1076
ANALYTICAL CHEMISTRY
a Zniinute reaction interval, small excess of mercaptan, use of a basic catalyst, and mixture of the mercaptan with acrylonitrile before addition of the base. This method with modification in some cases can be employed for accurately determining a,@-unsaturatedesters and aldehydes. The salts of the corresponding acids and other simple or conjugated unsaturated compounds do not react. LITERATURE CITED
(1) Bruson, H. .4., and Riener, T. W., J . A m . Chem. SOC.,65, 23 (1943). (2) Dinsmore, H.L., and Smith, D. C., ANAL.C H E M 20, . , 19 (1948). (3) Gershbein, L. L.,and Hurd, C. D., J . Am. Chem. SOC..6 9 , 241 (1947). (4) Harman, M. W. (to Monsanto Chemical Co.), U. S. Patent 2,413,917(Jan. 7, 1947). (6) Hurd, C. D., and Gershbein, L. L.,J . A m . Chem. Soc., 69,2328 (1947).
(6) Jacobson, R. A. (to E. I. du Pont de Nemours & Co.), U. 9. Patent 2,199,799(May 7,1940). (7) Kolthoff,I. M., and Harris, W. E., IND.ENQ.CHEM., ANAL.E D . , 18, 161 (1946). (8) Nicolet, B. H., J . Am. Chem. Soc., 53,3066 (1931). (9) Zbid., 57,1098 (1935). (10 )Nicolet, B. H., J.Biol. Chem., 95, 389 (1932). (11) Petersen, G.W.,and Radke, H. H., IND, ENQ.CHEM.,ANAL. ED.,16, 63 (1944). (12) Posner, T., Ber., 35,809 (1902). (13) Rosenmund, K.W.,Kuhnhenn, W., Rosenberg-Gruszynski, D., and Rosetti, H., 2. Untersuch. Nahr.-u. Gennussm., 46, 164 (1923). (14) Rowe, R. G., Funas, C. C., and Bliss, H., IND. ENG.CEEM,, A x . 4 ~ ED., . 16, 371 (1944). (15) Ruhemann, S.,J . Chem. SOC.,87,461 (1905). (16) Tamele, M.W., and Ryland, L. B., IND.ENO.CHEM.,ANAL.E D . , 8 , 16 (1936). RECEIVEDD r m u b e r 8. 1948.
Reduction of Error in Flame Photometry CHARLES E. BILLS', FRANCIS G . MCDONALD, WILLIAM NIEDER?vIEIER*,AND MELVIN C. SCHWARTZ Research Laboratory, Mead Johnson and Company, Evansville, Znd. In the determination of sodium and potassium with a Perkin-Elmer Model 18 flame photometer, much of the mutual interference of these elements was eliminated by the use of a newly designed amplification circuit. Further reduction in interference, both mutual and from foreign solutes, was effected by working at lower concentration ranges than have previously been recommended. With interferences thus minimized, most biological materials can be analyzed with no more than the instrument error of about 20/,, but when the test element is in low ratio to the total ash, it may be necessary to correct the determinations by the use of an empirically derived correction curve.
S
TUDENTS of flame photometry are agreed that this new analytical art is subject to numerous errors, some of which are understood and can be dealt with, whereas others remain obscure (1, 2, 4-7). Most of the published work has been done with the instrument described by Barnes, Richardson, Berry, and Hood ( I ) or its commercial counterpart, the Model 18 Perkin-Elmer flame photometer (8). This consists essentially of B Meker burner, an atomizer for spraying solutions into the flame, and a photoelectrir colorimeter for measuring the characteristic light emitted. The original work (1) was done with solutions of known composition. I t brought out the importance of constant conditions in the flame and atomizer, and the fact that foreign substances in the solution decrease or increase the light output. The interference of foreign substances or even of too much sodium or potassium ha4 troubled all subsequent workers, and various means have been proposed to overcome it. Thus Berry, Chappell, and Barnes (2) have suggested that, in suitable situations, counterbalancing quantities of the interfering substances be added to the standard solutions with which the photometer is calibrated. Obviously, this device would work only in routine analyses of a particular material. These authors also proposed, tor a corrective of more general applicability, the use of lithium tls an internal standard in flame photometry. They claimed That the internal standard was highly effective against errors caused by fluctuating gas pressure, and partially effective against errors caused by fluctuating air pressure and the presence of toreigri molecules. The present paper is based on nearly three years of expcrience with flame photometry, with emphasis on the reduction of errors. The study was made in connection with a survey of the 1 2
Present address, Route 12, Stringtown Road, Evansville, Ind. Present addreas, Hillman Hospital, Birmingham. Ale.
sodium and potassium content of foods and public water supplies (5).
INSTRUMENTAL IMPROVEMENTS
The Jyork was begun with a Model 18 Perkin-Elmer flame photometer. (Production of the Model 18 instrument was discontinued in 1946 i-n favor of a new model which the manufacturer considers greatly improved.) In this instrument the glass atomizing chamber was held in position around the base of the burner by a coiled spring. The arrangement was such that the chamber was readily displaced, as by touching the intake tube with the beaker holding the test solution. Such displacements occasionally caused large errors in reading, and the defect WBB corrected by clamping the chamber in a reproducible position. Less obvious, but more important, were certain defects in the electrical circuit. The instrument when calibrated for the most desirable range, 0 to 10 p.p.m., was operating a t practically ita limit of sensitivity, and therefore showed considerable drift and instability. Moreover, the amount of interference given by sodium with potassium readings and by potassium with sodium readings was both considerable and inconstant. These s h o r t comings were substantially reduced by employing a new electrical circuit, for which the authors are indebted to R. L. Schoene of the Schoene Electronics Laboratory, Evansville, Ind. The original circuit is shown in Figure 1. With this circuit, amplifier drift occurring between the time of setting the zero and taking the reading appears in the dial reading as an error. To obtain higher sensitivity 4,ithout undue drift a two-stage modified Wynn-Williams bridge circuit was adopted, giving en increase in electrical sensitivity of many times with only about the original amount of drift. To eliminate from the reading any error caused by this drift, a null balance push button was provided. The resulting circuit is shown in Figure 2. In use, the modified instrument is balanced by adjusting the appropriate
V O L U M E 2 1 , NO. 9, S E P T E M B E R 1 9 4 9
1077
appears that interference in flame photometry is not wholly an inherent shortcoming, but is to a considerable extent a function of instrument design. STANDARD1ZATION
For primary standards of sodium
L
\
Figure 1. Original Circilit
Figure 2.
Improved Circuit
control, so that pressing or releasing the button causes no change in the galvanometer deflection. When so adjusted, the dial reading will be proportional only to the amount of light striking the phototube, even though the galvanometer deflection has drifted since the zero balance was made. The original circuit used a phototube load resistor of 500 megohms, which made it necessary to do the switching from one phototube to the other (for sodium or potassium) in the anode circuit. The phototube not in use still had its cathode connected to the amplifier, giving one possible source of the interference of sodium with potassium and vice versa. With the modified circuit, the large increase in sensitivity allowed a reduction of this resistance to only 20 megohms, which is low enough to permit switching in the cathode circuit, thus completely disconnecting from the amplifier the phototube not in use. The new switching arrangement substantially reduced the amount of interference. Formerly the interfering action of sodium on potassium determinations was evident a t all sodiumpotassium ratios. Now the effect is nil a t ordinary operating ratios, and only slight a t extreme ratios. A typical example of the improvement is found in the effect of 4000 p.p.m. of sodium on the assay of a 4 p.p.m. potassium solution, wherein the error was reduced from 122 to 8%. Again, with 1000 p.p.m. of sodium and 4 p.p.m. of potassium, the error was reduced from 103% to zero. A general picture of the interferences under the new switching arrangement is given in Figures 3 and 4. Thus it
and potassium, the spectroscopically pure chlorides made by Johnson, Matthey and Co. were employed (distributed in the United States b Jarrell-Ash Co., Boston, Mass.). d e solutions were kept in bottles of No-Sol-Vit glass (T. C. Wheaton Go., Millville, N. J.). Perkin-Elmer standard solutions were used in routine work, after they had been checked against the primary standards. The fuel used was propane. A pressure regulator a t the propane cylinder delivered gas to the line a t 2 pounds per square inch (140 grams per square centimeter). A needle valve fitted with a gear drive enabled the operator to obtain the exact working pressure desired. A dial gage reading 0 to 30 ounces per square inch was located at the needle valve. For all sodium determinations the propane was delivered a t 20 ounces per square inch, and for potae sium a t 25 ounces per square inch. It is important that the final pressure reduction be obtained with a needle valve, rather than a regulator of the diaphragm type which is subject to flutter and lag. The compressed air required for aspirating and atomizing waa cleaned by passage through a Selas ceramic filter (Selas Corp. of America, Philadelphia, Pa.). The filtered air flowed to a pressure regulator set for 20 pounds per square inch. The needle valve and dial gage in the photometer itself were used to reduce the pressure further to the 13 pounds per square inch used for all determinations. The electric current available for operating the amplifier wm subject to surges from nearby machinery. Constant voltage was secured by the use of a Raytheon voltage regulator. Even with regulation and the improved amplifier, it was found desirable to turn on the amplifier and the burner for a 15-minute warm-up before use. The photometer was standardized for the range 0 to 10 p.p.m. of either sodium or potassium-Le., the range rheostats were adjusted a t the time of use SO that 10 p.p.m. of either element produced a dial reading of 100, and pure water a reading of zero. I n theory, the standardization is permanent, but in practice it should be done before and after each determination. There are several advantages of standardizing for the 0 to 10 p.p.m. range, rather than for the higher ranges commonly used: .4t the 0 to 10 p.p.m. range the relation between concentration and dial reading is linear, instead of curvilinear. Hence no calibration curve is required, and calculations are simplified. Smaller samp1t.s are required, with less preparatory work. Interferences from foreign solutes and the mutual interferenee between sodium and potassium are minimized. Errors due to viscosity effects in the atomizer are minimized. Pollution of the atomizer chamber with spray residues is minimized. In water analysis, advantage can be taken of the fact that most water samples naturally fall in the 0 to 10 p.p.m. range, and can therefore be run without concentration.
ANALYTICAL CHEMISTRY
1078
With the improved amplifier it was possible to standardize for ranges lower than 0 to 10 p.p.m., even as low as 0 to 1 p.p.m., at which level 1 dial unit corresponds to 1 part of sodium or potassium in 100,000,000 parts of water. At this extreme range the problem of contamination becomes serious, and instrumental defects are also magnified. The use of extreme low range standardization therefore appears to be limited to the analysis of minute samples and t o research on certain problems in the physics of flame photometry.
I
I
I
100
90
80
I l l
I l l / (
I
K=lOppm
1
'
I
"
I
1
'
I I l l /
4 1-
K=Sppm K=8 p pm
1
1
~
-
K=7ppm v)
0
-
70
K'6
2 60
p prn
1
4
50.
K=5 p p m
-
K=4ppm
2a 40
-
,
K = 3 p pm
I
PREPARATION OF SAMPLES
Biological fluids rich in sodium and potassium, such as urine, serum, sweat, saliva, whey, and fresh milk can be analyzed without ashing, provided no particulate matter is present which might clog the atomzer. It is merely necessary to dilute with distilled water, so that the sodium or potassium content falls in the range 0 to 10 p.p.m. Buttermilk, and milk which has been soured, evaporated, or dried, should be ashed because of particles present. Fruit juices, soft drinks, and alcoholic liquors usually contain so much interfering material (sugar or alcohol) in proportion to their content of sodium and potassium that ashing is required. Most water samples require no preparation whatever for potassium determination, but dilution with distilled water is sometimes required for sodium. All solid samples must of course be ashed. The following ashing technique, based on experience with huiidreds of diverse samples, is applicable to nearly all organic materials.
I
I
I
l 1 1 l 1
I
I
1
I I I11
I
I
I I
I I l l
I
Na=lO p.pm.
I
-
100 Na = 9 p. p.m. 90 p
80-
Na = 7p.p.m.
70 v,
gm a
5
e
Na = 8 pap.m.
Na = 6 p.p.m.
I
Na = 5 p.p.m.
-
50
$ - 40
n
30
i
Na=4 p.pm
-
N a = 3 p,p.m.
-
Na = 2 p.p.rn.
I
20 Na = 1 p.p.m.
10
I
-
Na = 0 p.p.m.
O 1
The sample, usually 1 to 50 grams according to the especteci sodium or potassium content, is weighed in a shallow fused quartz dish. If damp, it is dr$d in an oven a t j w t below 100 C. It is then placed in a cold mume furnace, and heated to 550" until the gray ash stage is reached. The furnace is then allowed to cool, and the dish removed. The ash is moistened with distilled constantboiling hydrochloric acid, and the excess acid is removed by warming. The sample is again heated in the furnace to 550" until the ash is white (generally 1 or 2 hours). If the ash fails to become white after several hours at 550", it is moistened with a 25% solution of distilled nitric acid, dried, and again heated to 550'. Color still remaining is generally due to iron, and cannot be eliminated. Finally the ash is moistened with a few drops of hydrochloric acid, dissolved in water, and made up to 100 ml. No measurable loss of sodium or potassium occurs in this procedure. AVOIDANCE OF CONTAMINATION
In practicing flame photometry one encounters contaminative eources of sodium and potassium in many and unexpected forms. For 2 days in January 1947 the atmosphere was hazy from a Texas dust storm, and during that period the photometer could not be operated satisfactorily. The handling of soap powder in a distant part of the laboratory may pollute the air for hours. Tobacco smoke contributes potassium in considerable quantity. It
10
100
1000
X 4000
has been said that operation of flame photometers near the seacoast is difficult during ocean winds. The air in a poorly ventilated room may become polluted from the chimney fumes of the photometer. Barnes ( 1 ) has cautioned against fingerprints on glassware, and the warning should be extended to filter paper folded with the fingers. Common filter paper, even if untouched, may contribute sodium and potassium, while acid-washed papers strongly adsorb these elements from trace solutions. The top lining of the muffle furnace should be inspected for loose particles which might drop into the ashing dishes. In handling glass-stoppered bottles and volumetric flasks one should cultivate the habit of inserting and withdrawing the stopper without giving it the customary grinding twist, the abrading action of which releases sodium and potassium to the solution. One should especially avoid the use of g h s stirring rods in scraping ash from quartz dishes. New glassware may exhibit a surface concentration of soluble alkali, most of which can be removed by soaking the glass for several days in distilled water. This treatment should be applied to glassware in which solutions for flame photometry are to be stored.
V O L U M E 2 1 , NO. 9, S E P T E M B E R 1 9 4 9
1079
INSTRUMENT ERROR
rate of aspiration or flow into the atomizer, the nebulosity of the spray, and the rate of propagation of the flame. These effects are related to the physical properties of the interferant in ways that are not understood. Some of the observations of others (8,7 ) on the interferences engendered by foreign solutes have been qualitatively confirmed. Thus it was found that methanol and ethanol enhance the light emitted by sodium, whereas urea and sucrose depress it. Numerous other substances were studied in this connection, but it will suffice here to consider only the extreme examples of tert-butyl alcohol, which enhances the light output, and glycerol, which depresses it. Both substances were prepared metal-free by distillation, which makes them better study materials than inorganic salts that might contain traces of sodium or potassium. It was of special interest to ascertain the effect of dilution on interference. Stock solutions were prepared to contain 50,000 p.p.m. (5% w/v) of the interferant and 50 p.p.m. of sodium or potassium. These were assayed first with the photometer standardized for the range 0 to 100 p.p.m. Then the solutions The mean deviations were calculated and are expressed as per were diluted 1 to IO and assayed a t the 0 to 10 p.p.m. range. cent error in Table I. It is evident that no claim can be made as Finally, the diluted solutions n-ere in turn diluted 1 to 10 and to instrument error except under specified conditions. When the assayed a t the extreme low range of 0 to 1 p.p.m. The findings photometer is standardized for the 0 to 10 or 0 to 100 p.p.m. are recorded in Table 11. range, and readings are made on the upper half of the dial, the I t is significant that the interference a t any given concentration error is about 2%, and this can be reduced considerably by taking was approximately the same for both sodium and potassium. two or more readings and averaging them. The instrument error However, when the concentration was decreased, the interference does not include error from interferences, which is a matter for also was decreased, even though the ratio of interferant to metal separate consideration. remained constant. I t therefore follows that, for minimum interference from foreign solutes, flame photometers should be INTERFERENCES operated a t the lowest feasible range. During this experiment observations were made on the rate The term “interference” covers various effects of dissolved of aspiration of the solutions containing 5 p.p.m. of sodium with substances on the light produced by the test substance. Even 5000 p.p.m. of glycerol and 5 p.p.m. of sodium with 5000 p.p.m. the test substance, when sufficiently concentrated, may interfere of tert-butyl alcohol, in comparison with a solution containing nith itself, as witness the curvilinear responses in calibrating a 5 p.p.m. of sodium in pure water. The glycerol, which depressed flame photometer a t the higher ranges. If the interferant colors the light output 24%, depressed the aspiration rate only 6%. the flame, the flame may absorb some of the light of the test subThe tert-butyl alcohol, which increased the light output 30%, stance, or, if the light filter is not perfect, the light of the interdepressed the aspiration rate 18%. The nebulosity of the spray t’crarit may register as that of the test substance. In the latter does not lend itself readily to quantitative measurement, but from case, the effect may be especially great when, a s shown above, a observation of the atomizing chamber it was evident that the faulty electronic system is also involved. All interfering subglycerol produced a coarse spray and the alcohol a fine one which stances, whether or not they color the flame, tend to alter the probably carried more sodium to the flame. It was noted incidentally that only about 3 to 5% of the atomized solutions __--____ entered the burner; the rest collected on the n-alls of the atomising chamber and went down the drain. The alcohol appeared to Table I. Instrument Error with Pure Solutions of Sodium and Potassium at Several Concentrations and decrease the flame propagation rate, making the flame less “hard” Standardization Ranges and increasing its area. StandardSodium Potassium The relation of standardization range to the interference of Mean Error, % Mean Error, % Concenization sodium and potassium with each other was determined in an Range, tration, €$ingle Duplicate Single Duplioate P.P.hl. P.P.M. readings readings readings readings experiment similar to the one with the organic materials. Stock 0 to 1 0.1 21.3 18.7 36.5 27.0 solutions were prepared to contain 40 p.p.m. of sodium with Otol 0.5 5.5 3.2 6.1 3.9 0 to 1 0.0 3.4 2.6 5.2 3.4 40,000 p.p.m. of potassium, and 40 p.p.m. of potassium with 0 to 10 1 .o 6.3 15.0 5.2 13.5 40,000 p.p.m. of sodium. Only spectroscopically pure chlorides 0 t o 10 5.0 1.6 1 .O 2.8 2.1 0 to 10 1.4 9.0 0.9 2.1 1.7 were used. These solutions were assayed first with the photom0 to 100 10.0 4.1 3.1 7.6 5.4 0 to 100 50.0 2.4 1.9 2.5 1.9 eter standardized for the range 0 t o 100 90.0 1.4 1.8 1.3 0.9 0 t o 100 p.p.m. Then they __ -. ~ ~ _ _ _ _ ~ were diluted 1 to 10 and asTable 11. Relation of Standardization Range to Interference by Organic Solutes sayed at the 0 to 10 range. Standardization Sodium, Potassium, tert-Butyl Glycerol, Interference, Finally, the diluted solutions Range, P.P.M. P.P.M. P.P.M. Alcohol, P.P.M. P.P.M. % were again diluted 1 to 10 and 0 to 100 50.0 ... 50,000 ... >+loo 0 to 100 assayed at the 0 to 1 range. 50.0 50,000 >+loo 0 to 100 50.0 ... ... 50,000 - 34 Thus in each range the inter0 to 100 ... 50.0 50.000 ... - 37 0 to 10 5.0 ... 5,000 ... +30 fering (dominant) element w m 0 to 10 ... 5.0 5,000 ... 32 0 t o 10 5.0 in a 1000 to 1 ratio to the ... . . . 5,000 24 0 t o 10 5 .O .. 5,000 - 22 element being assayed. Con0 to 1 0.5 ... 500 . . . 0 to 1 .., 0.5 500 ... centrations of the interfering 0 to 1 0.5 ... 500 - 8‘ 0 to 1 ... element alone were also sub0.5 500 - 6a a Several readings averaged to lessen instrument error characteristic of this standardization range. jected to assay for the test ele_ _ ~ ment, for thepurposeof demon-
To ascertain the instrument error-i.e., the error in repetitive readings-the photometer was standardized with sodium and potassium for the ranges 0 to 100,O to 10, and 0 to l p.p.m. Calibration curves were plotted for the 0 to 100 p.p.m. ranges of sodium and potassium. The 0 to 10 and 0 to 1 p.p.m. ranges were found to be so nearly linear that perfect linearity was assumed. Solutions containing 10, 50, and 90 p.p.m. of sodium and of potassium were assayed on the 0 to 100 p.p.m. range; solutions of 1, 5 , and 9 p.p.m. were assayed on the 0 to 10 p.p.m. range; and solutions of 0.1, 0.5, and 0.9 p.p.m. were assayed on the 0 to 1 p.p.m. range. Because the new amplifier was designed for the 0 to 10 p.p.m. range, it was necessary when working a t the other ranges to deviate somewhat from the established operating. conditions. Thus, for the 0 to 100 p.p.m. range the sensitivity control was turned down and the gas pressure was lowered from 20 for sodium, and 25 for potassium, to 16 and 20 ounces per square inch, respectively. For the 0 to 1 range the sensitivity control was turned up and the gas pressure raised to 30 ounces for both sodium and potassium. Each determination was made 20 times, and the instrument was qtandardized before and after each reading.
~
.
I
.
I
.
+
.
i;:
1080
ANALYTICAL CHEMISTRY
strating the amount of error due to light filter leakage. The findings are recorded in Table 111. At the 0 to 100 p.p.m. range a strong negative interferenee is evident, amounting to over 50% for both sodium and potassium. The positive error due to filter leakage is relatively small. At the 0 to 10 range the negative interference is declining and becoming offset by the positive filter error. At the 0 to 1range the negative interference has beome 80 small that the filter error dominates it. The leakage of sodium light through the potassium filter is especially noticeable. It would seem that if the filters employed to isolate the sodium and potassium radiations,performed perfectly, the amount of interference would decline sharply from the highest to the lowest ranges, just as it did in the case of glycerol and tertbutyl alcohol. But with filters io:! 100:1 1000:1 10000:1 less than perfect, the practical point is brought out ASH : S O D I U M RATIO in Table I11 that interference is minimal when Figure 5. Correction Curve the photometer is standardized for the 0 to 10 Baaed on ratio of ash (including added aoid) to amount of eodium found p.p.m. range. The sodium-potassium interference study was extended to cover widely different ratios of these elements within Table 111. Relation of Standardization Range to Mutual the limits of the 0 to 10 p.p.m. range. The findings are charted Interference of Sodium and Potassium in Figures 3 and 4, from which it is evident that the amount of Standardieation Interfering interference depends not only on the sodium-potassium ratio, Interferenee, -Test Element, P.P.M. Element, but on the absolute quantities present. For the ratios usually P.P.M., Range P.P.M. Present Indicated % encountered in biological materials the interference is small, if 0 to 100 K 40,000 Na 40.0 Na 18.6 -54 K 0 to 100 40,000 Ka 0.0 Na 0.6 any, although it may be considerable in the case of certain plant 0 to 10 K S a 4.0 4,000 Na 3.8 -6 0 to 10 K products containing much potassium and only a trace of sodium 4,000 Na 0.0 Na 0.5 K Na 0.4 400 Na 0 to 1 K Na 0.0 400 0 to 100 K 40.0 Na 40,000 0 t o 100 Na 40,000 0.0 K K S a 4,000 0 to 10 K K 4.0 S a 4,000 0 to 10 K 0.0 K Na 0 to 1 K 400 0.4 K Sa 0 to 1 K 0.0 400 K a Several readings averaged to lessen instrument this standardization range. 0 to 1
PRACTICAL ERROR
It remains to be shown how much error is involved in practical analyses of biological materials where the interfering substance, ash, is no definite chemical. Because the ashes of both plant and animal products are rich in potassium, the ash-potassium ratio is never large, and serious interference with potassium determinations is unlikely. The ashes of all animal products and some plant products contain sufficient sodium so that serious interference with sodium determinations is unlikely. But in certain plant products, especially fruits, grains, nuts, and legume seeds, sodium is a trace element in the presence of relatively enormous amounts of ash. This is the place to study interference at its worst. The ash of the bean, Phaseolus vulgaris, variety Great Northern, h a typical low-sodium plant ash consisting mainly of potassium, calcium, magnesium, phosphate, and sulfate. To obtain a supply for study, 400 grams of dry beans were ashed as described under Preparation of Samples. The ash, 14.872 grams, was made up to 200 ml. by the addition of 25 ml. of constant-boiling hydrochloric acid and sufficient water. A small amount of insoluble matter settled and was discarded. For purposes of flame photometry, the added hydrogen chloride must be considered part of the ash, for it exhibits a negative interference of about the same magnitude as ash proper. As constant-boiling hydrochloric acid contains approximately 2OY0 w/v of hydrogen chloride, the acid in this case contributed 5.0 grams, making a total of 19.872 grams of ash in 200 ml. Dilutions of this stock solution were used in a aeries of sodium recovery experiments.
A dilution of 25 ml. of stock solution with water to 50 ml. showed a sodium content of 5.225 p.p.m. A dilution of 25 ml. of stock solution and 5 ml. of a 10 p.p.m. sodium solution with water to 50 ml. showed a sodium content of 5.795 p.p.m. Because of the magnification of instrument error in the analysis of such a small recovery increment, both dilutions were read 20 times and the averages taken. The ash content of the recovery test dilution (ash proper plus added acid plus the added trace of sodium chloride) amounted to 49,683 p.p.m. The ratio of ash to sodium found was therefore 49,683 to 5.795, or 8573 to 1. The added sodium amounted to 1 p.p.m., of which 0.57 p.p.m. was recovered. In short, it was necessary to increase the reading
2
0.51‘ 0.13“ 19.3 2.6 4.3 2.9 0.84“
f280
- 62 +8
+110’
0.45O
error characteriatio of
for sodium by a correction factor of 74.4% when the ash-sodium ratio was 8573 to 1. Following the same general procedure, eight more recovery tests were made, the dilutions and additions being calculated so that each successive solution had an ash-sodium ratio approximately one half as great as the one before it. In this way it was possible to simulate all the ratios encountered in everyday work. The findings are shown as a correction curve in Figure 5. The use of this curve is justifiable only with flame photometers operated a t the 0 to 10 p.p.m. range and equipped with the electrical circuit described in this paper. As new models of flame photometers become available, similarly derived correction curves can be used as a measure of their improved performance. LITERATURE CITED
R. B., Richardson, D., Berry, J. W., and Hood, R. L., IND. ENQ.CHEM.,ANAL.ED.,17,605-11 (1945). (2) Berry, J. W., Chappell, D. G., and Barnes, R. B., Ibid., 18, 19-24
(1) Barnes,
(1946). (3) Bills, C.E.,McDonald, F. G., Niedermeier, W., and Schwerts. M. C., J. Am. Dietetic Assoc., 25,304-14 (1949). (4) Hald, P. M., J. BioZ. Chern., 167,499-510 (1947). (5) Howard, J. E.,and Bigham, R. S., chapter in “Conference on Metabolic Aspects of Convalescence, Transactions of the Eleventh Meeting,” p. 10, New York, Josiah Macy, Jr., Foundation, 1945. (6) Overman, R. L.,and Davis, A. K., J . BWZ. Chem., 168, 641-9 (1947). (7) Parks, T. D., Johnson, H. O., and Lykken, Louis, ANAL.CEEM., 20,822-5 (1948). (8) Perkin-Elmer Corp., Glenbrook, Conn., Instruction Manual, Flame Photometer Model 18, 1945. RECEIVED December 10, 1948. Presented in part before the American Institute of Nutrition, Chicago, Ill., May 21, 1947.
