The analysis of heavy water by infra-red spectrometry

Spectrochimica Acta, 1956,Vol. 8, pp. 57 to 65. Pergamon PressLtd., London

The analysis of heavy water by ix&a-red spectrometry J. GAUNT United Kingdom Atomic Energy Authority, Atomic Energy Research Establishment, Harwell, Didcot, Be&s (Received

15 March 1966)

AI&&-Methods are described for the analysis of the deuterium content of heavy water in theconcentrationranges 50% DsO, 3% D,O, 0.2% DsO, and the naturalrange. The accuracy of determination rises from f2% in the natural range to hO.O5o/o in the 50% range. The measurements are made directly on the samples without any previous dilution or enrichment, and in consequence the methods developed are suitable for application to continuously flowing plant streams. Instrumentation for the laboratory and the plant is discussed.

liltJ!OdUCtiOIl

IT has aheady been shown in an earlier paper [l] that the deuterium content of heavy water in the concentration range 99+--100~o W/W D,O can be estimated with speed and accuracy by means of an infra-red spectrometric technique, and in addition it was also shown that the low concentration range from O-l y. W/ W’ D,O could be covered by a similar technique. In the production of heavy water it is necessary to monitor over intermedmte concentration ranges, and hence suitable techniques have to be developed. Such ranges are likely to-be- as follows: (a) ca. 50% D,O (b) ca. 3% DQO (0)

c&. 0.2 o/o DQO

(d) Natural rsnge

with increased accuracy over that previously obtained.

In each csse en accuracy of measurement of f 1y0 of the armlytical figure is desirable. The work described in the following sections represents an attempt to fulfil such anslytical requirements.

Discussion of methods In the production of heavy water it would be desirable to have a series of instruments which could not only monitor the product from the various stages of the process but which could also be used as controllers. Such instruments must be rapid in response to be effective, and it is this factor beyond ill others which makes the infra-red technique highly desirable. In addition to the requirement for flow analysis, however, instruments are required in the control laboratory. Although such instruments do not require the robust construction and long-term stability as plant instruments, the preliminary investigations which are reported here have been

J. GAUNT made with the object of providing laboratory analytical techniques, at the same time bearing in mind the possible extension of these techniques to plant-control. For this reason every effort has been made to use samples of heavy water in the form in which they will be available from the plant. Better methods could probably have been devised by diluting or enriching the samples, as requked, to bring them all to a specific concentration range; such techniques, however, would be extremely difficult to carry out in a continuous-flow stream, and were ignored. It was also anticipated at the outset of this work that some difficulty would be experienced in continuously flowing representative samples from a bypass through very thin cells. Consequently, every effort has been made to use cells as thick as possible. One point of importance which has been observed is the effect of temperature on the absorption coefficient of H,O/D,O mixtures. This was first reported by TRENNER and WALKER [Z], and although in the present author’s previous work this factor was ignored, this was because all the samples used and the spectrometer employed were housed in a thermostatically controlled laboratory. In the present work, however, this has not been the case, and the day-to-day temperature of the As a consequence, the effect noticed by TRENNER and laboratory has varied. WALKER has been confirmed. It has, however, also been noticed that the effect of temperature decreases as the deuterium content of the heavy water increases, and samples of heavy water in the high concentration range are not noticeably affected Because of the fluctuating temperature, it by quite large temperature variations. would have been desirable to use thermostatted cells for this work; but as these were not available, the technique employed has in every case been one in which a complete calibration curve has been drawn up each time a sample was estimated. Although this may seem tedious, it shows very clearly the rapidity of the infra-red method, since it takes only 30 min to draw up a complete calibration curve using five or six standard samples and carry out the estimation of an unknown sample. During this relatively short period, the temperature changes were negligible. Although in some types of analysis the infra-red method can be made absolute, in the analysis of heavy water this is not the case, and a calibration curve must be drawn up from known standards. The technique involves observing changes in the absorption of a band characteristic of the material to be analysed. The choice of a suitable band is, however, very important and is complicated to some extent in heavy water by the equilibrium which is rapidly established when pure D,O and pure H,O are mixed. H,O

+ D,O + 2HOD

If x g D,O are mixed with (100 - x) g H,O, the resultant mixture is defined as containing x% W/W D,O. Although this “nominal” figure is often used to define heavy-water samples, the real conditions prevailing in the mixture are rather different from this, and in order to appreciate the problem more fully, calculation of the relative concentrations of the three species is desirable. The equilibrium constant for the reaction is given by [HODI K = LH20] LD20] = 3.80 at 25’C

58

The analysis of heavy water by infre-red

From the experimental

data ( W/W%),

the mole per cent D may be obtained when

[HOD1 + VW1 2[HODl + VW1 + VW1 = [HOD]

and

+ [H,O]

spectrometry

+ [D,O]

mole per cent D 100

= 100

The solution of these equations gives the mole per cent of each component. Typical examples of this are given in Table 1 for the region around 50% W/W. Table 1. Relationship

of H,O,

D,O, and HOD in heavy water. 50% W/W Range

% D,O

T HOD Mole y.

Mole y. D

w/w

(nominal)

56.00 54.00 52.63 52.00 50.00 48.00

49.13 49.32 49.36 49.35 49.22 48.94

53.39 51.37 50.00 49.37 47.37 45.38

i

w/w %

Mole y.

48.95 49.25 49.36 49.38 49.35 49.18

28.82 26.71 25.32 24.76 22.76 20.91

-

30

GO

w/w

Mole y.

%

30.23 28.07 26.65 26.02 24.03 22.12

-

w/w %

22.05 23.97 25.32 25.95 28.02 30.15

-

20.82 22.68 23.99 24.60 26.62 28.70

-

Similar calculations show that a solution which is nominally 3.0% W/W D,O comprises 94.377% H,O, O.O85o/0 D,O, and 5.538% HOD, by weight. When the deuterium concentration is less than this, the solution contains essentially only the two species H,O and HOD, and similarly when greater than 97% D,O, only D,O and HOD. It is thus obvious that just as measurements based on an HOD absorption band at 2.95 p gave the greatest sensitivity in the 99-lOOoh W/W Da0 range [l], so the maximum sensitivity will be achieved by making measurements on an HOD absorption band for the 3%, 0*2%, and natural concentration regions. From Table 1, it is clear that attempts to make measurements on HOD absorption bands will lead to little success in the 48-56% W/W D,O range, since over this concentration range the HOD concentration is practically constant. On the other hand, both the H,O and D,O concentrations vary quite markedly over this range, and measurements on either of these species should give the desired results. Details of the methods are described below. 50%

W/W D,O Range

In this range, the concentration of all three components of the mixture is high, and it consequently becomes necessary to use a weak absorption band in order to As a fairly high precision is required over a small obtain measurable intensities. range of concentration, a differential comparison technique is indicated. The most suitable band for this concentration range was found to be the H,O absorption centred at l-445 ,u, which is a weak combination band (vl + vs) and is not obscured 59

J. GAUNT

by overlapping HOD and D,O bands. An initial survey on a Hilger HSOO doublebeam spectrometer showed that a &mm glass cell filled with 50% W/W D,O transmitted about 2 y0 at 1.445 ,u. This transmission is adequate at this wavelength and the following technique wss for carrying out accurate measurements, worked out. Two &mm glass cells were filled with 51.93% W/W D,O and placed at the foci ,of the two beams in the spectrometer, the slits were opened to 1 mm, and the reference beam trimmed to give about 10% absorption at the peak wavelength. The absorption band was then scanned from 1.39 to l-47 ,u, and the maximum absorption recorded. This was repeated, using different samples of D,O in the sample cell but maintaining the reference sample at the focus of the reference beam throughout the investigation. In this way a calibration curve was obtained, the relevant figures being given in Table 2. Table 2. Calibration of spectrometer for 50%

W/W

D,O region Meen value

Recorder reading mV at 1.445 /I

% WIW

%O 8.74 8.37 7.93 6.90 6.25 5.53

51.93 51.55 50.71 49.74 48.76 47.81

I

8.80 8.45 7.80 7.05 6.25 5.54

8.70 8.47 7.80 7.11 6.27 5.54

8.90 8.50 7.83 7.00 6.17 5.55

I

I

I. I

8.76 8.44 7.81 6.98 6.21 5.52

845 8.43 7.69 6.84 6.12 5.42

I

The samples of heavy water used for this calibration were made up by direct weighing of D,O and H,O, and were kept and analysed in a thermostatically controlled room. In order to test the repeatability of the method, a fresh sample of heavy water was made up by weighing, and analysed as though it was an unknown. The figures are given in Table 3. Table 3. Analysis of “unknown” sample (Real value = 51.74% W/W D,O) Recorder mV

% W/W D,O

I

8.68 8.68 8.66 8.62 8.58

~ 51’70 51.70 1 j Mean = 51.73 5 0.06

8.62. 8.52 8.52 8.63

60

The analysis of heavy water by infra-red spectrometry

As an alternative to the double-beam differential method, just described, this analysis can also be carried out on a single-beam spectrometer. A Harwell Grating spectrometer [4] was used, employing a lead sulphide cell as detector and a 2400 line-per-inch grating in the second order (blazed for maximum intensity at 3 ,u in the first order). The filter for eliminating overlapping orders was an additively coloured alkali halide crystal of the type described by BURNS and GAUNT [5], the transmission starting at 1.2 ,u. The results obtained were *comparable to those outlined above. Although the changes in the absorption coefficient due to changes in temperature are small in this region, it is nevertheless still desirable to use thermostatted cells to achieve maximum repeatability. No special precautions were found to be necessary in handling samples of this concentration, and they were treated throughout in a straightforward manner. 3% W/W Da0 Range It has already been shown [l] that the range O-l y. W/W D,O can be covered by measurements on the O-D fundamental vibration band of HOD at 3.98 ,u, using a 0*25-mm cell. TRENNER and WALKER [2] have also shown that measurements can be made on samples of concentrations up to 5 o/o W/W D,O, using thinner cells. Because of the high concentration of H,O in samples in this concentration range, it was not possible to make measurements with any accuracy on the HOD band at 1.6 ,u, whilst the HOD band at 2.95 p was totally obscured by the H,O fundamental occurring at the same wavelength. Attempts were, therefore, made to improve the accuracy of measurements on the 3.98-p band of HOD. One of the difficulties encountered when working at this wavelength arises from the absorption of Ha0 at approximately 3.6 ,u. This band is normally weak, but the high concentration of H,O in the samples develops it into a strong absorption whose wings badly overlap the 3.98-p HOD absorption band. As a consequence, the sensitivity of measurements on HOD at 3.98 p is reduced because of the limitation of cell thickness imposed by this high-background absorption. Two alternative methods of attack were tried, both being single-beam methods. For this work, the Harwell Analytical Grating spectrometer was used, employing a filter which started to transmit at 2.2 p. 2400-line/inch grating with an “F-centre” A Hilger-Schwarz SW thermocouple was used as detector with a 300 : 1 matching transformer and Grubb-Parsons lo-c/s amplifier, the output being fed into a lo-mV Brown recorder. The slits were set at 1-Omm and the grating set to give 3.98 ,u in the first order. The amplifier was set to give the maximum workable gain (i.e. maximum tolerable noise). Under these conditions, a 0*25-mm cell containing 3% W/W D,O gave zero transmission and no stray light was detectable. Two alternative techniques were examined. (1) A standard technique in which various cells were tried, to find the conditions under which a 3% W/W Da0 mixture would give rise to a full-scale deflection on the recorder, followed up by sensitivity tests over the range 3*O-3*4o/o W/W D,O in that cell. (2) A technique in which a thinner cell than in (1) was used such that when filled with 3.0% W/W Da0 it would give rise to an output from the amplifier 61

J. GATJNT

equal to (x + 10) mV, and to apply a backing-off potential of x mV so that a full-scale deflection of the recorder was obtained, followed up by sensitivity tests over the range 3*0-3*40/O W/W D,O in the same cell, maintaining a constant back-potential. Because of the background absorption due to H,O, there is an optimum cell thickness for this range, and this was found to be 0.07 mm for the second technique, which was found to give much greater sensitivity than the more conventional method [l]; the cell windows were synthetic sapphires. It was found that measurements in this range were much more influenced by temperature fluctuations than in the 50% range, and it was not found possible to repeat absolute measurements from day to day. In the absence of thermostatted cells, this difficulty was overcome by making observations on a number of standard samples at the same time as the unknown sample and drawing up a fresh calibration curve for each set of readings. Although the absolute readings could not be repeated under these condit&s, the total number of points can be placed on a single calibration curve by means of normalizing to standard values. This is shown for five sets in Table 4. Table

%W/WD,O

A

4. Calibration points for 3% W/W (Normalized readings (mV)) ~

D,O

B

~

C

D

9.14 6.78 4.95 3.35 5.44

I

9.02 6.78 4.96 3.32 5.53

9.19 6.86 4.99 3.31 5.40

range

E

Mean

9.05 6.85 4.90 3.40 5.50

9.08 6.81 4.96 3.36 -

I

8.98 6.76 4.98 3.41 5.57

3.000 3.146 3.274 3.404 “Unknown”

)

~ i I

E%Ling-off

potential

=

i

10 mV

I A calibration curve was drawn “unknown” values were obtained.

Series

Normalized reading

A B c D E

The real value

5.57 5.44 5.53 5.40 G.50

through

these mean points

from

% D,O

3.23, 3.24, 3.23, 3.24, 3.23,

of the “unknown”

62

3.23, & 0.00,

sample was 3.23,%

W/W D,O.

which the

The analysis of heavy water by infrs-red spectrometry

O-2 y. W/W II,0

Range and samurai Range

Because of the low deuterium content of samples in the 0.2% range and the natural range, thicker cells than were needed for the 3 o/0 range must be used, and in consequence those factors which influence measurements in the higher range become even more pronounced. By experiment it was found that the identical spectrometric :conditions for the 3% range were required for the 0.2% and the natural ranges, with the exception of the cells. For the 0*2% range a 04%mm sapphire cell was required, and for the natural range a 0.25-mm sapphire cell was used, the techniques being identical in both cases with the technique used in the 3% range. Here again the influence of temperature on the absorption coef&ient of the samples was large, and the day-to-day variations were such that the absolute measurements could not be repeated. It was again found convenient to draw up a separate calibration curve for each unknown sample analysed, but all the points recorded can be placed on one curve, as was done for the 3% range. The results of these investigations are shown in Tables 5 and 6. Table 5. Calibration points for 0.2%

%WlW / D,O

“Unknown”

II,0

range (normalized readings (mV))

T I

A

/

--

I 0.186 0.221 O-251

W/W

/

8.62 5.11 2.51

8.61 5.24 2.55

1

-

-

__~__

~ 8*28 ! I

5.31 2.71

-

/

I

j

5.09 2.52

j

5.82

8.65 5.20 2.43

! 4.87

9.20

5.55

5.31

2.26

8.05 5.21 2.88

2.92

5.77

_ 5.72

8.48 5.16 2.60 -

-

I Racking-off potential = 25.5 mV

From the calibration

curve based on these points, the unknown

Series

D

E F c

H

was estimated.

% D,O

5.82 5.55 5.31 5.77 5.72

0.214 1 0.217 0.219 0.214 0.215

The real value of this “unknown” was 0217~o

Mean = 0.216

W/W

f

0.002

D,O

Finally, an attempt has been made to improve the sensitivity of measurement in the very low concentration range, from the natural concentration (O-0167 o/o The technique has already been described, W/W D,O) to 0.07% W/W D,O. and the results are tabulated below. 63

J. GAUNT Table

6. Calibration

% W/W%0

A

0.0167 0.0234 0.0301 0.0434 0.0667 0.0762

9.4 9.1 8.7 7.1 4.6

“Unknown”

8.0

points for natural

-

c D

E .F G

readir

3mV))

-

-

-

10.1 9.3 8.7 6.9 3.9

From the calibration

I3

Normalized

G

9.4 8.6 6.9 -

7.9

9.1 8.6 7.1 -

2.4

2.5

9.7 9.2 8.6 6.9 4.3 2.4

7.9

7.6

7.8

-

9.3 8.7 6.9

potential

=

30.0 mV

curve based on these points, the unknown

I j Normalized : reading

Mean

-

Backing-off

A

t:

-I

I

Series

range

/

8.0 7.9 7.9 7.9 7.9 7.6 7.8

The real value of the “unknown”

was estimated.

% 30

0.0355 0.0365 0.0365 0.0365 0.0365 0.0390 0.0375

Mean = 0.0368 & 0.0008

was 0.03670/6W/W D,O

Discussion of instruments It is obvious from the results of the investigations described in the previous sections that analytical control of the product from the various stages of a heavywater production plant can be achieved in the laboratory with a fairly high degree of accuracy by means of an infra-red technique. The methods devised are rapid, and require only a small quantity of material (ca. 3 ml) for each estimation. There is little doubt that the repeatability and accuracy of measurements can be improved by temperature control of the samples and absorption cells. Although analyses of the type described in this paper, and in the previous one relating to the high concentration range, can be carried out by means of a conventional infra-red spectrometer, to do so would mean th.e introduction of a number of unnecessary complications to the techniques and would immobilize an otherwise versatile and elaborate instrument. The simplest form of analytical device based on infra-red spectrometry is one which consists of a source, narrow band filter, and detector. Filters of this type are not readily available for most purposes, although it is possible under some circumstances to devise means for 64

The analysis of heavy water by infra-red spectrometry

doing this. In the case of the analytical problems in question, suitable filters have not been found, and in consequence some alternative must be sought. This requirement is fulfilled to some extent by the analytical spectrometer described previously [4]. Although this instrument is capable of a degree of resolution somewhat better than conventional prism spectrometers, it was originally designed as a simple monochromator to serve the function of a narrow-band filter, and, in the author’s opinion, is best used in that capacity. Instruments of this type have been in operation at Harwell and elsewhere for some considerable time and have proved to be both accurate and reliable. In order to obtain precision results, it is probably better to design a special instrument for each purpose than to use one instrument to cover a wide variety of problems. Therefore, to cover the five ranges of heavy water, as described, on a routine basis, a minimum of three laboratory instruments would be required. (a) 99% Range An instrument set at 2.946 p equipped with PbS cell and 800-c/s amplifier, with 0.2%mm silica cells. (b) 50% Range An instrument set at I.445 ,D(second order), equipped with PbS cell and 800-c/s amplifier, with 5-mm glass cells. (c) 3 %, 0.2 %, an,d Natural Ranges An instrument set at 3.98 ,LL,equipped wit,h thermocouple and lo-c/s amplifier, using 0*07-mm, 0*19-mm, and 0.25mm cells with windows of calcium fluoride or synthetic sapphire. Although instruments of the type envisaged are essentially single-beam devices, the cell-in/cell-out technique employed, in which a “grey” standard is compared against the sa,mple, gives results which are highly reproducible, because the short-period instability of the equipment is very small. If long-term stability is required, it is much better to use self-compensating double-beam devices. It is only within very recent years that attempts have been made to meet the very rigid requirements of stable plant-control equipment for liquid flow analysis, and this work has been almost exclusively the prerogative of the U.S.A. At present, two general types of equipment are available. In one case a dispersive system is used [6], and the other is of a nondispersive type [7]. This latter type has already been sensitized for use in the analysis of heavy water in the 99-100% range, and preliminary reports are very encouraging. Modified forms of this nondispersive type of instrument could probably be used for the other ranges of heavy-water concentration, as described in this present paper. 111 GAUNT J. Ii’&eAnalyst 1954 79 580.

References

PI TRENNER N. R. and WALKER R. W. Perkin-Elmer

News 1952, Vol. 4, No. 1.

r31 KIRSCHENBAUM I. Physical Properties and Analysis of Heavy Water. McGraw Hill, New York,

1951.

[41 GAUNT J. J. Sci. Inst. 1954 3 315. [51 GAUNT J. and BURNS W. G. Institute of Petroleum-Conference Fl [71

on Molecular spectroscopy, 1954, p. 66. WOODHULL E. H., SIEC+LERE. H., and SOBCOV H. Ind. Eng. Chem. 1954 46 1396. SAVITZKY A. and BRESKV D. R. Id Eng. Chem. 1954 46 1382. 65