electrical circuit of magnetic floatdensitometer showing second stage of operation (not given)

merely using a push button of this section. Ifbutton no. 1 is pressed down, it will connect theinstrument to 100 turns, while others remaindisconnected. Similarly button no. 2, if presseddown, connects the instrument to 200 turns, whileothers remain disconnected. In the presentdiagram, the selection of 700 turns is shown bypush button no. 3 pressed downward, others aredisconnected.

The resistance bridge used to change the currentin main solenoid has been inserted in the main linebetween main solenoid and push buttons. The

values of the components used in the bridge aresame as that used earlier by Millero".

Joint operation of main and pull down solenoidThis section of electrical circuit provides the

joint operation of the pull down and main solenoid.The section comprises of a set of three pushbuttons. Button no. 1, if pressed down, makes thepull down solenoid. 'ON' (shown in Fig. 2 but notgiven). Button no. 2, if pressed down, makes boththe coils 'ON' (shown in Fig. 3 but not given)while button no. 3, if pressed down, disconnects

PATHAK & SAXENA: MAGNETIC FLOAT DENSITOMETER

1.5V AND 12 IIBATTERY ON/OFFSECTION

JOINT OPERATION OFMAIN AND PULL DOWNSOLENOID

•

•...

. --...--- .•.+_ ...•+ •

•

Two 12 II batt.,lcaIn paralic I

SELECTION OF NO.OF TURNS OF MAIN

SOLENOID

•

Fig. 4--Complete electrical circuit of magnetic floatdensitometer showing third stage of operation.

---fJ.lL:~+IL ~====~-(K-2--~:~~~:~-----------m~

~~~82~~ ~8~LA~C~K ~oz

the pull down solenoid and only the main solenoidremains 'ON' (Fig. 4). The observations are takenat this position only.

Battery ON/OFF sectionThis section consists of a set of two push

buttons. Push button no. 1, if pressed down, makesboth the batteries 'OFF'. While push button no. 2,if pressed down, makes both the batteries 'ON'.Thus observations are taken at push button no. 2pressed down. The voltage for pull down solenoidis supplied by a 1.5V Eveready 6G Dry cell and formain solenoid, by two 12 V Exide batteries used inparallel. The complete modified electric circuit canbe obtained by the addition of Figs 2-4 to Fig. 5through the corresponding coloured wires.

The laboratory set-up for operation of magneticfloat densitometer is shown in Fig. 5.

WorkingSuppose V and Ware the volume and weight of

the float and v 'and ware the volume and weight ofplatinum respectively. If the float with a platinumweight floats, completely immersed, in a solutionof density p then according to Archimedesprinciple, the volume of the solution displaced will

281

WHITE

I, K,)-----,""IIz IJ

YELLOW

J:J>Z

~r:'"GREEN Z00

be equal to the sum of the volume V of the floatand the volume v of the platinum weight, and theweight of the solution displaced will be equal tothe sum of the weight W of the float and w of theplatinum weight. That is,

Volume of the solutiondisplacedWeight of the solutiondisplaced

= V+v

= density x volume

=p(V+v)

Thereforep(V+v) = W+w ... (1)Now when current is passed in the main

solenoid, a vertical attractive electromagnetic forceis developed that pulls down the float to the bottomof the solution container. The current is adjusted insuch a manner that the platinum point attached tothe float touches the bottom of the solutioncontainer. The force exerted by the main solenoidis given by a constant times J, the current passingthrough it. Let this constant be represented-by landnamed as 'weight equivaJent of currel)$:'- Now theeffective weight of the float will be W+w+(fxl)and Eq. (1) becomes

p(V+v) = W+w+(fxl) ... (2)

282 INDIAN 1. ENG. MATER. scr., OCTOBER 1998

PULL 00_SOLENOlO IOUO

L \7 c~L~--~~~.:-:-;'"t~-?c~~~

LI6HT 'OCUSSI ••C.OEVICE

TELESCOPE

RED DENSITOMETER

~HlTEN ~

'"0'" "''''101 SOLENOIDWHITE

"' 51 •••••0 •••110IIESISTA ••CE

•••••INSOLENOiO .0•••"0

TWO tEN TUIINSPOT£NTIOMETEIIS

(0-100 a , 0-2oo0Sl)

WHlTf

Fig. 5-Laboratory set-up for operation of magnetic float densitometer.

If PPtis the density of platinum weight, v=w/pPt.Therefore

p(V+w/PPt)=W+w+(fxl) ... (3)

Before measuring the density of solution, thedensitometer is calibrated with pure water. For itscalibration, platinum weights are added to the floatso that it is just immersed in pure water. Anattractive magnetic force is then exerted by passingcurrent through the main solenoid' so that theplatinum point of the float touches the bottom ofthe solution container. This current is measuredand is named as 'hold down current'. Eq. (3) canbe written for water as

PH,O (V+w/pPt) = W+w+(fxl)

On rearranging

w( 1- PH,O /pPt) = (-fxl)+ PH,O V-W ... (4)

orw= -[f7(l- PH,O /pPt)] x /+( PH,O V-W)/(I-PH,O /PPt).

This equation now resembles with Y = mX + Cwhere

m= -j7(I-PHzo/PPt) =Slope of the line

and

C» (PHzO V-W)/( \- PH,O /pPt) = Intercept of theline on Y-axis

The system is calibrated at 25°C by measuringthe hold down current when various platinumweights are added to the float. A graph is thenplotted between the weights, wand thecorresponding hold down current, 1. Using theknown value of density of water and the density ofplatinum at 25°C, the value of the weightequivalent of current, 1, is calculated by using theslope of the line. Similarly using weight of the

PA THAK & SAXENA: MAGNETIC FLOAT DENSITOMETER 283

5·1Table l---Calibration of magnetic float densitometer in pure 5-0

water at 25°C4·9

Weights Voltage drop Current, I'" 1,-8added, w across one ohm mA ~

g. resistor, mV "0 4·7•"04.500 670 670 "8 4'6

::I!4.550 640 640 ~ 4-54.600 606 606 ••~ 1,·44.650 582 5824.700 560 560

1,·3

4.750 524 524 1,·2420 1,60 500 540 510 S20 SSO

4.800 496 496 Curr..". mA4.850 460 4604.900 436 436 Fig. 6---Calibration curve

float (W=77.4240g.) and intercept of the line, thevolume of the float Vat 25°C is calculated.

Now the densitometer is run in the solution ofunknown density. The observations are taken asbefore. Various weights and corresponding holddown currents are noted. The density of thesolution is, then, calculated by using Eq. (3) in theform

p=[W+w+(fxJ)]/(V+w/pPt) ... (5)

The weight equivalent of current of themagnetic float densitometer is very important indeciding the range and sensitivity of theinstrument. It can be varied by using variousnumber of turns of copper wire in the mainsolenoid, for which the provision of differentwindings 100, 200 and 700 turns has been made init. Thus by selecting proper value of number ofturns, the suitable value off can be selected whichprovides necessary range and sensitivity of thesystem. The main solenoid being little farther fromthe float, is not able to pull the float down. Toassist in bringing the float in the region ofmagnetic field of main solenoid, another coil, justa little above it, is used. This coil, called the pulldown solenoid, when switched 'ON' pulls the floatdown and brings it near another coil. Then themain solenoid takes control over the float and theaction of pull down solenoid is finished. Theattraction force exerted by the coils was very weakin Millero's model as he had used coils withoutany core. This could not give sufficientconcentration of magnetic lines of force at thecenter. Consequently the magnetic force ofattraction was weak. We have inserted a ferrite

core by drilling a hole into the brass support so thatit fits exactly at the center and is surrounded by thecoils. This system provides sufficient magneticattraction for float. Though this instrument issuitable to measure the density of the solutions atdifferent temperatures, we have set it formeasuring the densities at 25°C only for ourresearch work.

\ ProcedureThe magnetic float was first weighed. The

instrument was calibrated with double distilledwater at 25°C. For this, the water was taken in thesolution container. The system was kept in'Toshniwal constant temperature bath' to maintainthe equilibrium of water at 25±.Ol "C. The weightswere added to the float and the corresponding holddown current was measured by the method givenbelow.

The keys K I and K2 were kept closed. Thenumber of turns in the main solenoid was selectedby the top section of the circuit. We selected 700turns by pressing push button no. 3 down (see.Fig. 4). Both the batteries were switched on byoperating the battery section (Push button no. 2pressed down). Then the middle section of thecircuit was operated. This section controls theoperation of both the coils. By pressing buttonno. I , pull down solenoid is 'ON'. Push button no.2 makes both the coils 'ON' and push button no. 3breaks the circuit of pull down solenoid and onlythe main solenoid remains 'ON'. The observationswere taken at push button no. 3. The resistancebridge was adjusted by selecting proper values ofthe components so that the float just touched thebottom of the solution container. This was viewed

284 INDIAN J. ENG. MATER. SCl., OCTOBER 1998

by telescope and light focusing arrangement. Thereading of the voltage drop (in terms of millivolts)across one ohm resistor was taken by PhilipsMultimeter. This directly gave the magnitude ofthe current (in mA) flowing in the circuit,resistance being unity. Thus observations weretaken for various weights on the float and notingthe corresponding hold down current. Theobservations recorded are shown in Table 1.

Calibration and measurement of density of solutionA graph was plotted in weights, w, versus

current, I and shown in Fig. 6. The weightequivalent of current, J, was calculated from theslope of the line (m= - 1.724 glA) and volume ofthe float V was calculated by the intercept of theline on Y-axis as explained earlier. Here Pl>t

25, the

density of the platinum and PH10 25, the density ofwater at 25°C were taken from the literature'?'!' asPPl25 = 21.482 g/mL and PH10 25 = 0.99707 g/mL.The values of the weight equivalent of current andthe volume of the float were found out to be f =1.644 g/A and V = 82.365 mL at 25°C. Aftercalibrating the system, the density of any unknownsolution can be determined by Eq. (5) using thevalues off and V. The instrument was verified byknown standard solutions of sodium chloride. The

results were found to be in good agreement withthe values reported earlier9,12. The densities ofseveral aqueous and non-aqueous solutions oftetraalkylammonium iodide salts were alsodetermined using this technique.

AcknowledgementThanks are due to the Head of the Department

for providing Laboratory and research facilities.

References1 Kohlrausch F & Hallwach W, Wied Ann, 53 (1894) 14.2 Kohlrausch F & Hallwach W, WiedAnn, 56 (1895) 184.3 Pisati S & Reggiani N, Rend Reale Acadei Lincei, 14(7)

(1890) 19.4 Lamb A B & Lee R E, JAm Chem Soc, 35 (1913) 1666.5 Geffchen W, Beckmann Ch & Kruis A, Z Phys Chem

(Leipzig120B (1933) 398.6 Hall N F & Jones T 0, JAm Chem Soc. 58 (1936) 1915.7 Mac Innes D A, Dayhoof M °& Ray B R, Rev Sci Instr,

22 (1951) 642.8 Spedding F H, Pikal M J & Ayers B 0, J Phys Chem, 70

(1966) 2440.9 Millero F J, Rev Sci Instr, 38 (1967) 1441.

10 Hand Book of Chemistry and Physics, (Chemical RubberPublishing Co., Cleve Land Ohio), 35 (1953) 2066.

II Dorsey N E, Properties of Ordinary Water Substances,ACS, Monograph No. 81, (Reinhold PublishingCorporation, New York), 1940.

12 Vaslow F, J Phys Chem, 70 (1966)2286.