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An interference refractometer to measure the pressure and temperature dependence of the refractive index of liquids
Specrrochmca Am, Vol Prmted I” Great Br~tam
40A
No
8 pp 785-788
1984 0
INSTRUMENTATION
058468539184 $3 00 + 0 Ot! 1984 Pergamon Press Ltd
NOTE
AN INTERFERENCE REFRACTOMETER TO MEASURE THE PRESSURE AND TEMPERATURE DEPENDENCE OF THE REFRACTIVE INDEX OF LIQUIDS ROGER APPLEBY,* DAVID W JAmst *Royal
Radar
Estabhshment,
and CAROL A BOWIE~
Malvern, U K and tchemlstry Department, Brisbane, Q, 4067, Australia
Umverslty
of Queensland,
(Recerved 4 Aprzl 1983) Abstract-The design of an Interference refractometer, using laser radlatlon, to determme the pressure and temperature varlatlon of refractive Index 1s reported The method does not attempt to determine the absolute refractive indices The performance of the refractometer was tested usmg water and methanol The results for a series of solutions of KBr m water are also reported and the results are compared with previous emplrlcal estimates
INTRODUCTION
ylelded a parallel beam of -4 mm diameter The fringe pattern, expanded usmg a 75 mm focal length lens, was lmpmged on a slit z l/10 of a fringe m width and the signal was detected using a photo transistor placed directly behmd the sht The amplified output from the transistor was recorded on the second channel of the two channel recorder This faclhtated the counting of the change m the number of fringes with either pressure or temperature change Typlcal generated frmge patterns are shown m Fig 1
There have been a limited number of studies of the pressure and temperature dependence of refractive index m hqmds and this IS undoubtedly due m part to the mtrmslc difficulty of these measurements Two techniques have been described m the recent hterature, the first of which uses a Raylelgh Interference refractometer m which the pressure dependence was determined at pressures less than atmospherlc[l] for D,O and methanol The second method[2] utlhzed an mterference refractometer [3] which was placed m a pressure vessel In this study only the changes m refractive Index with temperature and pressure were determined for benzene, water and carbon tetrachlorlde In a series of studies utlhzmg hght scattering to examme the vahdlty of a hydrodynamic treatment [4] of the LANDAU-PLACZEC ratio [5] it IS found to be necessary to know the plezo-optic and thermo-optic coefficlents [ (dn/dPh and (dn/dT),] for a variety of electrolyte solutions We report herem the design and performance of an interference refractometer, utlhzmg a laser source, smtable for operation with pure hqulds and electrolyte solutions DESIGN
OF REFRACT’OMETER
The Interferometer which 1s slmllar to the design of WAXLER et al [2] was machmed from a cube of stamless steel 7 cm per side The optlcal chamber was 34 mm long and 5 mm m diameter The end windows were of glass, 20 mm m diameter and 5 mm thick and were wedged to prevent addItIona reflections The mslde surface was gold plated m the central 4; mm diameter The plated surface was separated from the stamless steel to prevent electrolytic attack The optical cavity could be evacuated through a port which could be sealed by a steel sphere The hqtud was introduced through a second port through which gas pressure could be applied via a mercury seal which prevented dlssolutlon of the gas m the solution Around the optlcal cavity, machmed from the same block, was a water Jacket through which water was circulated from a thermostatted bath A stainless steel Jacketed Cr/Al thermocouple was Inserted mto the block with the tip adjacent to the interferometer cavity The temperature could be mamtamed constant m the block to OOI’C The gas pressure transmltted to the cell via the mercury plug was momtored usmg a cahbrated pressure transducer, the output of which was recorded on one channel of a two channel recorder The light source was a 15 mW He/Ne laser operatmg at 632 nm with a standard three lens beam expander which
Fig
785
1 A typlcal ment
frmge recordmg for a (dn/dPh A, Pressure change, B, fringes
measure-
Instrumentation
786 EXPERIMENTAL
TECHNIQUES
The hqmd under study was degassed usmg a standard freeze/thaw technique under vacuum The refractometer was evacuated and mamtamed under vacuum (10-l mm Hg) untd no further degassmg of the cavity took place The refractometer was then closed off and the degassed hqmd was bled into the evacuated chamber A plug of mercury was introduced by syringe into an external glass U-tube to isolate the hquld from the an or the pressurlzmg gas (nitrogen) The filled refractometer was then set up m the laser beam with the wmdows normal to the beam and was connected to the water bath and the pressure manifold The temperature was eqmhbrated to 15 C and a pressure of 2 76 x lo5 Nrn-’ (40 p s 1) was apphed and the system was allowed to eqmhbrate The change m the number of fringes was recorded as the pressure was released through a fixed orifice over - 2 mm This process was repeated three or four times at each temperature with temperature intervals of 5’C being examined The same procedure was followed for a change m temperature with the change m frmges for a temperature change of 5’C bemg recorded With a change m pressure there was a small change m temperature and with a change m temperature there was a change m cavity length and both of these varlatlons necessitated correctronz to the calculated refractive mdex change These correctlon procedures will be described later The change m retractive index 1s related to the recorded frmge pattern by the relationship [2] An = F
2r
sm 0
where An IS thechange m refractive index, Ajls the number of fringes passmg a fixed mark, I 1sthe spacing between the plates of the Interferometer and 0 1s the angle of Incidence of the laser beam (= 90- m this study) Typically for a 5 C temperature change Aj was -80 while for a pressure change of 2 8 x lo5 Nm-* Aj was - 5 Initial refractive indices were either taken from the literature or measured usmg an Abbe refractometer CORRECTION
PROCEDURES
When the temperature is raised the cavity ot the mterferometer mcreases m length due to the thermal expansion of the stainless steel This expansion causes the recorded number of fringes to have a consistent error This error was estimated by three separate methods (1) using the coefficient of expansion tor the particular composition of stainless steel the magnitude of the cavity change was calculated and the correspondmg value for Af was estimated (11) the refractometer was operated under vacuum and the
Table 1 Varlatlon
LIquld
of the refractlon
number of fringes for a 5°C temperature change was measured at intervals between 15 and 6o’C (111)using water, for whtch dn/dT~s known accurately, the correction factor for Aj was estimated The three methods to Aj” as being gave agreement to - 1% for the correction +183K-’ When the pressure 1s changed on a hqmd there 1s a change m temperature Although the temperature control from the isothermal bath IS good It must be reahzed that a temperature change of 0 02’ gives an error m Aj of 0 5 and smce the value of Aj’- 5 this gives an error of 10% Attempts were made to vary the rate of pressure changeand extrapolate to a dP/dt of 0 but mstablhty at very slow pressure change prevented useful corrections usmg this method The maximum value for the change m temperature can be obtained from the coefficient of thermal expansion and the adiabatic compresslblhty 1e /dT\
P
In the experiment the change of temperature 1s reduced because of the heat flow from the solution which will be influenced by the thermal conductivity of the liquid and the rate of pressure change We elected then to use a constant rate of change of pressure (pressure release through a fixed ordice) and make an empmcal correction based on a cahbrant liquid In this case
For water the values of k showed a linear increase with temperature with a, fl and (K) kappa values taken from the literature [l, 6, 71 When the k value for water at 20°C was used for methanol at 20°C excellent agreement was found with the literature value We therefore used the k values determined for water for all other liquids RESULTS
The results obtamed for the thermo-optic coefficient (dn/dT), and plezo-optic coefficient (dn/dPb for the pure liquids water and methanol are shown m Table 1 and for aqueous solutions of potassium bromide are shown m Table 2 The variation of (dn/dT), with temperature for the various liquids 1s shown m Fig 2 together with the hterature values of water DISCUSSIOh For water the agreement with the pubhshed values [2] of the thermo-optic coefficient and the piezo-optic coefficient are excellent, m the latter case, this 1s because the pubhshed values are used to determine k, and the differences noted are because the values at different temperatures were fitted to a
dn/dP (Bar-‘) (x 106)
Water
15 20 30 40 50 55
0 93 (0 90) 1 04(0 98) 123(1 21) 142(1 47) 1 67 175
15 15(15 17) 14 79(14 84) 1447(1443) 14 10 1395 137
Methanol
15 20 25 30
4 4 4 4
45 O(41 0) (41 0(42 0) 35 7(43 4) 3 1 4(44 7)
ll(39) 29(3 8) 34(3 9) 42(3 9)
B
\dp) => a
index of water and methanol
dn/dT K-l (x 104)
Temperature (‘C)
Note
with temperature
B (K?‘) (x 105)
(Bar-‘) (x 105)
15 20 30 38 45 49
1 7 3 5 8 1
467 4 59 448 442 442 4 43
1274 123 1 125 9 125 1
1188 12 27 1269 130
and pressure
kappa (x
to,,
141 143 147 1 50 1 53 1 54 048
k
(x 109)
1805 20 45 25 07 29 63 34 02 36 27 1805 20 45 22 76 25 07
Instrumentation
Table 2 Temperature
and pressure
Temperature (YJ)
Concentration
Note
variation of the refractive water
787 index of solutions
dn/dT (x 10e4)
dn/dP (x 10-S) (error +0 14)
( x 404,
of KBr m
(X~&
072M
25 35 45 55
131 1 57 168 179
1 73(1 44)t 172 164 1 52
29 37 45 53
4 51
108 M
25 35 45 55
145 171 1 83 192
1 6(142) 1 53 151 149
33 38 45 49
4 36
239M
25 35 45 55
1 69 1 84 1 89 194
1 50(1 32) 145 142 142
37 41 48 47
405
463M
25 35 45 55
1 85 193 2 02 202
16(1 18) 1 56 148 1 51
44 44 46 46
3 35
*The hypersonic compresslblhty was used m this calculation and at present only the value at 25°C has been determined tNumbers m brackets are calculated according to Eqn 1
/
0
/
/
’
/ / /
/
’
1’
,
,q
/
/
methanol with mcreasmg temperature IS m accord with the associated nature of the liquid The temperature variation of the refractive index of electrolyte solutions has been studled by several workers m a non systematic way [9] but there IS httle agreement between the different studies The pressure varlatlon of refractive Index of solutions has not been reported although It has been proposed that this parameter may be calculated from the results for water [l, lo] usmg the formula
I-
. 0
/’
/
/
n-l
Temp (“Cl Fig 2 Varlatlon of (dn/dT)p with temperature -----KBr 072M, - - - -KBr 108 M, _ _ _ - KBr 463 M
-Water, KBr 2 39 M,
straight line The performance of the refractometer 1s however validated by the excellent agreement m the (dn/dT)p values For methanol the values of (dn/dT)p which we determine are consistently above the literature values whde the value of (dn/dP)p IS correct at 2O’C but the gradient with temperature 1s m the opposite sense to that previously reported In the previous study a pure reagent, as supplied, was used wlthout purlficatlon It IS now well known that methanol takes up a small amount of water rapidly and that m order to study the water-free compound a fractlonal dlstlllatlon from calcium chloride or magnesmm 1s recommended[8] It IS possible that traces of water m the methanol previously used caused the difference with the results we now report For water, an associated lrquld, the plezo-optic coefficient decreases with mcreasmg temperature while for the non associated hqulds benzene and carbon tetrachloride [ 1 21 It Increases The decrease In this coefficient for
= ApBe-O
(1)
where A = 0 339, B = 0 972, C = 7 07 x 1O-5 and p 1sthe density The apphcatlon of this method to solutions of KBr at 25°C IS included m Table 2 It may be seen that the formula predicts a steady decrease m (dn/dPb as the concentration of salt Increases What 1s experimentally observed at 25°C IS an mmal rapld rise m the coeficlent followed by a small decrease, at higher temperatures there IS agam a rapid mcrease to a value which stays almost constant as the concentration Increases The formula proposed, which describes the temperature varlatlon of (dn/dPhfor water, relates the change m refractive Index to the change m density The varlatlons m Ion-Ion- solvent mteractrons with concentration and temperature are obviously more complex than the varlatlons m the hydrogen-bonded network structure of pure water and the simple density relatIonshIp 1s inadequate to describe them It may be noted that the mitral rise In (dn/dPh followed by a decrease mnrors the behavlour of the Intensity of the depolarized Raylelgh scattermg from salt solutions which has been described In terms of varlatlons m Ion-ran-solute mteractlons [ 1 l] There has been a recent report of the measurement of the thermo-optic and plezo-optic coefliclents m pure hqulds usmg an mterferometrlc apparatus[12] In this report the errors caused by change of temperature with pressure change were not consldered For the organic hqmds reported the correction necessary to (dn/dP) would be quite small because of the small thermal conductlvlty of the liquid We feel, however, that many techmque m which only the movement of Interference fringes IS observed the posslblhty of varlatlons m dn$dP due to temperature changes should be consldered
788
Instrumentation
Acknowledgement-The Austrahan Research Grants Scheme IS thanked for salary grants for R A and C A B
REFERENCES [l]
E REISLERand H EISENBERG, J them Phys 43, 3875 (1965) [2] R M WAXLER,C E WEIR and H W SCHAMP,J Res nafn Bur Stand 68A, 489 (1964) [3] J B SAUNDERS, J Res natn Bur Stand 35,157 (1945) [4] R D MOUNTAINand J M DELJTCH, J them Ph~s 50, 1103 (1969)
Note
[5l L LANDAUand G PLACZEC,Z Smletunm 5, 172 (1934) [6] G S KEEL, J them Engng Data 20,97 (1975) [7] CRC Handbook, 51st Ed, p E 4, (1970) [S] Techmques ofOrganrc Chermstry, Vol 7, p 333 (edlted bv A WEISSBERGER) InterscIence. New York (1955) [9] I&matmml Crrtrchl Tables, Vol III McGraw Hill, New York (1930) [lo] A R MARETand E YEAGER,J acoust Sot Am 54,668 (1973) [ll] D W JAMESand R IRMER, J Raman Spectrosc In press [12] A M EUTYUSHENKOV and Yu F KIYACHENKO,Opt Spectrosc (USSR) 52, 56 (1982) L
_
40A
No
8 pp 785-788
1984 0
INSTRUMENTATION
058468539184 $3 00 + 0 Ot! 1984 Pergamon Press Ltd
NOTE
AN INTERFERENCE REFRACTOMETER TO MEASURE THE PRESSURE AND TEMPERATURE DEPENDENCE OF THE REFRACTIVE INDEX OF LIQUIDS ROGER APPLEBY,* DAVID W JAmst *Royal
Radar
Estabhshment,
and CAROL A BOWIE~
Malvern, U K and tchemlstry Department, Brisbane, Q, 4067, Australia
Umverslty
of Queensland,
(Recerved 4 Aprzl 1983) Abstract-The design of an Interference refractometer, using laser radlatlon, to determme the pressure and temperature varlatlon of refractive Index 1s reported The method does not attempt to determine the absolute refractive indices The performance of the refractometer was tested usmg water and methanol The results for a series of solutions of KBr m water are also reported and the results are compared with previous emplrlcal estimates
INTRODUCTION
ylelded a parallel beam of -4 mm diameter The fringe pattern, expanded usmg a 75 mm focal length lens, was lmpmged on a slit z l/10 of a fringe m width and the signal was detected using a photo transistor placed directly behmd the sht The amplified output from the transistor was recorded on the second channel of the two channel recorder This faclhtated the counting of the change m the number of fringes with either pressure or temperature change Typlcal generated frmge patterns are shown m Fig 1
There have been a limited number of studies of the pressure and temperature dependence of refractive index m hqmds and this IS undoubtedly due m part to the mtrmslc difficulty of these measurements Two techniques have been described m the recent hterature, the first of which uses a Raylelgh Interference refractometer m which the pressure dependence was determined at pressures less than atmospherlc[l] for D,O and methanol The second method[2] utlhzed an mterference refractometer [3] which was placed m a pressure vessel In this study only the changes m refractive Index with temperature and pressure were determined for benzene, water and carbon tetrachlorlde In a series of studies utlhzmg hght scattering to examme the vahdlty of a hydrodynamic treatment [4] of the LANDAU-PLACZEC ratio [5] it IS found to be necessary to know the plezo-optic and thermo-optic coefficlents [ (dn/dPh and (dn/dT),] for a variety of electrolyte solutions We report herem the design and performance of an interference refractometer, utlhzmg a laser source, smtable for operation with pure hqulds and electrolyte solutions DESIGN
OF REFRACT’OMETER
The Interferometer which 1s slmllar to the design of WAXLER et al [2] was machmed from a cube of stamless steel 7 cm per side The optlcal chamber was 34 mm long and 5 mm m diameter The end windows were of glass, 20 mm m diameter and 5 mm thick and were wedged to prevent addItIona reflections The mslde surface was gold plated m the central 4; mm diameter The plated surface was separated from the stamless steel to prevent electrolytic attack The optical cavity could be evacuated through a port which could be sealed by a steel sphere The hqtud was introduced through a second port through which gas pressure could be applied via a mercury seal which prevented dlssolutlon of the gas m the solution Around the optlcal cavity, machmed from the same block, was a water Jacket through which water was circulated from a thermostatted bath A stainless steel Jacketed Cr/Al thermocouple was Inserted mto the block with the tip adjacent to the interferometer cavity The temperature could be mamtamed constant m the block to OOI’C The gas pressure transmltted to the cell via the mercury plug was momtored usmg a cahbrated pressure transducer, the output of which was recorded on one channel of a two channel recorder The light source was a 15 mW He/Ne laser operatmg at 632 nm with a standard three lens beam expander which
Fig
785
1 A typlcal ment
frmge recordmg for a (dn/dPh A, Pressure change, B, fringes
measure-
Instrumentation
786 EXPERIMENTAL
TECHNIQUES
The hqmd under study was degassed usmg a standard freeze/thaw technique under vacuum The refractometer was evacuated and mamtamed under vacuum (10-l mm Hg) untd no further degassmg of the cavity took place The refractometer was then closed off and the degassed hqmd was bled into the evacuated chamber A plug of mercury was introduced by syringe into an external glass U-tube to isolate the hquld from the an or the pressurlzmg gas (nitrogen) The filled refractometer was then set up m the laser beam with the wmdows normal to the beam and was connected to the water bath and the pressure manifold The temperature was eqmhbrated to 15 C and a pressure of 2 76 x lo5 Nrn-’ (40 p s 1) was apphed and the system was allowed to eqmhbrate The change m the number of fringes was recorded as the pressure was released through a fixed orifice over - 2 mm This process was repeated three or four times at each temperature with temperature intervals of 5’C being examined The same procedure was followed for a change m temperature with the change m frmges for a temperature change of 5’C bemg recorded With a change m pressure there was a small change m temperature and with a change m temperature there was a change m cavity length and both of these varlatlons necessitated correctronz to the calculated refractive mdex change These correctlon procedures will be described later The change m retractive index 1s related to the recorded frmge pattern by the relationship [2] An = F
2r
sm 0
where An IS thechange m refractive index, Ajls the number of fringes passmg a fixed mark, I 1sthe spacing between the plates of the Interferometer and 0 1s the angle of Incidence of the laser beam (= 90- m this study) Typically for a 5 C temperature change Aj was -80 while for a pressure change of 2 8 x lo5 Nm-* Aj was - 5 Initial refractive indices were either taken from the literature or measured usmg an Abbe refractometer CORRECTION
PROCEDURES
When the temperature is raised the cavity ot the mterferometer mcreases m length due to the thermal expansion of the stainless steel This expansion causes the recorded number of fringes to have a consistent error This error was estimated by three separate methods (1) using the coefficient of expansion tor the particular composition of stainless steel the magnitude of the cavity change was calculated and the correspondmg value for Af was estimated (11) the refractometer was operated under vacuum and the
Table 1 Varlatlon
LIquld
of the refractlon
number of fringes for a 5°C temperature change was measured at intervals between 15 and 6o’C (111)using water, for whtch dn/dT~s known accurately, the correction factor for Aj was estimated The three methods to Aj” as being gave agreement to - 1% for the correction +183K-’ When the pressure 1s changed on a hqmd there 1s a change m temperature Although the temperature control from the isothermal bath IS good It must be reahzed that a temperature change of 0 02’ gives an error m Aj of 0 5 and smce the value of Aj’- 5 this gives an error of 10% Attempts were made to vary the rate of pressure changeand extrapolate to a dP/dt of 0 but mstablhty at very slow pressure change prevented useful corrections usmg this method The maximum value for the change m temperature can be obtained from the coefficient of thermal expansion and the adiabatic compresslblhty 1e /dT\
P
In the experiment the change of temperature 1s reduced because of the heat flow from the solution which will be influenced by the thermal conductivity of the liquid and the rate of pressure change We elected then to use a constant rate of change of pressure (pressure release through a fixed ordice) and make an empmcal correction based on a cahbrant liquid In this case
For water the values of k showed a linear increase with temperature with a, fl and (K) kappa values taken from the literature [l, 6, 71 When the k value for water at 20°C was used for methanol at 20°C excellent agreement was found with the literature value We therefore used the k values determined for water for all other liquids RESULTS
The results obtamed for the thermo-optic coefficient (dn/dT), and plezo-optic coefficient (dn/dPb for the pure liquids water and methanol are shown m Table 1 and for aqueous solutions of potassium bromide are shown m Table 2 The variation of (dn/dT), with temperature for the various liquids 1s shown m Fig 2 together with the hterature values of water DISCUSSIOh For water the agreement with the pubhshed values [2] of the thermo-optic coefficient and the piezo-optic coefficient are excellent, m the latter case, this 1s because the pubhshed values are used to determine k, and the differences noted are because the values at different temperatures were fitted to a
dn/dP (Bar-‘) (x 106)
Water
15 20 30 40 50 55
0 93 (0 90) 1 04(0 98) 123(1 21) 142(1 47) 1 67 175
15 15(15 17) 14 79(14 84) 1447(1443) 14 10 1395 137
Methanol
15 20 25 30
4 4 4 4
45 O(41 0) (41 0(42 0) 35 7(43 4) 3 1 4(44 7)
ll(39) 29(3 8) 34(3 9) 42(3 9)
B
\dp) => a
index of water and methanol
dn/dT K-l (x 104)
Temperature (‘C)
Note
with temperature
B (K?‘) (x 105)
(Bar-‘) (x 105)
15 20 30 38 45 49
1 7 3 5 8 1
467 4 59 448 442 442 4 43
1274 123 1 125 9 125 1
1188 12 27 1269 130
and pressure
kappa (x
to,,
141 143 147 1 50 1 53 1 54 048
k
(x 109)
1805 20 45 25 07 29 63 34 02 36 27 1805 20 45 22 76 25 07
Instrumentation
Table 2 Temperature
and pressure
Temperature (YJ)
Concentration
Note
variation of the refractive water
787 index of solutions
dn/dT (x 10e4)
dn/dP (x 10-S) (error +0 14)
( x 404,
of KBr m
(X~&
072M
25 35 45 55
131 1 57 168 179
1 73(1 44)t 172 164 1 52
29 37 45 53
4 51
108 M
25 35 45 55
145 171 1 83 192
1 6(142) 1 53 151 149
33 38 45 49
4 36
239M
25 35 45 55
1 69 1 84 1 89 194
1 50(1 32) 145 142 142
37 41 48 47
405
463M
25 35 45 55
1 85 193 2 02 202
16(1 18) 1 56 148 1 51
44 44 46 46
3 35
*The hypersonic compresslblhty was used m this calculation and at present only the value at 25°C has been determined tNumbers m brackets are calculated according to Eqn 1
/
0
/
/
’
/ / /
/
’
1’
,
,q
/
/
methanol with mcreasmg temperature IS m accord with the associated nature of the liquid The temperature variation of the refractive index of electrolyte solutions has been studled by several workers m a non systematic way [9] but there IS httle agreement between the different studies The pressure varlatlon of refractive Index of solutions has not been reported although It has been proposed that this parameter may be calculated from the results for water [l, lo] usmg the formula
I-
. 0
/’
/
/
n-l
Temp (“Cl Fig 2 Varlatlon of (dn/dT)p with temperature -----KBr 072M, - - - -KBr 108 M, _ _ _ - KBr 463 M
-Water, KBr 2 39 M,
straight line The performance of the refractometer 1s however validated by the excellent agreement m the (dn/dT)p values For methanol the values of (dn/dT)p which we determine are consistently above the literature values whde the value of (dn/dP)p IS correct at 2O’C but the gradient with temperature 1s m the opposite sense to that previously reported In the previous study a pure reagent, as supplied, was used wlthout purlficatlon It IS now well known that methanol takes up a small amount of water rapidly and that m order to study the water-free compound a fractlonal dlstlllatlon from calcium chloride or magnesmm 1s recommended[8] It IS possible that traces of water m the methanol previously used caused the difference with the results we now report For water, an associated lrquld, the plezo-optic coefficient decreases with mcreasmg temperature while for the non associated hqulds benzene and carbon tetrachloride [ 1 21 It Increases The decrease In this coefficient for
= ApBe-O
(1)
where A = 0 339, B = 0 972, C = 7 07 x 1O-5 and p 1sthe density The apphcatlon of this method to solutions of KBr at 25°C IS included m Table 2 It may be seen that the formula predicts a steady decrease m (dn/dPb as the concentration of salt Increases What 1s experimentally observed at 25°C IS an mmal rapld rise m the coeficlent followed by a small decrease, at higher temperatures there IS agam a rapid mcrease to a value which stays almost constant as the concentration Increases The formula proposed, which describes the temperature varlatlon of (dn/dPhfor water, relates the change m refractive Index to the change m density The varlatlons m Ion-Ion- solvent mteractrons with concentration and temperature are obviously more complex than the varlatlons m the hydrogen-bonded network structure of pure water and the simple density relatIonshIp 1s inadequate to describe them It may be noted that the mitral rise In (dn/dPh followed by a decrease mnrors the behavlour of the Intensity of the depolarized Raylelgh scattermg from salt solutions which has been described In terms of varlatlons m Ion-ran-solute mteractlons [ 1 l] There has been a recent report of the measurement of the thermo-optic and plezo-optic coefliclents m pure hqulds usmg an mterferometrlc apparatus[12] In this report the errors caused by change of temperature with pressure change were not consldered For the organic hqmds reported the correction necessary to (dn/dP) would be quite small because of the small thermal conductlvlty of the liquid We feel, however, that many techmque m which only the movement of Interference fringes IS observed the posslblhty of varlatlons m dn$dP due to temperature changes should be consldered
788
Instrumentation
Acknowledgement-The Austrahan Research Grants Scheme IS thanked for salary grants for R A and C A B
REFERENCES [l]
E REISLERand H EISENBERG, J them Phys 43, 3875 (1965) [2] R M WAXLER,C E WEIR and H W SCHAMP,J Res nafn Bur Stand 68A, 489 (1964) [3] J B SAUNDERS, J Res natn Bur Stand 35,157 (1945) [4] R D MOUNTAINand J M DELJTCH, J them Ph~s 50, 1103 (1969)
Note
[5l L LANDAUand G PLACZEC,Z Smletunm 5, 172 (1934) [6] G S KEEL, J them Engng Data 20,97 (1975) [7] CRC Handbook, 51st Ed, p E 4, (1970) [S] Techmques ofOrganrc Chermstry, Vol 7, p 333 (edlted bv A WEISSBERGER) InterscIence. New York (1955) [9] I&matmml Crrtrchl Tables, Vol III McGraw Hill, New York (1930) [lo] A R MARETand E YEAGER,J acoust Sot Am 54,668 (1973) [ll] D W JAMESand R IRMER, J Raman Spectrosc In press [12] A M EUTYUSHENKOV and Yu F KIYACHENKO,Opt Spectrosc (USSR) 52, 56 (1982) L
_