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Colorimetric and photometric absorption analysis
VOL.
2
(1948)
ANALYTICA
COLORIMETRIC
AND
CHIMICA
PHOTOMETRIC
D.
ACTA
ABSORPTION
693
ANALYSIS
J. COUTsZOU
Research Laborafovy of the N. V. De Bataafse Pelrolewn Maatschappi~, Anrsterdam (Nefherfarlds)
I. INTRODUCTION
Among optical methods of analysis chemical calorimetry has become an important means in analytical chemistry. In physics, the term colorimelry indicates the measurement of colours as such and their expression in any system of colour specification, e.g., the I.C.I.-system. This may be very important in several branches of chemical industry, but this “calorimetry” cannot be regarded as chemical analysis. Calorimetric analysis does not aim at determination of colouv, but at a determination of quantities and concentrations. This dual meaning of the term “calorimetry” should be carefully borne in mind. Calorimetric analysis can be applied to coloured solutions or to substances which give a coloured solution when a second substance is added. In the course of time several reagents have been found, which give a sufficiently stable colour with metal ions or acid radicals. Procedures for carrying out the reactions necessary to obtain such coloured solutions are described in detail in several original publications and compiled in some handbooks (e.g. ,68). Several calorimetric analyses have a far greater sensitivity than chemical methods of analysis, so that micro quantities of substance can be measured. Many metal ions, for instance, can be determined with the aid of a suitable reagent in concentrations of about I microgram pro ml, with an accuracy of better than I %. In general thcseanalysescan be carried out in a very short time. 2. BEER’S
LAW
The possibility of calorimetric analysis is subject to the dependence of the absorption of light on the concentration of the absorbing solute. For monochromatic radiation and dilute solutions this dependence is governed by the law of LAMBERTBEER
where
:
-lOlogT=E=kcd T = transmission of the solution in depth of layer d C = concentration of the light-absorbing substance.
References p. 703.
(1) .
D.
694
J. COUMOU
VOL.
2 (1948)
E is called the cxlinction or the ofitical density of the solution, E/d is the extinction coefficient, k the specific extinction coefficient. If c is expressed in mol/l, k is the nLoLec2rZaror moZa7 extinction coefficient; k is a function of the wave length of the light used and is at a maximum when the absorption band in the transmission curve is at a minimum. Deviations from this law may occur in cases where a change in concentration is accompanied by a change in constitution, e.g., dissociation, association or solas is done in several measurements vatation of the dissolved molecules. If no monochromatic light, but a radiation from a certain range of wave lengths is used, the effective extinction coefficient is a mean value over this spectral range. Often this value is not constant either, but depends more or less on the total transmission. 3.
COLORIMETRIC
ANALYSIS
analysis” is especially used to indicate the At present the term “calorimetric original form of such an analysis, in which the coZou7 of the solution is compared with a number of standards. The simplest procedure is to employ as standards a number of solutions of the substance under test in known concentrations. Another method is to compare glass standards with a solution of analogous hue in a colour com$arator. For a given analysis these standards should first be calibrated. In measurements in which both colour and brightness are matched by changing the depth of layer either of the standard solution or of the solution under mcasurement a precision of a few per cent can already be obtained. The different types of Duboscq colovimcters as well as the wedge-cell coZorimete7s are built on this principle. Illumination usually takes place with white light; sometimes a colour filter with complementary hue to that of the solution is placed in the light beam. At equal transmission, the extinction coefficient is the same for both solutions and the concentration of the unknown solution is calculated from BEER’S law. On account of the rapid method of measuring with simple and cheap instruments, this type of calorimetric analysis is very attractive in all those cases where no great accuracy is required. Sufficient stability of the standard solutions during a long time is necessary. 4.
PHOTOMETRIC
ABSORPTION
ANALYSIS
Colorimctric analysis can be carried out with higher precision by direct photometric measurement of the transmission or the extinction of a solution of unknown concentration. Several instruments have been constructed in which this principle has been worlccd out. They all possess some kind of photometer, with which first the brightness in a light beam is measured through a cell containing a blank Rpfcretaces p.
703.
VOL.
2 (x948)
ABSORPTION
ANALYSIS
695
(e.g., the pure solvent) and thereafter through a cell of the same thickness, containing the solution. In addition they possess an equipment enabling these measurements to be made in a narrow spectral range. If a monochromalor is used for this purpose, the complete apparatus is called a spectrophotometer. If coZou7 filters are used, it is called a filter photometer, sometimes “abridged spectrophotometer”, absorptiometer, or simply “calorimeter”. There is no essential difference between these two types of instruments. In photometric absorption analysis the instrument should first be calibrated with a number of solutions of known concentration. At a linear relationship between extinction and concentration according to BEER’S law (I) the slope of the straight line is given by the coefficient k. With a greater k-value the same variation in concentration gives a larger variation in extinction. Thus the greatest accuracy is obtained at the maximum absorption. This proves the importance of a narrow spectral range: only the light that is really absorbed by the liquid should be measured. If BEER’S law is not valid, either on account of the properties of thesolution (Section z) or because of too wide a spectral range, a calibration curve can be made. In many analyses literature values of the extinction coefficient can be used. However, one should be prepared for disturbing effects of a chemical nature (impurities of reagents, presence of interfering constituents, etc.). Filter photometers present the possibility of inequality of the filters, also in the same type of apparatus. It is therefore advisable to make one’s own calibration, which can be done very quickly. Mixtures can be analysed by measurement in as many wave lengths as there are components. In calorimetric practice this analysis will be limited to mixtures of two substances, using two spectral ranges. Total extinction is equal to the sum of the extinctions due to each of the components. Thus two equations are obtained with the two unknown concentrations and four different extinction coefficients (for both components in each of both spectral ranges). These coefficients should be determined separately in solutions of known concentration. Transmission measurements in photometric absorption analysis are performed with a) visual, and b) photoelectric photometers. The importance of absorption spectrography, in which the transmission is determined with the aid of the photographic plate, has been decreasing in the last few years. For the sake of completeness we mention the MolI absorptiometer, in which the intensities of radiation are measured with a thermopile. 5. MONOCHROMATORS
AND
FILTERS
For a narrow spectral range the use of a monochromator is necessary. Prisms as well as gratings are applied to form the spectrum. The band width of the spectral range depends on the construction of the instrument, the dispersion of the prism, and the width of the slits. Generally these widths can be varied. We Refererrces p. 703.
VOL. 2 (1948)
D. J. COUMOU
696
the following examples: The Coleman Universal Spectrophotometer No. II has 35 rnp total bandpass; in Model No. IO a minimum bandpass of 2.5 rnp can be obtained. In grating monochromators there is a possibility of stray light from other wave lengths. TABLE I In the case of colour filters the half-intensity width is ‘St. width often mentioned. In Table I we give these values for some fi-Slters of the Zeiss Pulfrich photometer. Of course the 31 mCc total bandpass is larger (e.g., S 57-45 rnp; S 4770 mp). z; ra 0. These filters therefore give an essentially wider range than 50 26 ,, can be applied with a monochromator. However, the 24 8, 47 Lumetron Photo-electric Calorimeter, Model 402 E, is supplied with 14 colour filters of approximately 30 rnp bandpass, covering the whole visible spectrum. In combination with a discharge lamp monochromatic light of a single spectral line can sometimes be employed in filter photometers. give
%Y
0 403 F1g.
440
480
I. Transmission
520
580
curve, mcasurcd
600
wth
640
680
scvcrnl bandwt$[hs
For the exact determination of a transmission curve a spectrophotometer is preferable. Fig. I* shows the differences between the curves obtained with a band* Derived from MELLON~-". Refcrcmcs
p. 7‘03.
I
VOL. 2 (x948)
ABSORPTION
ANALYSIS
697
width of 5 rnp, of 35 mp, and with a filter photometer (no particulars of the filters are indicated). Transmission measurement gives a mean value over the spectral range. As a consequence, both the minima and the maxima are smoothed with a wider range of wave lengths. The determination of such a complete absorption curve may be of interest in constitution questions, in quantitative analysis it will seldom be necessary. Instability of the substance examined can be noted from the change in the shape of the absorption curve. Moreover in calorimetric determinations an absorption curve as in Fig. I seldom appears. Usually there is a broad absorption band and the analysis can be made with the larger spectral ranges of a filter photometer. XW% r
so .
Fig. 2. Filter
trnnsmlsslon
falllug over a steep part of the tr.msmlsslon
curve of a solution
Here, too, within the precision of the measurement, a. linear relationship between extinction and concentration is generally found. However, this does not hold, if a steep part of the absorption curve of the liquid falls in the transmission range of the filter (Fig. 2). 6. VISUAL
PHOTOMETERS
Although in future perhaps more and more photoelectric instruments will be used, several visual photometers are still regularly employed in photometric absorption analysis. The principle of all these instruments is to bring two light beams from a common light source in one field of view, observed by the human eye. The light intensity in one of these beams is influenced by the absorption of the solution; the intensity in the other beam can be regulated by means of a. a polarisation equipment (Kdnig-Martens-, spectrophotometer; Leifo-filter-photometer) References p. 703.
Hilger-Nutting-, ;
Bausch a. Lomb-
D.
698
J. COUMOU
VOL.
b. a variable rotating sector; c. a variable aperture (Zeiss-Pulfrich photometer) cl. a neutral density wedge; this should, however, absolute methods a-c.
2
(1948)
;
first be calibrated
by one of the
After photometric matching the transmission or the extinction can be read on the regulation apparatus. As it is possible to regulate the light beam used for comparison with a greater precision than the contrast sensitivity of the eye, this factor governs the reproducibility of matching the intensities. In favourable circumstances the contrast scrtsitivily
y
can amount
to about I %. This is only valid
in the middle
of the
visible spectral range, and largely depends on subjective factors (e.g., fatigue of the observer). Moreover it decreases considerably at the ends of the visible spectrum. From the contrast sensitivity the relative error in extinction, and thus also that in concentration, can be calculated: dE -___-E-cThe precision
dc
0.434 dI E I’
thus increases with increasing
values of dc for +I =
concentration.
Table
II shows the
I %. It is advisable to measure at concentrations
with E -
C TABLE II about 1.0; at greater absorption the intensity mostly becomes too low and the contrast sentitivity then also decreases. Under -dC E the most favourable conditions a precision of about 0.5 oh C can be obtained. Visual photometers have reached a high degree of optical 0.3 I.4 % 0.6 o/o perfection, and they are simple and easy to use. The double y-z 0.4 “/” * beam principle makes the results independent of fluctuations ? in the light source. In filter photometers (Leifo, Pulfrich) rather narrow spectral ranges can be employed when applying high intensity sources. A disadvantage in these instruments is the colour difference between the two parts of the field of view, when the transmission curves of filter and liquid are situated as in Fig. 2. This causes considerable diminution in contrast sensitivity and thus in the precision of the measurement.
7.
PHOTOELECTRIC
PHOTOMETERS
In photoelectric photometers the disadvantages of the subjective influences of the observer and the defects of the human eye are eliminated. In addition, photoelectric measurements can be made beyond the visible spectral range. With suitable radiation sources and instruments (monochromator with quartz prism, quarts
References 9. 703.
VOL.
2 (1948)
ABSORPTION
699
ANALYSIS
liquid cells) transmission can be measured of ultraviolet radialion, and several analyses are possible on substances with absorption bands in the ultraviolet region of the spectrum (vitamins, hydrocarbons, complex organic molecules). So photometric absorption analysis, in which the colouv of the solution no longer plays a part, covers a much wider scope than pure calorimetric analysis. Photoelectric photometers are supplied with ~ltolocells either of the barrier-layer type (generally selenium cells) or of the fihoto-emissive ty$e @hoto t&zs). The first mentioned cells are widely used on account of the possibility of measuring directly the generated current by means of a galvanometer or micro-ammeter. The photocurrent is, within a limited range, only proportional to the luminous flux on the cell surface for a low value of the external resistance. Moreover the cells show phenomena of fatigue and lag. Phototubes require an auxiliary e.m.f. between the electrodes. Though the response of these cells is inferior to that of the barrier-layer ceil, they are much better in constancy and in linearity of output against luminous flux. This output is determined from the potential drop over a high resistance (10~ to IO*O Ohms). In phototubes as well as in barrier-layer cells the sensitivity is not constant over the entire surface; the optical arrangement of the photometer should therefore be such that the same region of the cell is continually illuminated. Selenium cells show maximum response in the visible spectrum range, but this extends in the ultraviolet below 300 mp. Phototubes with different surfaces of alkali metal or alkali oxides can be chosen for several spectral ranges in which they have maximum sensitivity. The development of direct-current-amplifiers, which are exactly constant and linear over a large range has made it possible greatly to amplify the output of a phototube. Owing to this and their other properties these tubes are very useful in photoelectric spectrophotometers with a narrow bandpass. Some spectrophotometers, however, are provided with barrier-layer cells (Coleman Model No. II, spectral band width 35 rnp; Cenco spectrophotometer, with even 2.5 rnp minimum ’ bandpass). Filter photometers generally have cells of the last mentioned type. In general it cannot be said that a better precision is obtained in analysis with a photoelectric apparatus than with a visual photometer. Great precautions are necessary to attain this, both in constructing the instrument and in operating it. There is a large variety of these constructions and the choice of instrument will be largely a matter of price and the ease of operation with respect to the required accuracy. Very exact spectrophotometers make it possible to obtain a precision of 0.x %, but these are by no means simple instruments. In many cases a less complicated instrument, e.g., a filter photometer, will suffice. in which the In photoelectric photometry - as opposed to visual photometry, precision of matching the reproducibility Reyerences
9.
703.
intensities
is limited by the relative intensity
of the measurement
depends on the sensitivity
variation
y
-
of the photocell
D.
700
VOL.
J. COUMOU
2 (x948)
to absotute intensity variations d1 and the precision with which the corresponding variation in photocurrent can be read. From this the relative error in extinction or in concentration can be calculatedlo as dE dc 0.434 dI -=-P_ (2) E~Io-” E c 10 This is minimum at the maximum of the function E.Io-” for E = 0.434, or: T = 36.8 %. This may be understood by plotting the curve I = I, IO-~: at low concentrations (extinctions) dc is small for a given dl-value; but the relative error dc c is great. At high concentrations the relative error increases again, since the C
slope in the intensity curve changes very slowly and thus dc becomes large at the same dI-value.
T
4 1
Kg.
3. Rclatlvc
error in photoelectric
absorption
analysis
dc Fig. 3 shows that the minimum in the - - curve is rather flat, so about the C
same precision exists over a wide range of concentrations. It is evident from equation (2) that - as was to be expected - precision increases with increasing intensity I,. 8.
METHODS
OF
MEASURING
In siqrgle-cell filzotorneters the transmission is determined from the ratio of the deflections of the galvanometer indicating the photocurrents of phototubes with the blank and with the solution in the light beam after amplification between the source and the photocell. Generally the scale of the instrument directly indicates the extinction of the solution. Rs_fcrcnccs #. 703.
VOL.
2 (x948)
ABSORPTION
ANALYSIS
701
The main error in this method follows from fluctuations in the light source. This may be fed from a storage battery, or from the main voltage, provided this is well stabilised. Furthermore the properties of the photocell and the precision in reading the galvanometer deflection limit the precision in concentration measui-kments. Generally the error is not better than about z %. Instead of reading the deflection of a galvanometer the output of the photocell can be measured by a potcnliomeler. which compensates the potential drop over a resistance caused by the photocurrent. The galvanometer is only used as a null instrument. The ratio of the potentiometer settings with the blank and with the solution gives the transmission of the latter. With a suitable compensator it is possible thus to get a better reproducibility of instrument reading. With phototubes the zero adjustment may be effected with the aid of a thermionic voltmeter and amplifier. The well-known Beckman Photoelectric Quartz Spectrophotometer operates in this way 20. This apparatus can be used in the spectral range of about 200-2000 rnp. By very exact stabilisation of the light sources (tungsten filament lamp for the infrared and the visible, hydrogen discharge tube for the ultraviolet) a precision of 0.1 o/o can be obtained with this instrument. To be independent of fluctuations in the light source, as in visual photometers, the intensities in two light beams are compared by illuminating one of two photocells by each beam. Both cells are placed in a balnncing circd, in which a galvanometer operates as null indicator. With blanks in both light beams the galvanometer is adjusted to zero. Substitution of one blank by the solution gives the instrument a deflection. Compensation is now possible: a. opticilly by measurable regulation of the intensity in the other beam, according to one of the methods indicated in visual photometry; b. electrically by a suitable compensation circuit. The principle of two different
Fig. 4. Compcnsatmg arcuit for barrwr-layer photocells: Scrlcs-opposing type (voltage-bnlancmg)
circuits for barrier-layer cells are indicated in Figs 4 and 5. R, and R, may be two variable resistances or potentiometers. With the blank R, is adjusted to References 9. 703.
,
VOL.
D. J. COUMOU
702
2 (xg48)
IOO and I~, regulated at zero indication of the galvanometer G. After the solution is placed in the light beam of the measuring photocell P1 the balance is re-established with Rz, on which the transmission is read.
Fig.
5. Compcxwrting
circuit
for Inrricr-lnycr
photocells : Pdrnllcl
type
(current-bdnnclng)
The arrangement of Fig. 5, in several modifications, is especially often employed. The same principle can be applied with phototubes (~.g.,~~+*). In the balancing circuits mentioned above the luminous fluxes on the measuring photocell are different with the blank and with the solution. With the subslitzrtion method the circuit is first balanced with the solution, which thereafter is substituted by a regulation of the intensity not in the bundle on the comparing photocell, but in the same light bundle. The photocurrent is thus always the same and the measurement is independent of sources of error such as lag and non-linearity in the reponse of the photocell. Only with this method a high precision can be obtained, even with barrier-layer cells; 0.1 o/0 is mentioned in theliterature (KoRT~~M~, p. 110).
.
FIN. 6. DIngram
of I-Illget-Spckkcr-nbsorptiomctcr
Fig. 6 shows a diagram of the Hilger-Spekker absorptiometer operating on this principle. Zero adjustment is first performed with the diaphragm D,, while the References p. 703.
VOL.
2 (x948)
ABSORPTION
703
ANALYSIS
cell, C is filled with the solution. Thereafter, with the blank the circuit is rebalanced with the diaphragm D,. The resistance R only controls the sensitivity of the galvanome ter. SUMMARY A survey is given of colorlmetric mcasurmg m&hods and the main discussed.
and photometric absorption analysis. The princlplcs of pomts In the construction of various instruments arc
L’autcur rbsumc XX l’analysc d’absorption colorimGtriquc ct photom&riquc dlscusslon dcs prmcipcs dcs mCthodcs dc mcsurc ct dcs prmclpalcs qucstrons construction dc dlvcrs instruments.
ct il fait la rclntivcs & la
Einc Ubcrsicht dcr kolorlmctrischcn und photomctrlschcn Ahsorptlonsanalysc wird gcgehen. Dlc Grunclsatzc dcr Mcssmcthodcn SOWIC dlc hnuptslchllchcn Fragcn bczuglich dcr Konstruktlon vcrschlcdcncr Instrumcute wcrdcn crertcrt. REFERENCES WC refer to some handbooks and artlclcs In which surveys arc glvcn of the methods of colorlmetric and photomctrlc analysis. Some of them also ~lvc an.dytlcal proccdurcs. Bcsidcs this, in the text rcfcrcncc IS made to a few pubhcatlons on spcclal subjects or mstrumcnt constructions. Marc of the cxtcnslvc htcraturc may bc found In the publications mcntloncd. 0. J. WALKER, AOsorp;zov~ SpcclropirolomrLry and tfs Applrcattons, London 1939 (glvcs an ample blbhography of applications of Absorption Spcctrophotomctry from x932-1938). a F. MiiLLER, Phystkaluche Methoden der Analylrschcn Cherme, 3. Tell : Die photoclcktrlschcn Mcthodcn dcr Analyst, Lcipug 1939. 3 G. KoRTihr. Kolorimelne und SpcIILmIpholo,netrae, Berlin 1942. 4 Tn. R. P. GOB, Opfacal Methods of Chenrtcal Analyszs, New York 1942. ti B. LANGIS, Z
Received April 3rd, 1948
704
D. J. COUMOU
VOL.
2 (x948)
DISCUSSION In connection with the paper, the author showed some slides with diagrams and pictures diffcrcnt types of inst.rumcnLs for photometric absorption nnnlys~s, availnblc at the market. Miss N. 0. M. MAGETFIORN (Dclft) draws :Lttcntion to the fact that thcrc exists an cxccllcnt Dutch instrument with prism monochromator, the Gblccta colorimctcr, from Blcckcr, Utrccht.
of
Mr N. SIRAFFORD (Manchcstcr) strcsscs the need for an adcquntcly cooled lnmphousc in photoclcctric absorptiomctcrs, and also for a thcrmostntlcnlly controlled iackct for the absorption cells, CspcciiLlly when an accuracy of f 0.5 o/Ois rcquircd. Mr G. I-I. UEAVEN (Chcstcr) supports this remark and dcsircs more working space in the ;tbsorption ccl1 comp,Lrtmcnts. Mr J. G. REYNOLDS (Chcstcr) points out the ncccssity to rcchcck calibration CiLlly, bccausc of the dctcrior:tiion of the photocells.
graphs pcriodi-
Mr G. H. BEAVEN (Chcstcr) rccommcnds the USC of stable gl,Lss filters for checking the ;Lccuracy of photoclcctric instruments. \Vith a Wood’s glass, calibrated for the ncnr ultrnvrolct, hc found in a Bcclcmnnn spcctropholomctcr (with phototubcsl) over a period of five years continual USC the imlial st.Lbility of the Lnstrumcnt .Lnd the accur,.Lcy of the clcnsrty scale fully maintained.
2
(1948)
ANALYTICA
COLORIMETRIC
AND
CHIMICA
PHOTOMETRIC
D.
ACTA
ABSORPTION
693
ANALYSIS
J. COUTsZOU
Research Laborafovy of the N. V. De Bataafse Pelrolewn Maatschappi~, Anrsterdam (Nefherfarlds)
I. INTRODUCTION
Among optical methods of analysis chemical calorimetry has become an important means in analytical chemistry. In physics, the term colorimelry indicates the measurement of colours as such and their expression in any system of colour specification, e.g., the I.C.I.-system. This may be very important in several branches of chemical industry, but this “calorimetry” cannot be regarded as chemical analysis. Calorimetric analysis does not aim at determination of colouv, but at a determination of quantities and concentrations. This dual meaning of the term “calorimetry” should be carefully borne in mind. Calorimetric analysis can be applied to coloured solutions or to substances which give a coloured solution when a second substance is added. In the course of time several reagents have been found, which give a sufficiently stable colour with metal ions or acid radicals. Procedures for carrying out the reactions necessary to obtain such coloured solutions are described in detail in several original publications and compiled in some handbooks (e.g. ,68). Several calorimetric analyses have a far greater sensitivity than chemical methods of analysis, so that micro quantities of substance can be measured. Many metal ions, for instance, can be determined with the aid of a suitable reagent in concentrations of about I microgram pro ml, with an accuracy of better than I %. In general thcseanalysescan be carried out in a very short time. 2. BEER’S
LAW
The possibility of calorimetric analysis is subject to the dependence of the absorption of light on the concentration of the absorbing solute. For monochromatic radiation and dilute solutions this dependence is governed by the law of LAMBERTBEER
where
:
-lOlogT=E=kcd T = transmission of the solution in depth of layer d C = concentration of the light-absorbing substance.
References p. 703.
(1) .
D.
694
J. COUMOU
VOL.
2 (1948)
E is called the cxlinction or the ofitical density of the solution, E/d is the extinction coefficient, k the specific extinction coefficient. If c is expressed in mol/l, k is the nLoLec2rZaror moZa7 extinction coefficient; k is a function of the wave length of the light used and is at a maximum when the absorption band in the transmission curve is at a minimum. Deviations from this law may occur in cases where a change in concentration is accompanied by a change in constitution, e.g., dissociation, association or solas is done in several measurements vatation of the dissolved molecules. If no monochromatic light, but a radiation from a certain range of wave lengths is used, the effective extinction coefficient is a mean value over this spectral range. Often this value is not constant either, but depends more or less on the total transmission. 3.
COLORIMETRIC
ANALYSIS
analysis” is especially used to indicate the At present the term “calorimetric original form of such an analysis, in which the coZou7 of the solution is compared with a number of standards. The simplest procedure is to employ as standards a number of solutions of the substance under test in known concentrations. Another method is to compare glass standards with a solution of analogous hue in a colour com$arator. For a given analysis these standards should first be calibrated. In measurements in which both colour and brightness are matched by changing the depth of layer either of the standard solution or of the solution under mcasurement a precision of a few per cent can already be obtained. The different types of Duboscq colovimcters as well as the wedge-cell coZorimete7s are built on this principle. Illumination usually takes place with white light; sometimes a colour filter with complementary hue to that of the solution is placed in the light beam. At equal transmission, the extinction coefficient is the same for both solutions and the concentration of the unknown solution is calculated from BEER’S law. On account of the rapid method of measuring with simple and cheap instruments, this type of calorimetric analysis is very attractive in all those cases where no great accuracy is required. Sufficient stability of the standard solutions during a long time is necessary. 4.
PHOTOMETRIC
ABSORPTION
ANALYSIS
Colorimctric analysis can be carried out with higher precision by direct photometric measurement of the transmission or the extinction of a solution of unknown concentration. Several instruments have been constructed in which this principle has been worlccd out. They all possess some kind of photometer, with which first the brightness in a light beam is measured through a cell containing a blank Rpfcretaces p.
703.
VOL.
2 (x948)
ABSORPTION
ANALYSIS
695
(e.g., the pure solvent) and thereafter through a cell of the same thickness, containing the solution. In addition they possess an equipment enabling these measurements to be made in a narrow spectral range. If a monochromalor is used for this purpose, the complete apparatus is called a spectrophotometer. If coZou7 filters are used, it is called a filter photometer, sometimes “abridged spectrophotometer”, absorptiometer, or simply “calorimeter”. There is no essential difference between these two types of instruments. In photometric absorption analysis the instrument should first be calibrated with a number of solutions of known concentration. At a linear relationship between extinction and concentration according to BEER’S law (I) the slope of the straight line is given by the coefficient k. With a greater k-value the same variation in concentration gives a larger variation in extinction. Thus the greatest accuracy is obtained at the maximum absorption. This proves the importance of a narrow spectral range: only the light that is really absorbed by the liquid should be measured. If BEER’S law is not valid, either on account of the properties of thesolution (Section z) or because of too wide a spectral range, a calibration curve can be made. In many analyses literature values of the extinction coefficient can be used. However, one should be prepared for disturbing effects of a chemical nature (impurities of reagents, presence of interfering constituents, etc.). Filter photometers present the possibility of inequality of the filters, also in the same type of apparatus. It is therefore advisable to make one’s own calibration, which can be done very quickly. Mixtures can be analysed by measurement in as many wave lengths as there are components. In calorimetric practice this analysis will be limited to mixtures of two substances, using two spectral ranges. Total extinction is equal to the sum of the extinctions due to each of the components. Thus two equations are obtained with the two unknown concentrations and four different extinction coefficients (for both components in each of both spectral ranges). These coefficients should be determined separately in solutions of known concentration. Transmission measurements in photometric absorption analysis are performed with a) visual, and b) photoelectric photometers. The importance of absorption spectrography, in which the transmission is determined with the aid of the photographic plate, has been decreasing in the last few years. For the sake of completeness we mention the MolI absorptiometer, in which the intensities of radiation are measured with a thermopile. 5. MONOCHROMATORS
AND
FILTERS
For a narrow spectral range the use of a monochromator is necessary. Prisms as well as gratings are applied to form the spectrum. The band width of the spectral range depends on the construction of the instrument, the dispersion of the prism, and the width of the slits. Generally these widths can be varied. We Refererrces p. 703.
VOL. 2 (1948)
D. J. COUMOU
696
the following examples: The Coleman Universal Spectrophotometer No. II has 35 rnp total bandpass; in Model No. IO a minimum bandpass of 2.5 rnp can be obtained. In grating monochromators there is a possibility of stray light from other wave lengths. TABLE I In the case of colour filters the half-intensity width is ‘St. width often mentioned. In Table I we give these values for some fi-Slters of the Zeiss Pulfrich photometer. Of course the 31 mCc total bandpass is larger (e.g., S 57-45 rnp; S 4770 mp). z; ra 0. These filters therefore give an essentially wider range than 50 26 ,, can be applied with a monochromator. However, the 24 8, 47 Lumetron Photo-electric Calorimeter, Model 402 E, is supplied with 14 colour filters of approximately 30 rnp bandpass, covering the whole visible spectrum. In combination with a discharge lamp monochromatic light of a single spectral line can sometimes be employed in filter photometers. give
%Y
0 403 F1g.
440
480
I. Transmission
520
580
curve, mcasurcd
600
wth
640
680
scvcrnl bandwt$[hs
For the exact determination of a transmission curve a spectrophotometer is preferable. Fig. I* shows the differences between the curves obtained with a band* Derived from MELLON~-". Refcrcmcs
p. 7‘03.
I
VOL. 2 (x948)
ABSORPTION
ANALYSIS
697
width of 5 rnp, of 35 mp, and with a filter photometer (no particulars of the filters are indicated). Transmission measurement gives a mean value over the spectral range. As a consequence, both the minima and the maxima are smoothed with a wider range of wave lengths. The determination of such a complete absorption curve may be of interest in constitution questions, in quantitative analysis it will seldom be necessary. Instability of the substance examined can be noted from the change in the shape of the absorption curve. Moreover in calorimetric determinations an absorption curve as in Fig. I seldom appears. Usually there is a broad absorption band and the analysis can be made with the larger spectral ranges of a filter photometer. XW% r
so .
Fig. 2. Filter
trnnsmlsslon
falllug over a steep part of the tr.msmlsslon
curve of a solution
Here, too, within the precision of the measurement, a. linear relationship between extinction and concentration is generally found. However, this does not hold, if a steep part of the absorption curve of the liquid falls in the transmission range of the filter (Fig. 2). 6. VISUAL
PHOTOMETERS
Although in future perhaps more and more photoelectric instruments will be used, several visual photometers are still regularly employed in photometric absorption analysis. The principle of all these instruments is to bring two light beams from a common light source in one field of view, observed by the human eye. The light intensity in one of these beams is influenced by the absorption of the solution; the intensity in the other beam can be regulated by means of a. a polarisation equipment (Kdnig-Martens-, spectrophotometer; Leifo-filter-photometer) References p. 703.
Hilger-Nutting-, ;
Bausch a. Lomb-
D.
698
J. COUMOU
VOL.
b. a variable rotating sector; c. a variable aperture (Zeiss-Pulfrich photometer) cl. a neutral density wedge; this should, however, absolute methods a-c.
2
(1948)
;
first be calibrated
by one of the
After photometric matching the transmission or the extinction can be read on the regulation apparatus. As it is possible to regulate the light beam used for comparison with a greater precision than the contrast sensitivity of the eye, this factor governs the reproducibility of matching the intensities. In favourable circumstances the contrast scrtsitivily
y
can amount
to about I %. This is only valid
in the middle
of the
visible spectral range, and largely depends on subjective factors (e.g., fatigue of the observer). Moreover it decreases considerably at the ends of the visible spectrum. From the contrast sensitivity the relative error in extinction, and thus also that in concentration, can be calculated: dE -___-E-cThe precision
dc
0.434 dI E I’
thus increases with increasing
values of dc for +I =
concentration.
Table
II shows the
I %. It is advisable to measure at concentrations
with E -
C TABLE II about 1.0; at greater absorption the intensity mostly becomes too low and the contrast sentitivity then also decreases. Under -dC E the most favourable conditions a precision of about 0.5 oh C can be obtained. Visual photometers have reached a high degree of optical 0.3 I.4 % 0.6 o/o perfection, and they are simple and easy to use. The double y-z 0.4 “/” * beam principle makes the results independent of fluctuations ? in the light source. In filter photometers (Leifo, Pulfrich) rather narrow spectral ranges can be employed when applying high intensity sources. A disadvantage in these instruments is the colour difference between the two parts of the field of view, when the transmission curves of filter and liquid are situated as in Fig. 2. This causes considerable diminution in contrast sensitivity and thus in the precision of the measurement.
7.
PHOTOELECTRIC
PHOTOMETERS
In photoelectric photometers the disadvantages of the subjective influences of the observer and the defects of the human eye are eliminated. In addition, photoelectric measurements can be made beyond the visible spectral range. With suitable radiation sources and instruments (monochromator with quartz prism, quarts
References 9. 703.
VOL.
2 (1948)
ABSORPTION
699
ANALYSIS
liquid cells) transmission can be measured of ultraviolet radialion, and several analyses are possible on substances with absorption bands in the ultraviolet region of the spectrum (vitamins, hydrocarbons, complex organic molecules). So photometric absorption analysis, in which the colouv of the solution no longer plays a part, covers a much wider scope than pure calorimetric analysis. Photoelectric photometers are supplied with ~ltolocells either of the barrier-layer type (generally selenium cells) or of the fihoto-emissive ty$e @hoto t&zs). The first mentioned cells are widely used on account of the possibility of measuring directly the generated current by means of a galvanometer or micro-ammeter. The photocurrent is, within a limited range, only proportional to the luminous flux on the cell surface for a low value of the external resistance. Moreover the cells show phenomena of fatigue and lag. Phototubes require an auxiliary e.m.f. between the electrodes. Though the response of these cells is inferior to that of the barrier-layer ceil, they are much better in constancy and in linearity of output against luminous flux. This output is determined from the potential drop over a high resistance (10~ to IO*O Ohms). In phototubes as well as in barrier-layer cells the sensitivity is not constant over the entire surface; the optical arrangement of the photometer should therefore be such that the same region of the cell is continually illuminated. Selenium cells show maximum response in the visible spectrum range, but this extends in the ultraviolet below 300 mp. Phototubes with different surfaces of alkali metal or alkali oxides can be chosen for several spectral ranges in which they have maximum sensitivity. The development of direct-current-amplifiers, which are exactly constant and linear over a large range has made it possible greatly to amplify the output of a phototube. Owing to this and their other properties these tubes are very useful in photoelectric spectrophotometers with a narrow bandpass. Some spectrophotometers, however, are provided with barrier-layer cells (Coleman Model No. II, spectral band width 35 rnp; Cenco spectrophotometer, with even 2.5 rnp minimum ’ bandpass). Filter photometers generally have cells of the last mentioned type. In general it cannot be said that a better precision is obtained in analysis with a photoelectric apparatus than with a visual photometer. Great precautions are necessary to attain this, both in constructing the instrument and in operating it. There is a large variety of these constructions and the choice of instrument will be largely a matter of price and the ease of operation with respect to the required accuracy. Very exact spectrophotometers make it possible to obtain a precision of 0.x %, but these are by no means simple instruments. In many cases a less complicated instrument, e.g., a filter photometer, will suffice. in which the In photoelectric photometry - as opposed to visual photometry, precision of matching the reproducibility Reyerences
9.
703.
intensities
is limited by the relative intensity
of the measurement
depends on the sensitivity
variation
y
-
of the photocell
D.
700
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J. COUMOU
2 (x948)
to absotute intensity variations d1 and the precision with which the corresponding variation in photocurrent can be read. From this the relative error in extinction or in concentration can be calculatedlo as dE dc 0.434 dI -=-P_ (2) E~Io-” E c 10 This is minimum at the maximum of the function E.Io-” for E = 0.434, or: T = 36.8 %. This may be understood by plotting the curve I = I, IO-~: at low concentrations (extinctions) dc is small for a given dl-value; but the relative error dc c is great. At high concentrations the relative error increases again, since the C
slope in the intensity curve changes very slowly and thus dc becomes large at the same dI-value.
T
4 1
Kg.
3. Rclatlvc
error in photoelectric
absorption
analysis
dc Fig. 3 shows that the minimum in the - - curve is rather flat, so about the C
same precision exists over a wide range of concentrations. It is evident from equation (2) that - as was to be expected - precision increases with increasing intensity I,. 8.
METHODS
OF
MEASURING
In siqrgle-cell filzotorneters the transmission is determined from the ratio of the deflections of the galvanometer indicating the photocurrents of phototubes with the blank and with the solution in the light beam after amplification between the source and the photocell. Generally the scale of the instrument directly indicates the extinction of the solution. Rs_fcrcnccs #. 703.
VOL.
2 (x948)
ABSORPTION
ANALYSIS
701
The main error in this method follows from fluctuations in the light source. This may be fed from a storage battery, or from the main voltage, provided this is well stabilised. Furthermore the properties of the photocell and the precision in reading the galvanometer deflection limit the precision in concentration measui-kments. Generally the error is not better than about z %. Instead of reading the deflection of a galvanometer the output of the photocell can be measured by a potcnliomeler. which compensates the potential drop over a resistance caused by the photocurrent. The galvanometer is only used as a null instrument. The ratio of the potentiometer settings with the blank and with the solution gives the transmission of the latter. With a suitable compensator it is possible thus to get a better reproducibility of instrument reading. With phototubes the zero adjustment may be effected with the aid of a thermionic voltmeter and amplifier. The well-known Beckman Photoelectric Quartz Spectrophotometer operates in this way 20. This apparatus can be used in the spectral range of about 200-2000 rnp. By very exact stabilisation of the light sources (tungsten filament lamp for the infrared and the visible, hydrogen discharge tube for the ultraviolet) a precision of 0.1 o/o can be obtained with this instrument. To be independent of fluctuations in the light source, as in visual photometers, the intensities in two light beams are compared by illuminating one of two photocells by each beam. Both cells are placed in a balnncing circd, in which a galvanometer operates as null indicator. With blanks in both light beams the galvanometer is adjusted to zero. Substitution of one blank by the solution gives the instrument a deflection. Compensation is now possible: a. opticilly by measurable regulation of the intensity in the other beam, according to one of the methods indicated in visual photometry; b. electrically by a suitable compensation circuit. The principle of two different
Fig. 4. Compcnsatmg arcuit for barrwr-layer photocells: Scrlcs-opposing type (voltage-bnlancmg)
circuits for barrier-layer cells are indicated in Figs 4 and 5. R, and R, may be two variable resistances or potentiometers. With the blank R, is adjusted to References 9. 703.
,
VOL.
D. J. COUMOU
702
2 (xg48)
IOO and I~, regulated at zero indication of the galvanometer G. After the solution is placed in the light beam of the measuring photocell P1 the balance is re-established with Rz, on which the transmission is read.
Fig.
5. Compcxwrting
circuit
for Inrricr-lnycr
photocells : Pdrnllcl
type
(current-bdnnclng)
The arrangement of Fig. 5, in several modifications, is especially often employed. The same principle can be applied with phototubes (~.g.,~~+*). In the balancing circuits mentioned above the luminous fluxes on the measuring photocell are different with the blank and with the solution. With the subslitzrtion method the circuit is first balanced with the solution, which thereafter is substituted by a regulation of the intensity not in the bundle on the comparing photocell, but in the same light bundle. The photocurrent is thus always the same and the measurement is independent of sources of error such as lag and non-linearity in the reponse of the photocell. Only with this method a high precision can be obtained, even with barrier-layer cells; 0.1 o/0 is mentioned in theliterature (KoRT~~M~, p. 110).
.
FIN. 6. DIngram
of I-Illget-Spckkcr-nbsorptiomctcr
Fig. 6 shows a diagram of the Hilger-Spekker absorptiometer operating on this principle. Zero adjustment is first performed with the diaphragm D,, while the References p. 703.
VOL.
2 (x948)
ABSORPTION
703
ANALYSIS
cell, C is filled with the solution. Thereafter, with the blank the circuit is rebalanced with the diaphragm D,. The resistance R only controls the sensitivity of the galvanome ter. SUMMARY A survey is given of colorlmetric mcasurmg m&hods and the main discussed.
and photometric absorption analysis. The princlplcs of pomts In the construction of various instruments arc
L’autcur rbsumc XX l’analysc d’absorption colorimGtriquc ct photom&riquc dlscusslon dcs prmcipcs dcs mCthodcs dc mcsurc ct dcs prmclpalcs qucstrons construction dc dlvcrs instruments.
ct il fait la rclntivcs & la
Einc Ubcrsicht dcr kolorlmctrischcn und photomctrlschcn Ahsorptlonsanalysc wird gcgehen. Dlc Grunclsatzc dcr Mcssmcthodcn SOWIC dlc hnuptslchllchcn Fragcn bczuglich dcr Konstruktlon vcrschlcdcncr Instrumcute wcrdcn crertcrt. REFERENCES WC refer to some handbooks and artlclcs In which surveys arc glvcn of the methods of colorlmetric and photomctrlc analysis. Some of them also ~lvc an.dytlcal proccdurcs. Bcsidcs this, in the text rcfcrcncc IS made to a few pubhcatlons on spcclal subjects or mstrumcnt constructions. Marc of the cxtcnslvc htcraturc may bc found In the publications mcntloncd. 0. J. WALKER, AOsorp;zov~ SpcclropirolomrLry and tfs Applrcattons, London 1939 (glvcs an ample blbhography of applications of Absorption Spcctrophotomctry from x932-1938). a F. MiiLLER, Phystkaluche Methoden der Analylrschcn Cherme, 3. Tell : Die photoclcktrlschcn Mcthodcn dcr Analyst, Lcipug 1939. 3 G. KoRTihr. Kolorimelne und SpcIILmIpholo,netrae, Berlin 1942. 4 Tn. R. P. GOB, Opfacal Methods of Chenrtcal Analyszs, New York 1942. ti B. LANGIS, Z
Received April 3rd, 1948
704
D. J. COUMOU
VOL.
2 (x948)
DISCUSSION In connection with the paper, the author showed some slides with diagrams and pictures diffcrcnt types of inst.rumcnLs for photometric absorption nnnlys~s, availnblc at the market. Miss N. 0. M. MAGETFIORN (Dclft) draws :Lttcntion to the fact that thcrc exists an cxccllcnt Dutch instrument with prism monochromator, the Gblccta colorimctcr, from Blcckcr, Utrccht.
of
Mr N. SIRAFFORD (Manchcstcr) strcsscs the need for an adcquntcly cooled lnmphousc in photoclcctric absorptiomctcrs, and also for a thcrmostntlcnlly controlled iackct for the absorption cells, CspcciiLlly when an accuracy of f 0.5 o/Ois rcquircd. Mr G. I-I. UEAVEN (Chcstcr) supports this remark and dcsircs more working space in the ;tbsorption ccl1 comp,Lrtmcnts. Mr J. G. REYNOLDS (Chcstcr) points out the ncccssity to rcchcck calibration CiLlly, bccausc of the dctcrior:tiion of the photocells.
graphs pcriodi-
Mr G. H. BEAVEN (Chcstcr) rccommcnds the USC of stable gl,Lss filters for checking the ;Lccuracy of photoclcctric instruments. \Vith a Wood’s glass, calibrated for the ncnr ultrnvrolct, hc found in a Bcclcmnnn spcctropholomctcr (with phototubcsl) over a period of five years continual USC the imlial st.Lbility of the Lnstrumcnt .Lnd the accur,.Lcy of the clcnsrty scale fully maintained.