for any pressure between zero and p1 the equation of Glueckauf ( 1 ) can be used, viz:
where q
f(p) represents adsorption isotherm u = volume of pure helium passed betueen time nitrogen is shut off and pressure p of nitrogen is measured in elution curve 'I;= amount of nitrogen remaining on column when pressure p is measured 2: = weight of solid constituting column =
To illustrate t h e use of the equation, buppose that it is required to find the volume of nitrogen adsorbed a t a pressure p , which corresponds to the point G in Figure 1, then u p ( = up,) is given by the area ELGK and P; ( = pPg) is given by t h e area LFG. Thus the amount adsorbed a t pressure p , is given by t h e sum of t h e areas ELGK nnd LFG. The difficulty of measuring area LFG accurately can easily be overLFG) = come because area (ELGK (ABCD - G H J K ) and both ABCD
+
and G H J K lend themselves more readily to measurement. Any number of other points can be calculated in a similar way and used to obtain the usual B E T plot. Some workers prefer to use the Point B method for determining the monolayer capacity, V,, of the adsorbent. This involves estimating the volume adsorbed a t the point \There the isotherm becomes approximately linear (Figure 2). The point on the elution curve which corresponds to the Point B is also the point a t which the curve becomes approsimately linear; this is again G in Figure 1. Only one calculation is now required to determine T.,i A number of possible objections to t h e above methods may be mentioned: Surface areas measured in the n a y described will almost certainly differ from those obtained by static techniques because i t is assumed that equilibrium is attained during the measurement of t h e elution curve and this assumption is probably unjustified. However, if the results obtained by Gregg and Stock ( 2 ) using hydrocarbons as the adsorbates are any guide, discrepancies will not be large. Very low flow rates (10 cc. per minute) are recommended; diffusion effects will not be great provided a small column is used.
Strictly speaking, flution curves measure only the desorption isotherm; hence, i t is important that measurements are made only over those parts of the isotherm which are completely reversible. During measurement of the elution curve the flow rate of the helium-nitrogen mixture will be variable because of desorption of nitrogen. It will therefore be necessary to apply a correction to the elution part of the chromatogram to take this effect into account. The method will not be suitable for very fine powders because these v ill impede the gas f l o .~ It is hoped to test the method in the near future. LITERATURE CITED
(1) Glueckauf, E., J . Chem. SOC. 1947,
1302; Nature 156,748 (1945); 160, 301 (1947). (2) Gregg, S. J., Stp,ck, R., "Gas Chromatography 1958,. D. H. Desty, ed., p. 90, Butterworths, London, 1958. (3) Nelsen, F. &I., Eggertsen, F. T., ANAL.CHEV.30, 1387 (1958). (4) Roth, J. F., Elln-ood, R. J., Ibid., 31, 1738 (1959). RALPH STOCK Department of Chemistry, College of Technology, Byrom Street, Liverpool 3, England
Boron Contamination in Furnace Dry Ashing of Plant Material SIR: The accepted method of preparing plant material for boron analysis is by dry ignition in a muffle furnace (1, 4). Ignition is usually made on untreated plant niaterial or on material treated with a base such as Ca(0H)z to prevent loss of boron which takes place under acidic conditions. Vessels suggested for ashing are porcelain, quartz, or platinum dishes. Samples used vary from 0.25 gram to 2 grams. Ignition temperatures should not exceed 550" C. nith ignition time u p to 4 hours or as required to give a grey-white to nhite ash. A large number of barley plants, grown in culture solutions, were dried, ground, and analyzed for boron, using d r y combustion as a means of ashing and the colorimetric method of analysis developed by Dible, Truog, and Berger (2) for boron determination. This method uses cureuminoxalic acid solution as a reagent for developing t h e red boron complex. The samples were ashed in porcelain evaporating dishes in :in electric furnace for varying periods of time at 550" C. Some of the boron values obtained are reported in quadruplicate in t h e first section of Table 1. T'ariation in boron content ranged from 200 t o 800% of the lowest value for a
increased in each dish directly with teniperature in the muffle, time in the muffle, size of the dish, and area covered by the alkaline ash. Coors No. 2 high-form porcelain crucibles eliminated contamination from boron vapors. The height and narrow mouth of the crucible prevented the boron vapors from contacting the alkaline ash. The second section of Table I gives boron data obtained when high-form crucibles were used. Table I1 s h o w a comparison of values
given sample. Similar variation in boron values was noted n i t h four different furnaces. This large variation in boron content of duplicate samples of plant material was found t o be due to a contamination of the plant ash by boron coming from the furnace. The boron volatilized from t h e furnace walls was swept over the dishes,' condensing in the alkaline plant ash. Covering t h e evaporating dishes lessened the amount of contamination but did not stop it. Boron
Table 1.
I
5-0 6-0 7-0
8;
8-0
50
10-0 5-x
6-N 7-N
&N
9-iY 10-N
No. 2 Coors Hieh-Form Crucibles
Porcelain Evaooratine Dishes
Plant Sample
9-0
Boron in Barley Leaves P.P.M. Boron in Dry Plant Material
85 61 26 38 17 35 28 60 42 68
28 45 24 60 39 48 18 21 68 36 34 29
55
23 30
100
67 71 30
8
16 25 25 15
Y
52 67 67 86 60 59 ... 46 50
61 61 23
9.4 10.6 16.6 50
31.8 37.4 4.4 5.4
8.6 18.4 20.6 12.6
VOL. 33, NO. 7, JUNE 1961
9.8 11.4 16.0 50.8 31.2 38.0 5.4 6.0 9.8 19 4 22 13
967
Table 11.
Boron Analyses by Meeker and Muffle Ashing Ashed Over Meeker Ashed in Muffle
Porcelain evaporating dish (uncovered)
Porcelain high-form (uncovered)
Porcelain high-form (covered)
25.2 25.6 25.3 25.6 25.4
25.6 25.4 25.4 25.6 25.2
24.4 24.4 24.4 25.0 24.4
obtained with porcelain and platinum crucibles, heated in the muffle furnace, and with duplicate samples heated over a Meeker burner in open air. The data show close agreement between plant material ashed over a Meeker burner and that ashed in covered highform porcelain and platinum crucibles in the muffle furnace. Evidently the use of these crucibles with covers eliminated the error previously obtained when samples were ashed in porcelain evaporating dishes in electric furnaces.
Platinum (covered) 24.4 24.4 24.4
Boron values reported in the literature show good reproducibility of values even when ashed in electric muffle furnacese.g., the data of Hatcher (3). He compares boron analyses by wet and dry combustion, with much higher values obtained for dry than for wet combustion. The conclusion is drawn that the samples prepared by wet combustion are lower and more variable as a result of loss of boron during the acid digestion, and that boron analysis by dry combustion gives good reproduc-
ibility, and implies that dry ashing is more reliable. However, the fact that dry combustion figures are higher and more constant than wet digestion values does not obviate the possibility of a constant hidden error in dry combustion values. This constant error can come from a procedure standardized in time and temperature of ashing. I n the light of this work, boron analysis on plant or soil material following dry combustion in a muffle furnace should be re-examined. LITERATURE CITED
(1) Calif. Agr. Expt. Sta. Bull. 766, 64 (1959). (2) Dible, W. T., Truog, E., Berger, K. C., ANAL.CHEM.26, 418-21 (1954). (3) Hatcher, J. T., Ibid., 32,726 (1960). (4) U. S. Salinity Laboratory Staff, U. S. Dept. Agr. Handbook, pp. 60, 129, 135 (1954). D. EMERTON WILLIAMS ' JAMES VLAMIS University of California Berkeley 4, Calif.
Rapid Fusion Determination of Phosphorus in Gasoline SIR: Motor gasolines commonly incorporate organophosphorus compounds to reduce spark plug fouling and preignition or to combat rumble in high compression engines. T o permit rapid analyses for phosphorus content, a colorimetric method was devised bmed on use of the reduced phosphomolybdate complex (1). The color system mas used previously for determining higher concentrations of phosphorus in lubricating oils (3). Phosphorus in gasoline has been determined by emission spectroscopy ( 4 ) . It has also been determined colorimetrically following conversion of the organophosphorus compounds to inorganic phosphate, either by evaporating the gasoline and wet oxidizing (6) or by ashing in zinc oxide (2, 4, 6). By the more rapid method described below a single sample can be analyzed in less than 30 minutes. Under routine conditions, 30 to 35 samples can be analyzed in a n 8-hour day. The method is applicable to phosphorus concentrations as low as 1 mg. per kg. (p.p.m.).
add 2.00 ml. of gasoline sample a t room temperature. (For samples containing more than 60 p.p.m. of phosphorus, use 1.00 ml. of sample.) Cover the sample with another measure of sodium carbonate. Ignite the gasoline by passing a flame over the top of the crucible. After burning has ceased, heat the crucible strongly in a Meker burner flame until a clear melt is obtained. Allow the crucible to cool for several minutes, and then place i t in a 400-ml. beaker containing 50 ml. of 1 to 11 sulfuric acid. [This amount of acid is
RESULTS
Table I.
The apparatus and reagents are similar to those used earlier (3). I n addition, a 15-ml. centrifuge tube was marked to contain 3 5 0.1 grams of sodium carbonate, anhydrous powder. Standard form 15-ml. platinum crucibles were found suitable for the fusion. Place one measure (3 i 0.1 gram) of sodium carbonate in a 15-ml. platinum crucible. By means of a pipet 968
ANALYTICAL CHEMISTRY
Determination of Phosphorus in Gasoline
Phosphorus, Mg./Kg. zinc oxide method Proposed Additive Present (2, 4, 6) method Tris( chloroisopropyl) thionophosphate 36, 35 35, 35 Tris( chloroethvl) - . phosphate 19, 19 19, 19 Trimethyl phosDhate
EXPERIMENTAL
sufficient to react completely with the sodium carbonate and provide the standard excess required for the colorimetric method ( J ) . ] After several minutes, when most of the sodium carbonate has dissolved, rinse down the walls of the beaker with 50 ml. of molybdatehydrazine reagent, and develop and measure the color as described previously. Measure the specific gravity of the gasoline and calculate the weight of sample. I n most cases a n average density of 0.74 gram per ml. may be assumed.
Alkylamine phosphate Tri-n-propyl phosphate hfethyldiphenyl phosphate Mixed methylphenyl phosphates
42. 43
42. 43
30, 30
29, 29
32, 32
33, 33
33, 33
33, 33
34, 18, Tricresyl phosphate 33, Cresyldiphenyl 10, phosphate 26,
35 18 35 10 27
34, 17, 33, 10, 26,
34 17 35 11
27
T o test the proposed method, a series of representative gasoline samples was analyzed by both the conventional zinc oxide (2, 4, 6') and proposed procedures, Table I. The standard deviations of the zinc oxide and proposed methods are 0.60 and 0.56 p.p.m., respectively. This represents no significant statistical difference. Table I1 shows phosphorus recoveries obtained, under various conditions, for gasoline blends of the more volatile additives being used currently. Except for the results on a blend using trimethyl thionophosphate, recoveries mere acceptable for these additives using the recommended procedure. DISCUSSION
I n preliminary experiments, attempts were made to adapt the oxygen-flask method ( S ) , but it was not possible t o burn sufficiently large samples to com-
