V O L U M E 25, N O , 1 2 , D E C E M B E R 1 9 5 3 and sample cells with cyclohexane and determine the cell absorbance corrections a t 309, 311.5, and 318.5 mp with the spectrophotometer using a slit width of 0.40 mm. Empty the sample cell and fill it with the sample solution. Measure the absorbance a t 311.5 mp and 0.40-mm. slit width. If the absorbance is greater than 0.600, dilute 50 ml. of the solution to 100 ml. with cyclohexane. Using the same slit width, measure the absorbance of the final sample solution a t 309, 311.5, and 318.5 mp. Correct the observed sample absorbances for CALCULATIONS. the cell absorbance corrections previously made. Using the corrected sample absorbances, calculate the base line absorbance of the naphthalene a t 311.5 mp by means of the following formula:
+
A B = (0.74 X A ~ O B ) (0.26 X
A318.j)
Subtract this absorbance from that read a t 311.5 mp to give the absorbance from naphthalene only:
-4 = A
3,I.b-
A B
G = -1543 A 9.15 V where G = grains of naphthalene per 100 cubic feet of gas and TT = volume of gas sampled, cubic feet If dilution of the initial 100-ml. sample volume was necessary, the proper dilution factor must be incorporated into the above calculation. Interferences. No adverse effects on the determination of
1821
naphthalene by this method were observed from the presence of light oils (benzene, xylene, and toluene) when added to naphthalene-cyclohexane solutions. These light oils are the most likely interfering compounds present in coke oven gas. Accuracy and Precision. The accuracy of measuring naphthalene in cyclohexane by this method was determined by analyzing five samples, each containing a known weight of naphthalene in cyclohexane. The 95% confidence limits as determined from this dat,a are =k0.42%. Precision was determined by analyzing 10 separate portions of one of the standard samples which contained about 10 mg. of naphthalene per 100 ml. of cyclohexane. The results show a relative error for precision of 1.67,. The data for the accuracy and precision tests are hbulated in Table V. LITERATURE CITED
Altieri, V. J., “Gas Analysis and Testing of Gaseous Lfaterials,” pp. 400-3, New York, American Gas Association, 1945. (2) Altieri, V. J., “Gas Chemists’ Book of Standards for Light Oils and Light Oil Products,” pp. 232-4, Kew York, dmerican Gas ,issociation, 1943.
(1)
(3) Ibid., p. 235. ( 4 ) Reichardt, P. E., and White, D. L., IND. ENG.CHEM.,h . 4 1 . . ED., 18,286 (1946). ( 5 ) Schubert, S.,Gas-u. Tt‘ussrrfuch, 92, 277-8 (1951).
RECEIVED for review
April 20, 1953. ilccepted September 17, 1953.
Flame Spectrophotometric Determination of Sodium and Potassium In Viscous Solutions or Plant Extracts HOWARD M. BAUSERMAN AND ROB ROY CERNEY, JR.’ Research Laboratory, American Crystal Sugar Co., Rocky Ford, Colo.
A new method was needed for the routine determination of sodium and potassium in plant extracts, using the Beckman Model DU spectrophotometer and its flame attachment. The principle of operation of this instrument is such that viscosity and other characteristics of the fluid under test affect the flame color intensity. When properties of the fluid carrier alter the flame color intensity, this instrument is ideally suited to the application of a lithium internal standard method. Synthetic organic mixtures in solutions containing known quantities of the elements in question were tested. The apparent concentrations of sodium and potassium found spectrophotometrically could be corrected by use of concentration correction factors. Based on this work, a controlled analytical method is developed for solutions having characteristics differing from water.
T
HE instrument used in all the following experimental work was the Beckman Model DU spectrophotometer and the Model 10300 flame attachment, described by Gilbert et al. (6). It is set to measure the intensity of one of the characteristic wave lengths of light produced by exciting the element in question in an oxygen-natural gas flame. In this instrument the rate of introduction of the solution containing the element is controlled by causing it to pass through a glass capillary tube under a constant pressure differential. When the unknown solution has the same physical characteristics as those of a known standard solution, the rate of introduction vi11 be the same, and the line intensities can be directly related to concentrations (3). Often the material to be tested will have various kinds of organic matter as contaminants. The resultant changes in viscosity and surface tension may cause the unknown sample to be introduced a t a rate different from that of the known standard solution (4,IO). Various authors (1, 2, 7 , 9. 11-13) have proposed wet or dry aahing of the sample to remove organic matter 1
Present address, Ides1 Cement Research Laboratory, Boettcher, Colo.
or have proposed to ignore its presence. To avoid ashing, other Rorkers (4, 8, 10) have proposed that the same amounts of organic matter be added to the standard solutions as were contained in the unknown solution. If this method is used, the samples must contain a relatively constant concentration of the contaminants. When the unknown solutions have variable physical characteristics, particularly in regard to viscosity, the rate of introduction varies and a problem presents itself, as the concentrations are not directly correlated to line intensities. Such a problem, x-hich becomes acute in the case of certain plant extracts, can be solved by the introduction into the solution of a rarely occurring element, in this case lithium. Lithium has been used as an internal standard in other t \ pes of flame photometers for a number of years (4, IO). Its use, however, with the Beckman instrument for this purpose has been discouraged (6) because of uncompensated errors that may arise. As these errors may be significant in precise work, they should be appropriately compensated by the means of the conditions stipulated below. Errors due to interference caused by the presence of ions other
ANALYTICAL CHEMISTRY
1822
than sodium and potassium are Table 11. Instrument Settings Used to Prepare Calibration Curves not considered here. This phase has been covered ( 1 , 3 - 7 , 9 , 1 2 ) . Pressures This laboratory has had the Sensitivity, (950 Natural B,t.u.), gae project for the past 3 years of Concn. of Turns from Slit Waye Oxygen, Air, em. isoStd. PS. Selector Full ClockWidth, Length, inches Ib./sq. propyl determining sodium and potasElement 100% T Switch wise him. w water inch alcohol sium in solutions with slightly Li 375 0.1 2.5 0.030 670.8 36 25 4.0 Na 200 0.1 2.5 0.065 589 42 25 4.0 varying sugar content. This K 150 0.1 2.: 0.037 767 36 25 4.0 problem was partially overcome by adding sugar to the standTable 111. Emission (Transmittance Dial Readings) ards in amounts equal to the Obtained from Solutions Used to Prepare Calibration average sugar content of the solutions to be tested and standardizCurves ing the instrument under these conditions. This method was (Transmittance units) Element Element satisfactory until it was decided to analyze some sugar factory DlUS 47" DlUS 8% juices of widely varying organic content. The problem was even Element Element Element Sucrose; Sucrose; Concn., Element plus 4 % plus8% plus 10% 10% 10% more difficult when alcohol was present. If the direct method P.P.hI. Only Sucrose Sucrose Alcohol Alcohol Alcohol employing compensated calibration curves were used, the preparaLithium tion of calibration curves to fit all concentrations of sugar and 0.0 0 0.1 0.0 1.0 1.0 1.0 alcohol would be a practically impossible task. A search of the 35.2 28.2 57 1 75 51.3 43.2 36.2 5 4 . 2 7 8 . 5 6 3 . 5 5 6.2 150 7 1 . 2 4 5 . 3 literature indicated that, with care to avoid noncompensated 75.1 66.0 300 92.7 103.1 85.9 76.0 8 2 . 3 7 3 . 0 111.0 9 3 . 2 8 4 .0 375 1 0 0 . 0 errors, an internal standard might be used with the Beckman instrument (d), although no actual experimental work has been Sodium reported which uses the method described here. An investiga0 0.0 2.2 2.6 2.3 2.2 2 0 40 46.2 34.0 28.2 53.5 40.2 35.0 tion of this problem w&s undertaken and a controlled precision 55.8 48.8 84.2 65.8 57.2 100 73.7 140 85.8 67.0 59.1 96.9 78.1 68.0 method has been developed which is unusual in that the emission 112.0 78.8 70 0 91.2 80.8 200 100.0 of the unknown ion is not related to the emission of the internal Potassium standard ion. Rather, concentration factors are used. The 0 0.3 0.3 0.3 0.5 0.7 0.8 method has been successfully applied to beet root water extracts 16.8 11.3 35.8 20.8 30 26.5 16.8 41.2 31.7 75.3 51.0 41.2 75 62.7 clarified with lead acetate, factory diffusion juice, carbonated 55.3 45.8 92.3 66.8 57.2 105 81.0 factory juices, and sulfured factory juices preserved by the ad73.8 60.3 116 0 86.2 75.1 150 100.0 dition of alcohol. The method has not been applied to, but would seem equally pertinent for, any plant extract material not containing large particles of suspended matter. The concentrations each curve. To do this six sets of solutions were prepared, each of the sodium and potassium should be adjusted t o be in the upper composed of five standard solutions having final Concentrations range of the calibration curves, by proper selection of the highest as shown in Table I. These were made by using appropriate standard solution for setting the 100 T point of the calibration amounts of stock solutions of lithium, sodium, and potassium curve. This laboratory has had better success when the concenin 100-ml. volumetric flasks. trations range between 50 and 500 p.p.m. Table I. Standard Solutions
Concentrations of Elements in Solutions Used to Prepare Calibration Curves Final Concentrations, P.P.M. Element K Li Na 0 0 0 75 40 30 150 75 100 300 105 140 375 150 200
EXPERIMENTAL
The physical characteristics of each of the six sets were made different by introducing varying amounts of sucrose and alcohol according to the following directions: To set 1, add 10 ml. of alcohol to each of the five flasks comprising it; to set 2 add nothing; to set 3 add 4 grams of sucrose and 10 ml. of alcohol to each flask; to set 4 add 8 grams of sucrose and 10 ml. of alcohol; to set 5 add 4 grams of sucrose only; and to set 6 add 8 grams of sucrose only. Finally, make all solutions t@ 100-ml. volume with distilled water.
I
Li+lO%alcohol
The reagents used in the following work were made up from materials of the highest purity obtainable, with the exception of ethyl alcohol. Solox, a specially denatured ethyl alcohol, was employed, as a suitable substitute for the pure alcohol, because of its low cost. The other reagents were lithium chloride, potassium chloride, sodium chloride, and sucrose. The manufacturer's assay reported 0.02% sodium in the potassium chloride, 0.01% potassium in the sodium chloride, and 0.13% other alkali salts as chlorine in the lithium chloride. The sucrose contained about 0.001% sodium. All these amounts of trace impurities are unconsequential except the sodium in the sucrose, which would amount to less than 1 p.p.m. in the final solution. Lithium chloride is a difficult reagent to use for a standard concentration solution. The chloride was chosen to eliminate interference effects that result from a second anion. The errors due to the presence of secondary ions are not considered here. Small errors in lithium concentration are of no consequence if the same lithium stock solution is used throughout a test series. PREPARATION OF CURVES
To study the effects of varying viscosity, eighteen curves, six for each element, were prepared as determined by five points for
Li+4%suc+IOXalc. +4% suc rose /Li+B%sucror
oi
0
,
75
I
,
150 225 300 PPM L I T H I U M
Figure 1.
I
375
Lithium Curves
e
VQ,LUME
25, NO. 1 2 , D E C E M B E R 1 9 5 3
1823
'
tion factor could be derived, which, when applied to the apparent sodium and potassium concentrations found, would give the true concentrations of these elements. This correction factor value is calculated as a ratio value obtained by dividing the actual parts per million of lithium added by the parts per million of lithium found from the transmittance reading. This ratio value is referred to as the concentration correction factor. To use the curves for calculating the experimental values, the values of the ratios of concentrations a t comparable transmittance readings are first calculated. Examination of the lithium only curve (Figure 1) a t transmittance 100 shom that the concentration of lithium is 375 p.p.m., having been so set. Following the 100 transmittance line across to the lithium 10% alcohol curve, the corresponding concentration of lithium is 277.5 p.p.m. The concentration correction factor is obtained by finding the value of the ratio 277.5/375, which is 0.74. This factor will convert the apparent parts per million found in the 10% alcohol solution to the true concentration. I t will be shown that this factor, 0.74, holds very closely, regardless of what transmittance reading is chosen, provided that the lithium only and the lithium plus 10% alcohol are the curves under con-
+
I
I
I
50
;oc
200
I50
PPM. S O D I U M Figure 2.
Sodium Curves Table I V . Calculated Ratios or Concentration Correction Factors
120r
Curl-es
A. Li plus 10% alcohol/Li only
K+B%iucrose
I
I
I
120
I
I50
Figure 3 . Potassium Curves
Lithium, sodium, and potassium were then read on the flame spectrophotometer, using the inst'rument set,tings as s h o m in Table 11. The aniount of light emitted by the characteristic spectral line is reported here as per cent transniit,tance or transmittance unite, as it is t,he reading taken from the transmittance dial on the colorimeter port,ion of the instrument. The initial 100% transmittance setting on the instrument is nlwiys made using the solution containing 375 p.p,m. of lithium only, 200 p,p.m, of sodium only, and 150 p.p.m. of potassium only, respectively. The transmittanre units found are recorded in Table 111. Each value represents an average of five readings. These values are not absolute. Among other things, they depend upon the capillary tube used. All transmittance units shown in Table 111 are plotted in Figures 1, 2, and 3.
0.74 0.77
0.7;
367.5/293 142,5/112,5
1.25 1.27
1.26
Li plus 8% sucrose, 10% alcohol/Li only
82 60
359/217.5 172.5/105
1.65 1.64
1.6:
Li plus 4 % sucrose/Li only
82 62
372/217.5 193/111
1.71
1.74
1.72
Li plus 8% sucrose/Li only
72 32
368.5/157.3 190/79
2.34 2.41
2.37
l31/200 72/96
0.75 0.75
0.73
Sodium 100 72
S a plus 4% sucrose, 10% alcohol/Ns, only
90 66
195/155 100/79.5
1.26 1.26
1.26
Na plus 8% sucrose, 10% alcohol/Na only
80 64
197/126 124/7\5
1.63 1.65
1.65
Na plus 4 % sucrose/Na only
76 54
185/107.R 95/54
1.72 1.76
1.74
Na plus 8% sucrose/Xa only
70
200/90 120/52
2.22 2.31
2.26
100 60
118/150 55/70
0.79 0 79
0.79
K plus 4% sucrose, 1 0 % alcohol/K only
86 52
149.5/117 77/60
1.28 1.28
1.28
K plus 8% sucrose,
74 56
147.5/93.0 103/64
1.59 1.61
1.60
K plus 4% sucrose/K only
72 52
146/89.3 98/59
1.63 1.66
1.05
K plus 8% sucrose/K only
60
149/70 75/35
2.13 2.14
2 I4
54
C. Potassium K plus 10% alcohol/K only
10% alcohol/K only
Table V.
32
Comparison of Calculated Ratios
-
V" T Ir
DISCUSSIOIV
In a solution containing unknown amounts of organic material, accurabe determination of sodium and potassium using the Becknian flame spectrophotometer presents a problem due to bhe different, rates a t which the samples are introduced into the flame. This problem has been satisfactorily overcome by the addition of a known amount of lithium to the solution. Alcohol and sugar were added to known solutions to determine whether a correc-
Average Value of Ratio
92 62
Ka plus 10% alcohol/S-a only
30 60 90 PPM. POTASSIUM
Ratio
Lithium 100 277.5/375 70 112.5/147
of Ratio
Li plus 4% sucrose, 10% alcohol/Li only
B.
0
Valtir .
Transmittance
Curve 10% alcohol/element only 4% sucrose plus 10% alcohol/element only 8% sucrose plus 10% alcohol/elernent only 4% sucrose/element only 8% sucrose/element only
373 P.P.M. Largest Lithium .4=rage Value of Largest ~ ~ f r o~m Ratio Del-iation, Table Li Ka K tion % 111 0.04 111.0 0 . 7 5 0.75 0 . 7 9 5 2 93.2 1 . 2 6 1.26 1 . 2 8 0.02 1 6 1.65 1.60
0.05
3.1
84 0
1.72 1.74 1.65 2 . 3 7 2 . 2 6 2.14
0.09 0.23
5 3 10 2
82.3 73.0
l,65
i
~
-
ANALYTICAL CHEMISTRY
1824
sideration. Likewise, the concentration correction factor should be nearly 0.74 when similarly calculated from either the sodium only and sodium plus 10% alcohol curves, or the potassium only and potassium plus 10% alcohol curves. In Table IV a tabulation of concentration correction factors is found. Arbitrary but high transmittance readings were used in order to improve the accuracy of the calculations of the factors. A comparison of the average ratios for the three elements is given in Table V. As the concentration of the various organic matters is altered, so that the rate of introduction and flame temperatures become less similar t o the conditions holding during introduction of the standard solutions, the error increases significantly. The last column in Table V shows the transmittance values obtained for the 375 p.p.m. lithium plus the organic mixtures, after the instrument has been previously set at 100 transmittance units with the 375 p.p.m. lithium only solution. Such values, established by preparing known solutions similar to the unknown solutions, can be used as the criteria for establishing the accuracy of analysis that may be anticipated when working with organic mixtures of unknown composition. In sample analysis it is preferable to bring the lithium reading of the sample as close to the standard lithium reading of 100 as possible. The principle of operation of the ordinary internal standard technique presupposes that the emission-concentration curve of the internal standard element will be similar in shape to the curve of the unknown element. That this is not true has been shown by many workers and can be seen b y reference to Figures 1, 2, and 3. If emissions are used to calculate the correction factors rather than using concentration correction factors, gross errors will be found.
A calculated example will make this point clear. Assume 375 p.p.m. lithium is added to a sample and that it then contains 8% sucrose and 10% Solox. The instrument would first be set on 100 T , using a 375 p.p.m. lithium only solution. Reference to the appropriate curve of Figure 1 shows that the sample would report 84.0% T . The emission factor would be 100/84.0 = 1.19. If the sample contained 150 p.p.m. of potassium, i t would report 75.1% T (Figure 3). 75.1 X 1.19 = 89.4. Reference to the potassium only curve (Figure 3) shows 89.4% T is 123 corrected p.p.m. of potassium, an error of 27.0 p.p.m. or 18%. The method proposed here would use a concentration correction factor of 1.65 as obtained from Table IV. 75.1% T is 92 p.p.m. 92 X 1.65 = 151.8 corrected p.p,m. K, an error of 1%. The values found in Table 5’ are not absolute values, but depend upon relative concentrations and settings of the instrument. They are comparable within themselves, but would be different if other instrument settings or other concentrations of either positive or negative ions were used, in part because of ion interaction. PROCEDURE
Place a measured amount of the unknown solution in a 100ml. flask, add 25 ml. of a 1500 p.p.m. lithium stock solution to produce 375 p.p.m. lithium in the final solution, then make to the mark with water. Having set the instrument a t 100% T , using the standardization procedure with a solution of 375 p.p.m. lithium only, read the per cent T for lithium in the sample. If this per cent T is farther from 100 than will give the required degree of accuracy (Table V) a further dilution
of the sample is necessary or either sucrose or alcohol is to be added, until a transmittance reading indicating adequate accuracy is obtained. The instrument is now set to measure sodium, then potassium flame color intensities. The apparent concentrations obtained from these readings are then corrected by means of the concentration correction factor already described. Analyses of Actual Sample. Fifty milliliters of an unknown solution was transferred to a 100-ml. volumetric flask, 25 ml. of the 1500 p.p.m. lithium stock solution was added, and water was used to make to the mark. Another solution containing only 375 p.p.m. lithium was first used to set the instrument a t 100% T , and the unknown solution containing lithium was then read. The transmittance was found to be 91.2%. The 91.2% T indicates (Table V) that the error of the analyses will be less than 2.5%, which is considered satisfactory, so the analyses were continued by finding the per cent T for sodium and potassium, which were 77.2 and 69.3, respectively. Referring to the sodium only curve (Figure 2) 77.2% T is 112 p.p.m. sodium, and referring to the potassium only curve (Figure 3) 69.3% T is 85 p.p.m. potassium. The concentration correction factor is obtained by reference to the lithium only curve (Figure 1). The lithium transmittance is 91.2%, which corresponds to 300 p.p.m. lithium. Thus the concentration correction factor is 375/300 or 1.25. Correcting sodium, the true concentration of sodium in the solution is 112 X 1.25 or 140 p.p.m., and the true concentration of sodium in the sample is 280 p.p.m., owing to the 1 to 1 dilution previously made, Following this through for potassium, the true potassium concentration in the solution is 85 X 1.25 or 106 p.p.m., which multiplied by 2 gives 212 p.p.m. potassium in the sample. For sample analyses it is not necessary to duplicate all the curves shown. These curves were developed only to justify the technique and to show what magnitude of error may be anticipated under these experimental conditions. Three curves are necessary for the analysis of unknown solutions: the lithium, sodium, and potassium only curves in Figures 1, 2, and 3. ACKNOWLEDGMENT
The authors of this paper wish to thank F. W. Weitz for suggestions made during the preparation of the manuscript. LITERATURE CITED
(1) Attoe, 0. J., and Truog, E., Soil Sci. SOC.Am., P r o c . , 11, 221
(1946). (2) Bartstra, E. A. C., and Verburgt, J. W., Inst. Rationeele Suikerproduct., 17, 111-22 (1948). (3) Bauserman, H. M,,and Cerney, R. R., Jr., Proc. Am. SOC. Sugar Beet Technol., 1952, 681-7. (4) Berry, J. IT,, Chappell, D. G., and Barnes, R. B., IND.ENQ. CHEW., AN.4L. ED.,18, 19-24 (1946). ( 5 ) Broderick, E. J., and Zack, P. G., ANAL.CHEM.,23, 1455-8 (1951). (6) Giibert,’P. T., Jr., Hawes, R. C., and Beckman, il. O., Ibid., 22, 772 (1950). (7) Hald, P. M., J . Bid. Chem., 167, 499-510 (1947). (8) Mosher, R. E., et al., A m . J . Clin. Pathol., 19, 461-70 (1949). (9) Myers, A. T.. Dyal. R. S., and Borland, J. W., Soil Sci. SOC.Am., PTOC., 12, 127-30 (1947). (10) Perkin-Elmer Corp., Glenbrook, Conn., “Flame Photometer Instructional Manual, Model 52 -4,” 1948. (11) Rogers, L. H., Soil Sci. SOC.Am., Proc., 12, 124-6 (1947). (12) Toth, S. J., and Prince, A. L., Soil Sci., 67,43945 (1949). (13) Toth, S. J., et al., Ibid., 6 6 , 459-65 (1948). RECEIVED for review December 18, 1952.
Accepted October 8 , 1953.
