Determining Only One Compound in a Mixture Short Spectrophotometric Method EUGENE ALLEN1 AND WILLIARI RIEhIAN 111 School of Chemistry, Rutgers University, New Brunswick, 1V. J . In setting up a routine spectrophotometric method for a mixture containing many radiation-absorbing compounds, often only one of them is to be determined. In such a case, it is wasteful to take enough readings to determine all the compounds by simultaneous equations. Mathematical treatment of this situation results in some novel equations which make it possible to take readings at only two or three wave lengths, no matter how many compounds are present. This method will not always work; certain relationships must hold. Because the number of readings and the calculations are both cut, the analysis becomes much simpler. The well-known base-line technique, when used algebraically, is a special case of the method described in this paper.
I
N SPECTROPHOTOMETRY, readings are usually taken a t
as many different wave lengths as there are absorbing compounds present, and are then substituted in a set of simultaneous equations. In this way, the concentrations of all the unknown substances in the test solution can be learned. Stearns ( 4 ) gives an excellent discussion of the method of conducting such an analysis, and also describes the various ways of calculating the results. Often it is not necessary to determine all the components of a miuture; there are many cases where only one is of interest. This paper deals with the problem of determining one component in a multicomponent system in the light of a new mathematical treatment. The mathematics shows how such a determination can be made by taking absorbance readings a t only two or three wave lengths, no matter how many compounds are present Certain relationships must hold in order for the method to work. This simplification should prove useful when the analysis is to be run on a routine basis, Shurcliff ( 3 ) has proposed an ingenious short-cut graphical method for determining one component in a three-component mixture, using only two wave lengths. The two-wave-length method described in this paper is really an extension of Shurcliff’s method, even though the method of derivation is different. The “base-line” method is familiar to many spectroscopists (1, I, 5 ) . This method is used when the compound to be determined has a sharp absorption peak and the other compounds have a linear absorbance-wave-length curve in this region. The height of the peak of the sample curve above the background line is a measure of the concentration of the compound being determined. However, the base-line method can be applied algebraicallyas well as graphically(1). When soused, it is really a method for determining one component in a multicomponent system by taking readings a t three wave lengths. The base-line method is a special case of the method presented here. This paper provides a theoretical treatment of both the twoand the three-wave-length methods, and offers experimental evidence of the validity of the two-wave-length method. Experimental work on the three-wave-length method will be presented in another paper.
lengths. The length of light path will be considered to be unity throughout , The basis of the conventional method of conducting multicomponent spectrophotometric analyses lies in the following set of simultaneous equations:
++ ++ .. .. .. .. .. ++ ............................ + ..... A, = cIain + -41 =
cIaI,
AZ =
CIUI~
cNaN,
]
CNUN~
...
i-’eNaNn
(1)
...
This series of n equations has n unknowns, CI, CII, C N , and ran be solved simultaneously to give values for the unknowns, In a conventional spectrophotometric analysis on a multicomponent system, a single component can be determined by taking a reading a t only one Rave length, provided that all the other components have zero absorptivity a t this wave length. From the first of the set of foregoing simultaneous equations, A I = CIUIi
+
CIIUIIi
+ .....+
CNUNi
(2)
I
if
Thus, the simplest method of conducting a multicomponent spectrophotometric determination of one component is to find a wave length a t which only the component to be determined, I, has any absorptivity, and then make use of Equation 3. However, if such a wave length cannot be found, this simple approach vannot be used. Let us define tn-o new symbols, 9 and 9, as follows:
(4)
where 01, (3, and y are positive constants.
THEORETICAL CONSIDERATIONS
Development of Basic Equation. Let I, 11,111,. . . . . . N refer to radiation-absorbing components, 1, 2, 3, ...... n to wave lengths, A to absorbance, a to absorptivity, and c to concentration. Thus, U I I ~means the absorptivity of component I1 a t wave length 3, and similarly for the other components and wave
cIIaII1 cIIaIIz
9 =
C Y A~ p-42 - ?AI
(5)
It is never necessary in the analytical procedure described in this paper to take readings a t more than three wave lengths. Combining Equations 5 and 1, 77-e find that
+
Present address, American Cyanamid Co., Caloo Chemical Division, Bound Brook, N. J. 1
1325
= a(cIa1,
++
P(CIUI,
cIIaIIl
+++ .. .. ... .. +++ . . . . . +-CYUN,) CNUN~)
CIIUII~
~(CIUI,
CIIUII~
CNUN~)
1326
ANALYTICAL CHEMISTRY
Rearrangement of Equat,ion 6 gives 0
Application to Method Involving Readings at Two Wave Lengths. If y = 0, Equation 4 becomes
= Cdaaxl - Pare - YUIJ 3. cIr(O1aIII - Barr* - yarx,) f ..................... -t c.v(aa.v, - BaSz - ? a x 3 )
- Pur2 1 . . . . . . . . . . .. . palrg ....
$1
611
+
CII+II
+ . . . . . + cs4s
t
(9)
6.v = anz-, - pas?j
or, in view of Equation 4, 0 = cr+r
= sax, = LYaIII
and Equatioii 5 beconies
+ = O1.41
(7)
+
Thus, the function has a formal resemblance to absorptivity, and the 0 function has a formal resemblance to absorbance. This can be seen from the formal resemblance between Equations 7 and 2.
- PA,
(10)
Combining Equation 9 with the restriction 611, 6x11.. . +.s = 0 # 41, wc find that awrr1
- Pun, = 0 - Pa1112 = 0
ffa111,
iti
Equation 8 that
1i
................ aa,vi - pax2 = 0 j ~ U I BRI, ~ Z 0 J
(11)
Rearrnngrment yields am = azl = ..... atr? arrr?
= -c(.Sl
n .s,
-- K # -
:: I
(12)
where I< = p / a Thus. thc restriction that +IX,+III . . . 4~ = 0 # 61 can be expressed by saying that the ratio of the absorptivity a t one wave lengt,h to the absorptivity at t,he other wave length must be the same for all the eomponrnts except that nhich is to tie determined. Substitution of Equatioiis 9 ant1 10 in Equation 8 yirlds COMPONENT
CI =
II
n.41 - P A ? a n 1 , - par,
Di~itlixrgnumerator and denominator by a,n r gcAt .l,
P -4.
- -
= ___-
CI
ax, -
rrr:
I n view of Equation 12
COMPONENT 'II L
I
WAVELENGTH
Figure 1. Two Suhstances of Same K Value
This formal relationship can now be carried still further, to derive an equation analogouq to (3), as follows: From Equation 7, we see that
Equation 8 is the fundamental equation of this paper.
I t tells
us that one component, I, in a misture of components, I, 11. I11 . . . N, can be determined by dividing the quantity ip by the constant +I, on the condition that the constants 4x1, 6x11. . . $.v can be made equal to zero. 9 is calculated from the absorbanrc readings as indicated by Equation 5. The problem is how to choose wave lengths 1, 2, and 3, and constants 01, p, and y , so a s to make + I X , + I I I . . . @N = 0. The treatment is divided into two parts: (1) the method by which readings are taken a t two wave lengths; and (2) the method by which readings are taken a t three wave lengths. In the case of the two-wave-length method, y must he zero.
Equations 12 and 15 are the working equations for the simplified determination of one component in a multicomponent system by taking readings a t two wave lengths. T o use these equations, i t is necessary first to find, if possible, two wave lengths such that, the ratio of the absorptivity a t one wave length to the absorptivity a t the other wave length is the same for all the eomponents except that n-hich it is desired to determine. The valucx of this ratio, K , is subst,ituted in Equation 15 togrther with the values for the absorptivities of component I a t thwe same wave lengths. ax, and ax,, and the measured absorbariws of the solution a t these Rave lengt,hs, A 1 and A ? . Solving t,hr ('(Iuiition will give t,he concentration of the component sought. The location of such two imve lengths can be effected simply ti>- a graphical method which is described in detail in a lat.er section. Figure 1 shows an esample of two substances which have the) same K value, 0.5, at the two wave lengths designated. Once the equations have been derived, they may be iiimlilified ns follows for use in the control lalmratory: CI =
PA,
-
Q.42
wlif,rv P nnd Q are constants which ('a11l w calculated as follo\v-::
Equation 15 is identical in form with the convention;il equation used to calculate the, 1vsulti of txo-component :iiinlyqes
V O L U M E 25, NO. 9, S E P T E M B E R 1 9 5 3 For the two-romponent case, K = conies
UIIJUII~
1327
and Equation 15 be-
i
(19)
Rearranging and dividing by a,we have or
CI =
(LIIn-il
aII,ur,
- UII,A? - aII,arz
This rquation is identicd with that obtained by solving =
CIaIl
A2 =
CIUI,
A1
++
......................
i
CIIUII, CIIaII*
simultaneously, Equation 18 being Equation 1 rewritten for two components. Thus, a multicomponent analysis by this method is really considered to be a two-component analysis, th(1 first component being the substance which is being determined, and the sccond componcwt being everything else. It also follon-s that for the analysis of two-component mixtures, Equation 15 offers no advantage ovcr thr c~orivoiitionalmethod. bring, in fact, identi(~11 ~ i t it. h
Define JI a3 follows:
or
I -
'J
-p
.t!l
ABSORPTIVITY
Substitution of Equation 21 in Equation 20 yields 1 - 111
+ 1 -JI M
[a,,,:
=
nlll.
a],,.
............................
] 1 -JI
COkPONENT
II
llultiplyirig the cxpression within brackets by iM and dividing the cocfficient outside the brackets by the equivalent of M , or
&,
we have
40
((111,
30 -
P+r [ N a I l I %+ (1 - *%I)a11131 ...................... = B+r [-ll(c,s, + (1 - Jl)u'val z 8--*'+ [Ala,ri + ( I - J J ) ~ ~ J
=
CY
a" (11,
or
_ -+ MUHI?+ (1 - M ) ~ I I I , Ma.,-? + (1 - Xin.vv,- jT' # Mal, + (1 - M)a1, =
UIII
a1111
X ~ I I (1 ~ - JI)arIa
WAVELENGTH
Figure 2.
Two Substances of Same k" Yalue
Application to Method Involving Readings at Three Wave Lengths. l I a n y multiconiponent systems cannot be treated by the method of the previous section because two wave lengths cannot be found which satisfy the requirements of Equation 12. Then the system must be examined to see if it can be set up so a b to determine the desired constituent by taking readings a t three wave lengths. The location of these three wave lengths is somenhat more cumhersomr> than the location of two wave lengths described in the previous wetion. Yet, once the proper wave lengths have been found and the equation for the calculation derived, thr performanrc of the analysis and the calculation are simple and well adapted to routine use. Whcn readings a t three wave lengths arc taken, y # 0, and Equations 4 and 5 are used as such. Combining Equation 4 with the restriction of Equation 8, we find t h a t
a11
U.V, _____.__.--
h"
where
I,,
-i
I
=B __ +Y
a
Sirice 3f, by definition, is a fraction which is less than 1, the expression MUN, (1 - J I ) u N ~ can be considered to be a wightcd average of the absorptivitiw of .V at. wave lengths 2 and 3. Thus, the restriction that 411,+III . . . +N = 0 f 61 can be expressed in the case of the three-wave-length method by saying that thc ratio of the absorptivity a t one wave length to the weighted average of t,he absorpt'ivities a t the other two wave lengths must be the same for all the componerlta except the one which it is desired to determine. Substitution of Equations 4 and 5 in Equation 8 yields
+
e1
=
aAi aml
-b4s - Y A - Barr - Y a l a
~
ANALYTICAL CHEMISTRY
1328 Dividing numerator and denominator by
01,
For use in routine analyses, the equation can be simplified as was done before for the case of readings a t two wave lengths:
n e have
CI = P’Ai
- Q’Az - R’A3
where P’, Q’,and R’ are constants calculated as follows:
P
Y
ff
01
Since -’= K’M and - = S’(1 - M), CI
A1 = aI,
‘’ UI,
- K‘ [ M A z+ (1 - M)A31 - K’ [XUI,+ (1 - A!f)arg1
K‘M - K’ [MuI, (1
+
- Mll)a1,1
The “base-line technique,” long known among spectroscopists, can be applied either graphically (6,5 ) or algebraically ( I ) . When applied algebraically, it can be shown to be a special case of the foregoing more general treatmrnt, in which A,
(27)
= XI
K‘ = 1
and
(28)
From 25 and 27 we obtain, for the base-line method,
\ 40
i I
‘ It I1
Figure 3 shows two materials which have linear absorbance between wave lengths XI, XZ, and X B ~for which these equations a p ply. The entire curve of the substance need not be linear in this region-it is necessary only that the absorptivities a t the three wave lengths in question lie on a straight line. Substituting Equations 28 and 29 in Equation 22, we have
4
COMPONENT
A2
I-
a11,
m I A l=x.lL
A3
WAVELENGTH
Figure 3.
Two Substances for Which K = 1
Equations 22 and 24 are the working equations for the determination of one component in multicomponent systems by taking readings a t three wave lengths. In using these equations, three wave lengths must first be found such that, for all the components except that to be determined. the ratio of the absorptivity a t one wave length to the weighted average of the absorptivities a t the other two wave lengths is constant, in accordance with Equation 22. (Of course, the same weight must be used in calculating all the weightrd averages.) Then the values of K’, the ratio, and M , the weight applied to the average, are substituted in Equation 24 together with the absorptivities of I at the three wave lengths and the three absorbance readings. This will give an expression for the concentration of I. A convenient method of locating the three wave lengthe to be used is under investigation a t the preeent time. Figure 2 d l l serve to illustrate thc meaning of these relationships more clearly. I t can hr seen graphically that
Rearrangement of Equation 30 gives a111 -_U I I J -=_ an, - UII, Ae - X3 U I I I l - U I I I ; - A1 - A3 a1112 - UIIIg X? - X3 1 .................... i a.v1 - X3 _ - -A1 U_ N2 _ - aN3 a.v3 X? Xa
-I
I
- a13 a12 - UI, a11
A, - [ ‘&-42
+
- A3 A3
1
1
J
Thew equations, except the last, are equations of straight lines, and shon that a t the three wave lengths XI, X p , and X3 the absorptivities of all the compounds except I lie on a straight line. This is the condition for the application of the base-line method. Substituting Equations 28 and 29 in Equation 24, we have
CI =
where A,, is the same for all the components. For both substances in the figure the ratio, K’, of al to Mu2 (1 - M ) a t is equal to 2.
AI
#XZ -
- A3 a11 - [ A1 x,-^ A3 ,a12 Xz
+ (1 + (1
- -3)Aj] x> - Xa - *2)a13]
(32)
This equation corresponds to Equation 9 in ( I ) , and is one form of mathematical expression of the base-line method.
1329
V O L U M E 2 5 , NO. 9, S E P T E M B E R 1 9 5 3
ments of x and y and use an equation of the type of 15, even though there may be many compounds present in the mixture. By the conventional procedure we would have to determine as many physicochemical properties as there are unknown compound s .
Rearrangement of Equation 32 yields
PRACTICAL METHOD USING TWO WAVE LENGTHS
Equation 33 shows that the concentration of the desired constituent is proportional to the height of the peak of the absorbance curve a t wave length I over the straight line drawn between the absorbances a t wave lengths 2 and 3, a relationship also characteristic of the baseline method. Figure 4 will hplp to make this clear. Thus, the baseline method is a special case of the more general treatment of determining one component by taking readings a t three wave lengths.
Method of Location of Wave Lengths. In order to apply the method described in the preceding section, it is necessary to locate t n o wave lengths such that the ratio of the absorptivity a t one wave length to the absorptivity a t the other wave length (the K value) is the same for all the components except that n-hich it is desired to determine. Fortunately, it is not necessary t o pick pairs of wave lengths on a trial-and-error basis and then calculate ratios of absorptivities for each pair of wave lengths for each compound. There is a much simpler graphical method for selecting the wave lengths, One can proceed as follows:
...
For each component in the system, I N , plot the logarithm of the absorbance as ordinate against the wave length as abscissa on transparent paper. The solutions used to obtain the absorbance data need not be of any particular concentration, nor need all the solutions be of the same concentration; any convenient concentrations will do. Then, assuming that I is the component N to be determined, superimpose the curves of components I1 on each other so that they may be viewed simultaneously over a strong light and align as to wave length. Now move the curve8 vertically with respect to each other until two wave lengths are found such that all the curves intersect each other a t these two wave lengths. These are the required wave lengths, provided that the curve for I cannot be made to intersect the others at both wave lengths. These two wave lengths, then, will satisfy the r q u i r e d conditions of Equation 12.
...
22
'hl
WAVELENGTH
Figure 4.
Calculations of Base-Line Method
The utility of the base-line procedure lies in the fact that it is very easy to test an absorbance curve for linearity-simply lay a straight edge along it. Thus, it is only a matter of minutes to determine if Equations 31 hold. However, the more general method is of wider utility. After the wave lengths have been located and the method of analysis set up, it is no easier to conduct analyses algebraically by the base-line method than by the more general method embodied in Equations 22 and 24. The equation for calculating the results by the base-line method boils down to the form of Equation 26, just as does the equation for the more general method. One point about the baseline method will bear mentioning here-the method will work just as well if the wave length of high absorption is located outside the other two wave lengths instead of between them. Extension beyond the Field of Spectrophotometry. These relations are not limited to spectrophotometric measurements, but may be used in many situations where analyses are conducted by simultaneous equations. Suppose, for example, that x and y are two additive physicochemical properties and that the ratio of z to y is constant for all the compounds in a certain class. Assume that we have a mixture of several compounds in that class plus one compound not in that class. Then, if nre wish to determine the amount of the compound not in that class, we would only have to take measure-
WRVLLLNGTH, m p
Figure 5.
-- --- --
Transmittance Curves
Pyridine @-Picoline 2-Aminopyridine 2-Aminopyrimidine 2-Amino-4-methylpyrimidine 6;00 Z-Amino-4,6-dimethylpyrimidine
---
To prove this, let us call the two wave lengths 1 and 2. The fact that the curves of compounds I1 . . N intersect a t these two wave lengths means that the difference between the two respective ordinates is the same for all the components from I1 to N or log AIII, - log A I I I ~= . . . . . log A111 - log AII, log ANI - log AN^ # log AI, - log AI, (34) Srbstitution of ca for A at each wave length, for each component, gives log cI1aII. - log cIIaI12 = log cIImI1, - log C I I I ~ I I I=~ . . . -
.
. .
log CNUNl - log CNUNZ # log CIUI,
or
an, am log-a m = log-aIm =
- log CIUI,
a1
.. . . . = log - # log -'arr
Removing logs, we have Equation 12.
aN1 aNp
(35)
ANALYTICAL CHEMISTRY
1330
Compound Pyridine 8-Piooline 2-Aminop yridine 2-Aminop yrimidine 2-Amino-4-methylyrimidine 2-X mino-4,6-dimethylpyrimidine
...
...
4,1476 1,1532 0.8054
These calculations are shown in Table I. The absorptivities of pyridine a t both wave lengths were also determined as shown in Table I. Insertion of the values K = 0.3070, UI, = 34.98, and axp = 10.74 in Equation 15 gives the equation used for ralculating the concentmtion of pyridine.
3 .i6
...
1.2115
cpSr(gram,?per liter)
3 82
...
1.3194
Determination of K , a,, and arl
Table I.
Concn., G . / Liter
K AM
dm
=
A~cc/A265
0.02207 0.01729 0.05000 0.05000
0.722 0.237 0.243 0.823 0.197 0,673 0.138 0.473
0.2952 0.2927 0.2918
0.06000
0.178 0.523
0.3403
0.05000
0.191
0 553
0 3454
CTG., ;.?&.,
Cm.
Cm.
34 14 3 2
98 05 94
10.74
70
,..
aPM,
( A d Azd
....
0.03157 A,,,
Zarct = 28.13
Z { azu(Az~/Azss)1 = 8.6370 Kay
= 8.6370/28 13 = 0.3070
Table 11. Results on Synthetic Mixtures for Six-Component System
8-Picoline 16.5 5.0 5.0
5.0 5.0
Per Cent in Mixture. b y Synthesis 2-Amino- 2-Amino2-Amino- 4-methyl4,6-di2-Aminopynrnipyrimimethylpypyridine dine dine rimidine 16.5 62.3 5.0 5.0 5.0 13.0 5.0 18.0 5.0 15.0 5.0 10.0 27.0
16.5 5.0 62.3 5.0 5.0 13.0 5.0 18.0 5.0 15.0 53.5 10 0 5 0
16.3 5 0 5.0 62.3 5 0 13.0 5.0 18.0 71.2 15.0 5.0 10 0 5.0
16.3 5.0 5.0 5.0 62.3 12.7 44.7 19.2 5.0 13.5 5.0 7.0 5,o
Pyridine Found,
Pyri-
dine
A944
Am
%
17.7 17.7 17.7 17.7 17.7 35.3 35.3 8.8
0.566 0.507 0.470 0.497 0.503 0.823 0.783 0.429 0.361 0.689 0.620 1 075 1.055
0.870 0.722 0.609 0.637 0.657 0.805 0.650 0.884
18.9 18.0 17 9 19.0 19.0 36.4 36.8 10 0 10.5 27.5 27.1 53.4 53.3
=
- 0.00969 Aads
Several synthetic solutions were now prepared as shown in Table 11. The total concentration in each synthetic solution was 0.05000 gram per liter. Several different concentrations of pyridine were chosen, and for each concent,ration the amounts of the other components were varied several waj-s. Each solution was measured at 244 and 266 mp against 0.1 iV hydrochloric acid as reference. Results Fere calculat'ed as per crnt pyridine in the mistuw, as follows:
*
%pyridine = 0.05000
loo
Results of thcsr experiments arr shown in Table 11. The results in Table I1 show that the values for per cent pyridine are fairly close to the truth, 0.632 8.8 5.0 0.827 26.6 15.0 irrespective of the concentration of pyridine or 0.620 26.5 5.0 0.747 53.0 10.0 the composition of the remainder of the solu0.684 53.0 5.0 tion. The average relative error is 5.5%. The results are better a t the higher concentrations of pyridine than a t the lower concentrations. It is felt that if there had not been so much deviation in the K valThis is an illustration of the usefulness of log absorbance plot., ues of the components (Table I), the results would have been as described by Stearns ( 4 ) . closer to the truth. Experimental. The determination of pyridine in a six-comI t was now decided to investigate the determination of pyridine ponent system consisting of pyridine, ,&picoline, 2-aminoin a three-component system consisting of pyridine, 6-picolinr. pyridine, 2-aminopyrimidine, 2-amino-4-methylpyrimidine,and and 2-aminopyridine, because the K values of @-picolineand 2Zamino-4,6-dimethyIpyrimidine was investigated. A l l work was done in 0.1 N hydrochloric acid solution. Solutions of each of these substances a t a concentration of A 0.0125 gram per liter were prepared. Transmittance curves on I \ these solutions were run on the Beckman Model DU quartz spectrophotometer with a hydrogen-discharge lamp and silica cells. These curves are shown in Figure 5 . The same curves were plotted 011 a scale implicit in log -4 (esplicit in 2'). These curves were then aligned as to wave length and adjusted vertically until two wave lengths were found at which the curves of all the components except pyridinr intersected. As can be seen from Figure 6, 244 mp and 266 rnp are two such wave lengths, Some of the curves do not quite pass through these two points, but the amount of deviation is not great; 244 mp was considered to be wave length 1 and 2GG infi wave length 2. I n order to determine the K value as piecisely as possible, somewhat more concentrated solutions of each of thr compounds were prepared so as to get absorbance readings on a convenient portion of the scale. Each of the solutions was now rend at 241 mp and 266 mp, and the ratio of the two absorbances, . 4 2 4 4 , " l 2 ~ . was determined for each. As the values for A214/A266were not exactly equal for all thr compounds, it was necessary to calculate an average to use as 6. T o do this, it was assumed that all the compounds (except pyridine) would be present in equal amounts, and accordingly the amount of light absorbed bv each compound would depend on its absorptivity. Therefore, the individual K values were weighted WQVELENGTH, r n ~ by a244to calculate the average K , a$ follows: Figure 6. Transmittance Curves 13.0 5.0
18.0
__
------
Pyridine 8-Picoline 2-Aminopyridino
-. .-.,
2-Aminopyrimidine 2-Amino-4-methylpyrimidine
0 0 0 2-Amino-4.6-dimethylpyrimidinn
V O L U M E 25, NO. 9, S E P T E M B E R 1 9 5 3 Table 111. Totai Concn., G./Liter
0,03824 0.01808 0,03876 0 04001 0.02026 0.01985
1331
Results on Synthetic -Mixtures for ThreeComponent System Per Cent in Mixtiire, hrSyntliehij 2-4minopyridine Pyridine 52.0 27.4 20.6 27 5 29.0 43.5 20.3 I4 1 25 6 39.8 a2 4 7.E 77.5 7 .i 1$.7 79 1 15 9 , 0
8-Picoline
~
.
~
iiw
A266
0.575 0.318 0 887 0 855 0.579 0 SQl
0.779 0.520 0.757 0.625 0.304 0.347
Pyridine Found. “0 28 4 28.6 53.9 52.7 76.0 77 4
The average relative error of these values is 1.7%. These results show somewhat better accuracy than those in Table 11, as may be expected in view of the better K value. ACKNOWLEDGMENT
The authors are indebted to R. €1. Kienle and William Seaman for making available to them the facilities of the American Cyanamid Co., Calco Chemical Division, for the experimental work. They also wish to thank E. I. St,earns for helpful criticism and suggestions.
__
_. . ~
LITERATURE CITED
aminopyridine are close to each other (Table I). Therefore, a8 there is little deviation of K values of the components not being determined, the accuracy of the results for pyridine should be much improved. The equation used for calculation of the pyridine concentrations are derived from the same u values as were given in Table I and a K value of 0.294. The equation is cpyr (grams per liter)
- 0 . 0 3 1 4 3 A ~- 0.00924Am
Several synthetic solutions were made, as shoivn in Table I11 hhqorbance readings and percentages of pyridine found are also given in Table 111.
(1) Banes, F. W., and Eby, L. T., ISD.ESG.CHEM.,ANAL.ED., 18, 535 (1946). (2) Heigl, J. J., Bell, BI. F., and White, ,J. U . , AXAL.CHEM.,19, 293 (1947).
(3) Shurcliff, W.A,. unpublished information. (4) Steams, E. I., in Mellon, M. G., “Analytical Absorption Spectroscopy,” Chap. 7 , Xew York, John Wiley & Sons, 1950. ( 5 ) Wright, N., IND.ENG.CHEM..ANAL.ED..13, 1 ( 1 9 4 1 ) . RECEIVED for review January 8, 1953. Accepted J u l y 6, 1953. Abitracteri from a portion of the thesis submitted by Eugene Alien in partial fulfilliiient of t h e requirements for t h e degree of doctor of philosophy at Rutgers University. J u n e 1952. Presented in part a t the Meeting-in-Miniature of the North .Jmsey Section, AMERICAN C ~ ~ l i i cSOCIETY, .4~ Newark, N. J.. .Janilarq. 2 8 . 19z2.
Spectrophotometric Determination of Zirconium A. D. HORTOK Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. Development work is being continued on the use of 2-(2-hydroxy-3,6-disulfo-lnaphthy1azo)benzenearsonic acid [l-(o-arsonophenylazo)-2-naphthol-3,6-disulfonic acid, Thoron] as a spectrophotometric reagent. Its previous use for the determination of thorium(IV), together with experimental results on its formation of complexes with tin(IV), titanium(IV), hafnium(IV), and zirconium(IV), indicates that it may form complexes with all quadrivalent metal ions. The reagent has been used successfully for the determination of zirconium. The method covers a range of 10 to 1007 of zirconium with a precision within 5.75% in the 10-7 range and 2.9yo in the 100-7 range. The method is simple and rapid and does not require a skilled analyst. The reagent, although complex, is now- available commercially, and requires little preparation. There are very few interferences which cannot be reduced to noninterfering states, or removed from the sample without loss of zirconium.
I
N
1038, Liebhafsky and Winslow ( 3 ) developed a method for det.ermining 10 to 1007 of zirconium. This method makes use of t,he stable lakes formed Kith zirconium by alizarin, quinnlizarin, or purpurin. Hapes and Jones ( 1 ) have developed a procedure for zirconium in steel, which is dependent upon the color produced whrn thp zirconium salt of p-dimet~hplaminoazophenylarsonicacid is di+ solved in ammonium hydroxide. This method requires 30-minute digestion after precipitation, plus a tloublr filtration. It will determine 0.02 to 1.0 mg. of zirconium. 2-(2-Hydroxy-3,6-disulfo-l-1iaplith~lazo)benzenearson~c acid (Thoron) shows promise as a reagent for the spectrophotometric determination of most quadrivalmt metal ions. It has been used for the determinat,ion of thorium ( 5 ) and fluoride (b), the latter made possible by its diminution of the rolor of the thorium-2-(2hpdroxy-3,6-disulfo-l-naphth~lazo~henzrnearsonic acid comples. EXPERIMENTAL
Aliquots of 0.25, 0.50, and 1.0 ml. of the zirconium oxide standard, which contained 1 0 0 ~of zirconium per ml., were pipetted into separate 10-ml. volumetric flasks. To each flask
\Tere added 7 drops of concentrated hydrochloric acid and 1.0
ml. of 0.2% 2-(2-hydroxy-3,6-disulfo-l-n~phthylazo)benzenearsonic acid reagent. These samples \\-ere diluted to 10 ml. with distilled water. A reference was prepared by diluting 7 drops of concentrated hydrochloric acid and 1.0 ml. of 0.2% 2-(2-hydrox\ $6-disulfo-1-naphthy1azo)benzenearsonic acid reagent to 10 ml ivith distilled water. The spectral absorption curve was determined over a navvIrngth range of 522 to i20 mp. The absorbances of thc compl=vs were checked a t 30-minute intervals, and the development of thtb color was found to be completp after 2 hours I t was found also that the color of the zirconium-2-(2-hydroxj 3,6-disulfo-l-naphthylazo)benzenearsonicacid complex could be developed completely by heating the solutions to 7 5 ” C., and maintaining them a t 75” to 80” C. for 5 minutes. The sample. %erecooled to room temperature before absorbance readings were made. The spectral absorption curves for zirconium and hafnium complexes of 2-(2-hydroxy-3.6-disulfo-l-naphthylazo)benzenearsonic acid are shown in Figure 1. Standard calibration curves piepared by either of the above methods are identlcal, and they conform to Bec.i’g Ian.
