8
A,YALYTIC'AL EDITIO-AT
9 'i
RELAY IIOYOLT LINE
]
v MOTOR
contact and an adjustable contact is connected in series with a relay and a battery as shown in the accompanying figure. The armature of the relay is connected to the motor which drives the vacuum pump. When the manometer circuit is closed by the mercury rising t o the end of the adjustable contact, the relay is actuated and the motor is stopped. When the pressure within the system rises, thus breaking the manometer circuit, the pump is started and continues t o operate until the pressure is reduced t o the desired magnitude.
T'ol. 1, s o . 1
Satisfactory regulators have been constructed using commercial laboratory pumps driven by '/c-horsepower, 110-volt, a. c. motors. In the writer's laboratory, one of these maintained a constant reproducible pressure of 15 mm. with the pump operating less than 1per cent of the time. Any desired pressure between atmospheric and approximately 2 to 3 mm. may be maintained if care is taken t o prevent leaks in the system. Obviously, this device is limited to the use of mechanically operated pumps which may be stopped without allowing air to leak back through the pump mechanism into the system. The manometer should have an internal diameter of about 8 mm. in order t o minimize the curvature of the mercury meniscus. The use of a reservoir between the pump and the distillation system reduces the fluctuation in pressure to such an extent as to render it almost unobservable on the manometer. Obvious changes in the construction and the use of a pressure pump will enable pressures above atmospheric pressure to be readily obtained. The apparatus as described was designed by the writer when a t the laboratories of the Standard Oil Company (Indiana) a t Whiting, Ind., in 1923, and has given satisfactory results during the past four years.
Analysis of Maple Products',' D. E. Fowler and J. F. Snell MACDONALD COLLEGE, MCGILLUNIVERSITY, MACDONALD COLLEGEP. O., P.
X-Study
Q.,CANADA
and Modification of the Canadian Lead Method
ETHODS for the detection of adulteration of maple products, dependent on measurements of the quantity of precipitate produced by treatment of the diluted sirup with basic lead acetate, were first proposed in 1904 by Jones3 and by Hortvet14who measured the volume after centrifugal collect,ion. In 1906 the more accurate methods at present recognized by the L4ssociat'ionof Official Agricultural Chemists3 were originated by Winton and Kreidern and by t'he chemists of the Laboratory of the Canadian Inland Revenue Department17particularly A. valin. Studies on three sirups by Snell and Scott8 indicate that in falling off more rapidly in adulterated sirups than the maple content, the Canadian lead value has an advantage over all the other values used for the detection of adulteration with refined sugar. But that the method lacks in precision, as evidenced by comparison of duplicates, is well known,8 mean differences of 0.10 and 0.09 between duplicates by a single
M
1 Received August 16, 1928. For reference to previous papers, see IND.ENG.CHEM.,19, 275 (1927). 2 The experimental work reported and much of the comment are derived from a thesis presented b y Mr. Fowler t o the Faculty of Graduate Studies and Research of McGilI University in May, 1928, in partial fulfilment of the requirements for the M . Sc. degree. 3 Jones, Vermont Agr. Expt. S a . , 17th Ann. Rept., 1903-4, p. 446. 4 Hort%et,J . Am. Chem. Soc., 26, 1523 (1904). :Assocn. Official Agr. Chem., Methods, 1926, p. 204. I n 112 t h e quantity of water t o be used in the preparation of the basic acetate solution should be 600 cc. e Winton and Kreider, J . Am. Chem. Soc., 28, 1204 (1908). 7 Laboratory Inland Revenue Dept. (Ottawa), Bull. 120 (1906); 140 (1907). The Food and Drugs Laboratories of the Department of Health are successors t o this laboratory. 8 Snell and Scott, J. IND. ENG.CHEM.,5, 993 (1913). Laboratory Inland Revenue Dept. (Ottawa), Bull. 140 (1907); 228 (1911).
observer, of 0.13 between the means of two observers, of 0.16 between duplicates in another series of experiments (printed figures give 0.19) and occasional differences as great as 0.20 (equal t o 10 per cent in a lead number of 2.0) being found. The experiments of Valin, reported by Snel1,lo indicated that actually different quantities of lead mere precipitated in the duplicate determinations. On the other hand, Lancasterll has expressed the opinion that the variations between duplicates are probably due mainly t o irregularities in the washing. I n the present work the effects of temperature and quantity of reagent, as well as methods of washing, have been studied with a view t o improving this simple and useful determination. Materials The lead subacetate solution was made from Horne's salt according to the A. 0. A. C. directions. It contained 0.2243 gram lead per cubic centimeter. The ratio of neutral to basic leadlo was 1.88. The sirups were collected in Ontario and Quebec by district representatives of the Provincial Departments of Agriculture, instructed to collect only samples of unquestionable purity. A few (4 to 9) were supplied by A. Valin, analyst of the Dominion Department of Health in charge of the Montreal Laboratory. Time Required for Maximum Precipitation Except for the time of standing, the prescribed method6 was closely followed. The results are recorded in Table I. Except in sirup 2 a t one-half hour, the results show no varia10 11
Snell, J . Assocn Oficial A g u Chern , 4, 428 (1921). Lancaster, Ibzd , 8, 372 (1926).
INDUSTRIAL AA'D ENGINEERING CHEMISTRY
January 15, 1929
tions exceeding the range of those between duplicate determinations. Table I-Effect of Varying Time of Standing on Canadian Lead Number --SIRUP 2---SIRCP 11----SIRUP 13-TIME A B Mean A B Mean A B Mean Hours 1/z 2 . 5 4 2.64 2.59 2 . 4 1 2.37 2.39 3.19 2.99 3.09 1 2 . 7 9 2.78 2.79 2.35 2.41 2.38 3.20 3.34 3.27 ll/n 2.69 2 . 7 3 2 . 7 1 2.44 2.46 2.45 3.34 3.26 3.30 2 2.74 2.84 2.79 2.42 2.28 2.35 3.35 3.46 3.40 2'/1 2.78 2.90 2.84 2.38 2.38 3.31 3 . 1 0 3.20 2 . 8 1 2 . 9 1 2 . 8 6 2 . 3 7 2 . 3 7 3 .03 3.14 3.08 3 2.24 2.28 2.26 3.16 3.29 3 . 2 2
Precipitation at Various Temperatures
The aliquot sample was heated to the required temperature on a water bath, the subacetate added, and the temperature maintained within 2 degrees for 2 hours. The results in Table I1 are the averages of duplicate determinations. They are represented graphicaJly in Figure l. At 100" C. the precipitates were light brown, coagulated immediately, and darkened to deep chocolate on standing. At 80" C. the precipitates curdled immediately; on standing, a clear sirupy layer formed a t the surface, in which a fine secondary precipitate developed, similar to that formed a t room temperature. After 2 hours the color of the precipitate resembled that of the room-temperature, rather than that of the 100" C., precipitate. Table 11-Effect of Temperature of Precipitation on Canadian Lead Number
TEMPERATURE SIRUP 9
(Averages of duplicates) SIRUP 11 SIRUP 12
SIRUP
13
9
not exhausted by the 0.5 cc. of reagent. It is clear, therefore, that the solubility of the precipitate in the liquid present cannot be very low. This solubility appeared to the authors greater than was t o be expected if the water present were the sole solvent. Table 111--Effect of Varying Amounts of Lead Subacetate on Canadian Lead Number VOLUME OB
(Averages of duplicates) REAGENT SIRUP 3 SIRUP 1
cc.
....
0.5
SIRUP
2
SIRUP
4
SIRUP 7
1 , 9 3 6 2 . 3 6 8 2 506 2.066 2 . 7 1 0 3 074 4 136 2 , 5 9 4 2 , 7 2 7 3 . 1 5 2 4 658 2.0 2 , 5 7 7 2 , 7 4 4 2 . 9 6 2 5 042 2.5 2 . 4 5 3 2 , 5 9 4 2 . 8 5 2 4 916 3.0 2 . 1 5 0 2 . 3 4 3 2 , 7 2 8 4 928 4.0 1.978 2,442 4 722 1.674 .... .... 5.0 1 . 9 1 8 4 294 1.121 4 . 0 reduction from maximum 0.915 0.959 0.766 0.710 0.320 Per cent reduction of maximum 3 6 . 5 36.4 27.9 22.5 6.3 4 . 0 reduction from 2 . 0 0.815 0 . 9 0 3 0 . 7 6 6 0 . 5 2 0 0 . 3 2 0 Per cent reduction of 2 . 0 cc. number 33.8 35.1 27.9 17.5 6.3 5 . 0 reduction from 2 , 0 .., . 1.456 . . . . 1,044 0.748 Per cent reduction of 2 . 0 cc. number . . . . 56.5 .. 35.2 14.8 Range of values in the five sirups: Using 1 .O cc. reagent, 4 . 1 3 6 - 2 , 5 0 8 = 1 , 6 2 8 = 6 4 . 9 % of minimum 2 , 4 7 0 = 2 . 1 8 8 = 8 8 . 6 Yoof minjmum Using 1 . 5 cc. reagent, 4 . 6 5 8 Using 2 . 0 cc. reagent, 5 . 0 4 2 - 2 , 4 0 8 = 2 . 6 3 4 = 1 0 9 . 5 % ' of minimum 2,508
1.0 1.5
2.470 2,408 2.250 2,132 1,593
2.633
. .
-
Work on the Winton method had led RossI3 to the conclusion that the precipitates produced by interaction of maple sirups and basic lead acetate solutions were soluble both in sucrose and in excess of the subacetate. As this would account both for the incomplete precipitation of lead by excess of sirup and for the falling off of lead values in presence of excess of subacetate, it was decided t o seek confirmation of Ross's conclusion.
c. 100 80 60 40 20
2.826 3.284 3.620 4,484 4.630
1.590 1.512 1.539 2.230 2.640
2.174 2.240 2.139 2.696 2.840
2.386 2,500 2,800 3,600 3,676
In all four sirups there was a great falling off in amount of precipitate between 40" and 60" C., and a smaller change between 20" and 40" C. Either the substances constituting the precipitate are more soluble in hot solution than in cold, or the precipitate obtained a t high temperatures is differently constituted from that formed a t room temperature. Differences in the physical nature of the precipitates favor the second supposition. Precipitation a t room temperature appears most advantageous.
I 20
I 40
CO
80
Tempe r o t urc -~
/do
Figure 1-Effect of Temperature of Precipitation upon Canadian Lead Numbers
Quantity of Reagent Effect of Sucrose
Results with 0.5 to 5 cc. of lead subacetate solution are given in Table 111. Maxima occur a t 1.0 to 2.0 cc., the amount of precipitate falling off rapidly when more than 2.0 cc. are added. I n no case is the lead number such as to indicate that nearly all the lead of the reagent has been precipitated. Thus, assuming the precipitate to contain 70 per cent of lead,I2 the largest precipitate obtained with the smallest quantity of reagent (0.5 cc.)-via., that from sirup 7-would represent 2.506 X 70 + 2000 = 0.08771 gram of lead, which is only 78 per cent of the 0.1122 gram contained in the 0.5 cc. of reagent. A similar calculation on sirup 2 indicates that the precipitants of that sirup took down only 60 per cent of the lead of' the 0.5 cc. of reagent. On the other hand, in view of the greater amounts of precipitate obtained with 1.0 to 2.0 cc. of reagent, i t is evident that the precipitants of the sirup are 1 2 Scott found 67 t o 70 per cent lead in precipitates produced by the Canadian lead method. See Snell, J. IND.ENG.CHEM, 8, 144 (1916).
(A) Four aliquots from sirup 11 were precipitated as in the Canadian lead method. After 2 hours' standing, as much of the supernatant liquid as possible was decanted onto tared Gooches. In two of the containers the decanted sirup was replaced by an equal volume of 25 per cent sucrose solution; in the other two by an equal volume of water. The mixtures were stirred and allowed to settle. This was repeated three times. Then the precipitates were transferred to the Gooches, washed with hot water as usual, dried, and weighed. Expressed as lead numbers, the average of the sucrose duplicates was 1.932, that of the water duplicates, 2.138. (B) To 20-cc. aliquots of a solution from sirup 13 containing 5 grams of dry matter, 20 cc. of a 60 per cent sucrose solution and 2 cc. of the standard lead subacetate were added. A check determination was run with 20 cc. of water in the place of the sucrose solution. The determinations, completed la
Ross, U S Dept. Agr
, Bur. Chem. Czrc 63 (1910).
ANALYTICAL EDITION
10
in the usual way, averaged 2.672 for the sucrose duplicates and 3.786 for the water checks. The results of both series of experiments evidence a considerably greater solubility of the precipitate in sucrose solution than in water. Effect of Excess of Lead Subacetate
~
Four aliquots from each of three sirups were precipitated in the usual manner. After standing 1 hour-a time found sufficient for complete precipitation (see Table 1)-an additional 3-cc. portion of the subacetate was added to each of two of each set of four aliquots. All were then allowed to stand 1 hour longer and the determinations completed as usual. The results, recorded in Table IV, show clearly that the precipitate (or some constituent thereof) is freely soluble in excess of the reagent. Both Ross's conclusions are thus confirmed. The solvent effect of sucrose will probably not vary much from sirup to sirup. The same quantity of dry matter, as indicated by the refractometer, is used in all the determinations of Canadian lead value, and the proportions of invert sugar and other constituents in this dry matter are small in comparison with the sucrose. The proportions of the various substances which react with the basic lead acetate no doubt differ also to some extent in different sirups, and the solvent effect of the sucrose may vary to some extent with the composition of the precipitate. Inasmuch, however, as the range of variation of lead value in genuine sirups is determined by the same procedure as is used in testing for adulteration, the solvent effect of the sucrose would not appear to be a factor of much significance in reference to the detection of adulteration. Experimentation upon concentrations other than 25 grams dry matter per 100 cc. might possibly be fruitful, but the prospect is not so good as for modifications of the method in other directions. Effect of Lead Subacetate on Canadian Lead Precipitates (Averages of duplicates) CANADIAN LEADNUMBERS METHOD Sirup 36 Sirup 13 Sirup 40 Usual procedure with 2 cc. subacetate 2.216 3.324 5.280 With addition of 3 cc. extra subacetate 1.216 1.860 4.440 after 1 hour 1.000 1.464 0.840 Reduction 45.1 44.1 16.9 Per cent reduction of 2 cc. number Table IV-Solvent
As regards the basic lead acetate, it is very evident that the solvent effect will not be identical for all sirups. The lower the content of precipitable matter, the greater will be the excess of basic acetate and, therefore, the greater its solvent effect on the precipitate. Consequently, low lead numbers will be reduced much more than high ones. The range of the Canadian lead number in genuine maple. goods will therefore be wider than the range of content of the substances upon which the determination depends. (The data of Tables I11 and IV show a greater effect of excess of subacetate on low than on high lead values.) In spite of this disadvantageous effect of the excess of reagent, however, the range of variation of Canadian lead number appears to be narrower than that of the majority of analytical values of maple products.14 I n particular, it appears to be distinctly narrower than that of the malic acid value-a value determined by a method (precipitation by calcium and alcohol) designed to measure the constituent which is largely responsible for the precipitation of the lead of the subacetate reagent, viz., the malate radical. The solvent effect of the lead subacetate might probably be diminished and the range of lead numbers in genuine goods somewhat narrowed by using a smaller quantity of the reagent than 2 cc. Table I11 shows that the two sirups with lowest 14
Trans. Roy. SOC.C a n , [3], 13,111, 221 (1919).
Vol. 1, No. 1
lead numbers (1 and 3) give a greater precipitation with 1 than with 2 cc. It will be noted also that, in €he five sirups involved, the range of variation of the lead number is much narrower when 1 than when 2 cc. of reagent are used-vie., 65 per cent of the minimum of value as compared with 110 per cent. This point deserves study on a greater number of samples. The fact that in sirups of hi,gh lead value the maximum precipitation would not be reached with less than 2 cc. of reagent is not important, as i t is those of low lead value that are likely t o be under suspicion. Regulation of Washing
I n the current method filtration becomes slow as soon as the precipitate is transferred to the Gooch, but as soon as all the filtrate has passed through, cracks form in the precipitate, and added wash water runs through very quickly. Hot water tends to cause the cracking, eveii when the precipitate is kept covered. As the surface of contact and the time of contact of the precipitate with the wash water vary considerably from experiment to experiment, differences in duplicates are to be anticipated and the magnitude of these differences will depend upon the solubility of the lead-maple precipitate in hot water. Table V-Solubility of Lead-Maple Precipitate in Hot Water CANADIAN PRECIPITATB DISSOLVED LEAD TOTALSOLIDS IN In 260 CC. In 500 CC. SIRUP NUMBER 100 cc. ALIQUOTS water water Gram Per cent Per cent 36 2.2 0.0173 38.05 58.08 41 3.1 0.0266 43.50 60.62 40 5.2 0.0373 34.12 60.49
To get an idea of the magnitude of this solubility, the wellwashed precipitates from the Canadian lead number determinations of three sirups together with the asbestos were transferred to flasks, heated on a steam bath 5 hours with 250 cc. water under return condensers, and brought to boiling. One hundred cubic centimeter aliquots of the filtered solution were evaporated a t 100" C. The results are shown in Table V. Portions of the filtered solutions also were allowed to cool. In these a fine white precipitate formed, showing that the solubility is greater in hot than in cold water. The residues from the first treatment were again treated with 250 cc. of water in the same manner, with the results shown in the last column of Table V. It is clear, then, that the solubility of the precipitate in hot water is great enough to demand careful regulation of the washing process. Washing by Centrifugal Decantation
It appeared that more uniform conditions of time and contact ought to be obtainable by a decantation process. The slow settling of the precipitate, however, offered a difficulty, and centrifugal settling was adopted. The precipitation was performed in centrifuge tubes. After 2 hours' standing the tubes were whirled for a few seconds in a Babcock milk-test centrifuge. The supernatant liquid was decanted to the Gooch and 17.4 cc. of hot water were delivered into the tube from an automatic pipet, connected by asbestos-insulated tubes with a flask of boiling water. After mixing and a few seconds' standing, the tube was again whirled in the centrifuge and the supernatant liquid decanted. This was repeated to a total of four washings. After the second washing the use of the centrifuge was usually superfluous as the precipitate had become granular and so settled well. A fifth and a sixth portion of wash water were used in transferring the precipitate to the Gooch, making 104.4 cc. of wash water in all. The results of eight determinations by the established method and six determinations by the centrifugal decantation method on each of three sirups are summarized in Table VI.
IilrDL-STRIAL AND ENGINEERING CHEMISTRY
January 15, 1929
It will be observed that washing by decantation reduces the range of variation to about half that shown where ordinary washing was practiced. The variations between duplicates in the writers’ determinations by the established Canadian lead method are of the same general magnitude as in those reported by the Laboratory of the Inland Revenue Department.15 The latter were 0.10 in ten sirups of average lead number 1.92 = 5.2 per cent; 0.13 in 10 sirups of average lead number 1.97 = 4.6 per cent; 0.16 (or, according to the figures of the table as printed, 0.19) in twenty-four sirups of average lead number 3.11 = 5.1 (or 6.1) per cent. Table VI-Comparison of Canadian Lead Numbers, Decantation Method with Ordinary Method SIRUP31 SIRUP44 SIRUP48 Variation ma:. to
METHODO F WASHINGPPT. Decantation : 6 detns. In crucible: 8 detns.
Variation max: to min.
Variation ma:. to min.
min. Per cent of mean
Mean
2.353
2.61
3.682
3.04
4.721
1.44
2.107
5.79
3.472
5.88
4.325
2.68
Mean
Mean
Per cent of mean
Per cent oj mean
Duplicate determinations made on nine sirups by the decantation method gave the results reported in Table VII. It will be observed that the average variation of duplicates amounts to only 0.027 or 0.8 per cent of the mean, and the maximum differences to 0.056, equal to 1.9 per cent of the mean. Table VII-Canadian SIRUP
Lead Numbers-Decantation
A
B
2.336 2.296 4.132 5.680 3.632 3.544 4.608 2.936 2.688
2.360 2.280 4.100 5.716 3.682 3.528 4.600 2,992 2.692
Method
DIFFERENCEDIFFEBENCE MEAN OF DUPLICATESIN MEAN
Per cent 31 36 38 40 44 47 48 49 50 Average
2,348 2.288 4.116 5.698 3.657 3.536 4.604 2.964 2.690 3.545
0.024 0.016 0.032 0.036 0.050 0.016 0.008 0.056 0.004 0.027
1.0 0.8 0.7 0.6 1.4 0.45 0.2 1.9 0.15 0.8
In all the nine sirups of Table VI1 higher results were obtained by the decantation method than by the ordinary Canadian method. This was unexpected, but may be due to the facts (1) that the wash water after passing through the automatic pipet has a lower temperature than when poured directly from a wash bottle into the crucible, and (2) that it cools still further during the centrifugation. I n other words, though more thorough contact of precipitate with wash water is secured than in the regular method, that contact is with water of lower solvent power than in the regular method. It would seem probable that centrifugal washing with water of a temperature equal t o that of the water used in the regular method would give lower, rather than higher, lead values; but such washing is obviously difficult to secure in practice. Volume of Wash Water
Sets of three aliquots of solutions of two sirups were treated by the decantation method with, respectively, 3, 6, 9, and 12 portions of hot water from the 17.4-cc. pipet, making the approximate total volumes of wash water 50, 100, 150, and 200 cc. The results are recorded in Table VIII. Table VIII-Effect of Volume of Wash Water on Precipitate. Canadian Lead Numbers-Decantation Method [Means of tridicates) Volume, cc. 50 100 150 200 Sirup 38 4.320 4,026 3.874 3.769 Sirup 54 2.496 2.279 2.133 1.843
The steady decrease of lead number with increasing amount of wash water is attributable to the dissolving of the precipi16
Laboratory Inland Revenue Dept. (Ottawa), Bull. 228, 11 (1911).
11
tate in the hot water. Using the regular method in a similar series of experiments, Scotts failed to detect this difference and concluded that it was indifferent whether 100 or 150 cc. of hot water were used in the washing. Time Required for Complete Precipitation
Determinations by the decantation method were made upon triplicate aliquots from each of three sirups with, respectively, l/6, I/*, 1, 2, and 5 hours’ standing. The results (Table IX) show a steady increase in lead number with time of standing. The original method failed to detect such a time effect both in the writers’ hands (Table I) and in those of Valin.10 This is to be attributed to the superior precision of the decantation method. Table IX-Effect
of Time on Precipitation. Canadian Lead Num-
bers-Decantation Method (Means of triplicates) 2 HOURS SIRUP 10 MINUTES30 MINUTES 1 HOUR 35 2.581 2.608 2.685 2.752 36 1.960 2.024 2.076 2.080 25 3.324 3.467 3.496 3.496
5 HOURS 2.778 2.147 3.577
Washing with Cold Water without Centrifugal Decantation
It was found that cracking of the precipitate in the Gooch crucible could be prevented by washing with cold water, taking care to keep the precipitate continually covered. (It has been observed that when the rate of filtration exceeds 40 drops per minute, cracking $may occur even under cold water.-J. F. S.) Eight aliquots from each of three sirups were treated by this method and eight by the regular method. In the cold washing the crucibles were filled four times with cold water, each portion being added just before the last of the preceding liquid passed through. The results are summarized in Table X. They show ,marked superiority for the cold water method as regards precision. It may be pointed out that greater variations among multiplicate determinations by the regular method are shown with sirup 42 than with those included in Tables VI and VII, and that in this series of experiments on sirup 48 a considerably wider range (though the same mean) was found than in the series reported in Table VI. Table X-Comparison of Canadian Lead Numbers Cold Water Method of Washing with Ordinary Methbd SIRUP42 SIRUP43 SIRUP48 METHODO F WASHINQ
Mean
Variation max. t o min.
Mean
Per cent
Mean
Per cent of mean
of mean
Cold water: 8 detns. H o t water: 8detns.
Variation max. to min.
Variation max: to min.
Per cent of mean
3.310
4.11
2.933
3.55
4.495
2.49
3.180
11.07
2.710
4.30
4.329
8.59
From the eight determinations on sirup 48 by the ordinary method, reported in Table X, the lowest value obtained was 4.152 and the next lowest 4.252 (which coincides with the minimum reported for the same sirup by the same method in Table VI). If we reject the lowest value, the mean becomes 4.354 and the range (4.252-4.524 = 0.272) 6.25 per cent of the mean, which is still considerably more than twice that of the values by the cold washing method. , Compared with washing with hot water by centrifugal decantation, washing with cold water appears to be somewhat less precise but more convenient. I n the sirups to which it has been applied it has, like the decantation method, yielded lead values somewhat greater than those by the regular method. It should be pointed out that the ranges of variation discussed here are those between determinations made upon aliquots of the same dilution of a sirup by the same observer on the same day. While, therefore, well adapted to their
ANALYTICAL EDI TIOW
12
covered with water and filtering a t a rate not exceeding 40 drops per minute. Dry a t 100" C., weigh, and multiply the weight by 20.
purpose, they no doubt represent a degree of precision much beyond that to be expected of results by different observers or even by the same observer a t different times. In Table VII, for example, we have two determinations on sirup 48 by the decantation method which both give values far below the minimum found by the same observer on another day in the series reported in Table VI.
Summary
1-In the current Canadian lead method, large experimental errors occur due to the formation of fissures in the precipitate and consequent irregularities in the solvent action of the hot wash water. 2-Washing by decantation, using centrifugal settling, eliminates the error due to cracking and greatly reduces the difference between duplicates. With the greater precision attained by this procedure, it becomes evident that increasing the volume of wash water decreases the lead number. 3-The lead-maple precipitate is soluble in sucrose solution and in excess of the basic lead acetate solution. In different sirups maximum precipitation is realized with different quantities of the reagent. 4-Much lower results are obtained by precipitating a t 40" C. than a t room temperature. Results a t BO", 80°, and 100" C. are lower than at 40" C. 5-The amount of precipitate increases slightly with the interval between addition of reagent and filtration. 6-Washing with cold water, keeping the precipitate covered therewith, and avoiding too rapid filtration, prevents cracking and gives results much more precise and somewhat higher than those obtained in the current procedure. The adoption of cold water washing is proposed.
Proposed Modification of Method
Further study of the advisability of using 1.0 or 1.5 cc. of subacetate reagent is being made under direction of the senior author (J. F. S.). Retaining for the present the use of 2 cc., the directions for the cold washing method (Canadian lead method, Fowler modification) are as follows: PREPARATION OF LEADSUBACETATE SoLuTIon--Boil 280 grams of Horne's dry basic lead acetate with 500 cc. of water. Pour off from slight undissolved residue, allow to cool, and dilute with recently boiled water to a density of 1.25 a t 20" C. DETER?rIIxATIos-Weigh the quantity of prepared sirup containing 25 grams of dry matter, transfer to a 100-cc. volumetric flask, and make up to mark a t 20" C. Pipet 20 cc. into a large test tube, add 2 cc. basic lead acetate solution, cork, and allow to stand 2 hours. Filter with suction on a 25-cc. tared Gooch having an asbestos mat a t least 3 mm. thick. Before quite all the liquid has passed through, fill the crucible with cold water. Repeat the washing to a total of four times, taking care t o prevent formation of fissures by keeping the precipitate completely
XI-Composition
of the Canadian Lead Precipitate
No detailed analysis of the precipitates produced upon addition of basic lead acetate solutions t o diluted maple sirups, as in the determination of the lead numbers employed in the detection of adulteration,16 has been published. As salts are known to constitute the bulk of the non-sugar solids of the sirup and malates t o be the most abundant of these, it would appear probable that lead malates, normal or basic, would be the chief constituents of the precipitate. Scott's analyses, reported by Snell,17 show a lead content of 69.4 per cent in the Canadian lead precipitate from a composite of fifty-four Canadian sirups, one of 70.1 per cent in another mixture of sirups, and from 66.95 to 69.62 (average 68.4) in six individual sirups, having Canadian lead numbers 1.79 t o 5.08, the lead content of the precipitate being higher the lower the lead value. These results suggest a composition approximating that of a hydrated basic salt with equal quantities of neutral and basic lead. (Anhydrous normal lead malate would have 61.07 per cent; the monohydrated basic salt, 2 PbO. C4H404.H,O, 71.39 per cent; and the anhydrous basic salt, 2 PbO. C4H404,73.68 per cent Pb.) In the present investigation lead, basic lead, carbon, hydrogen, and total organic acids were determined in the precipitates from three Quebec sirups of the 1927 crop, and a polarimetric determination of malic acid was made in a composite of precipitates from some twenty or thirty sirups. For all the determinations except that of the malic acid the sirups mere precipitated in the usual manner of the Canadian lead method, the precipitates were washed with hot water by the decantation method described in Part X, and weighed as well as analyzed. Methods of Analysis
TOTAL LEAD-The precipitate was dissolved from the asbestos filter with hot 2 N nitric acid. The solution was 18
17
Assocn. OfficialAgr. Chem , iMethods, 1925, p 204. Snell, J. IND. ENG.CHEM.,8, 144 (1916).
Vol. 1, y o . 1
,
neutralized with sodium hydroxide, acidified with acetic acid, and precipitated with potassium chromate. The precipitate was dried and weighed. Determinations were made in duplicate. BASICLEAD-Determinations were made in quadruplicate. The filtrate and washings from the Canadian lead precipitate were treated with 10 cc. of 0.5 N oxalic acid and filtered. The filtrate was titrated with 0.1 N sodium hydroxide. Blank determinations were run on 2 cc. of the lead subacetate reagent, 25 per cent sucrose solution being substituted for the maple sirup. The difference between the amounts of the alkali neutralized in the blank and in the experiment proper, corrected for the acidity found in the maple sirup itself, represents the alkalinity of the precipitate. This is calculated to per cent basic lead in the precipitate, which was dried and weighed as usual. Sirup 41, for example, gave 0.1660 gram of precipitate. From 40.9 t o 41.2 cc. of 0.102 N sodium hydroxide were required to neutralize the residual acidity from 10 cc. 0.497 N oxalic acid. The average titration was 41.05 cc. That for the blank was 34.63 cc.; that for the sirup itself, 0.36 cc. The difference in alkalinity of the liquid due to formation of the precipitate is therefore equivalent to 41.05 - 34.63 - 0.36 = 6.06 cc. 0.102 N = 6.18 cc. 0.1 N = 0.0640 gram lead. Therefore, the precipitate contains 38.53 per cent of basic lead. In the other two sirups the variations of the titrations were less than in sirup 41. ORGANIC AcIDs-Determinations were made in duplicate. The dried precipitate was ground in the crucible, transferred with the asbestos to a flask, covered with 100 cc. water, and transformed with hydrogen sulfide. To effect complete transformation it was found necessary to alternate heating to boiling and passing the gas to coldness several times. CARBON AND HYDROGEN-FrOm two to four determinations were made on precipitates from each sirup. The precipitate and asbestos were transferred to a combustion boat and
January 15, 1929
I N D LTSTRIAL A,VD EhTGIA7EERIhTGCHEMIXTRY
13
The wide variations in the quantities of total lead precipitated by a given quantity of dry matter of sirup are attributable, a t least in part, t o tha,t tendency of the precipitate to dissolve in excess of reagent discussed in Part X. What chemical action is involved in such dissolving is not known. The two experiments on the combined precipitates from miscellaneous sirups using diff ereiit decolorizing carbons give results indicating, respectively,that the malic acid constituted 95.93 and 98.08 per cent of the total acids liberated by hydrogen sulfide.19 The tabulated figures for normal lead malate are therefore the equivalents of 97 per cent of the acids so liberated. Added to the lead oxide as base, these account for only 83 to 84 per cent of the weight of the precipitate. The carbon of these quantities of lead malate is little more than half that actually found in t'he combustion analysis. Even were we t o assume the neutral lead as determined by difference bet'weeii total and basic lead to be all present as lead Discussion of Results malate, the carbon of this salt would be considerably short of that found by combustion. Similarly, the hydrogen of the The results are summarized in the accompanying table. malate, calculated either way, falls far short of that found by The lclad content of precipitates is higher, the lower the combustion. It is to be inferred that, in addition to basic Canadian lead number. For two of the sirups the results on lead malate, the precipitate contains a considerable quantity total lead are higher than any of Scott's; for the third, lower of highly carbonaceous organic matter. than a n y but one of Scott's. It should be borne in mind, I n the transformation of the lead precipitate with hydrogen however, that the method of washing and also the method of sulfide it was observed t'ha,t the acid filtrate was colorless. determining the lead were different in the two investigations. On neutralization a brown color developed. When the The basic lead is higher, the lower the Canadian lead number. neutralized, but not when the acid, solution was concentrated, maple odor could be detected. The flavoring and coloring C o m p o s i t i o n of C a n a d i a n L e a d P r e c i p i t a t e s of T h r e e M a p l e S i r u p s matters of the sirup appear, therefore, to be contained in the SIRUP36 SIRUP41 SIRLP40 lead precipit'ate. Middle Middle Early Run of sap Iron Modern ,Modern Sitrogen and sulfur were overlooked in the ult'imate analyEvaporator 19 5 8 Color sis, Previous investigations20 suggest that these may possibly 3 09 5 15 2 19 Canadian lead value Total carbon, per cent 11 13 11 7 1 12 23 constitute 1 or 2 per cent of the precipitate, provided the Total hydrogen, per cent 1 46 1 22 1 15 lead subacetate precipitates all or nearly all the compounds Oxygen, nitrogen, sulfur, e t c , by difierence 14 77 16 54 19 04 containing them. I n further work on this line the writers Total lead, per cent 72 64 70 53 67 58 would suggest t'he determination of these elements and also 100 00 100 00 100 00 that of reducing sugars in the filtrate from the hydrogen Basic lead, per cent 38.62 38.53 35.44 Neutral lead, by difference 34.02 32.00 32.14 sulfide treatment.
burned in oxygen in an electrically heated combustion furnace in the usual manner, a copper spiral being used. By use of blank determinations the hydrogen results were corrected for water given off by the asbestos. The residue in the boat contained metallic lead and litharge. M A i x ACID-TO determine generally what proportion of the tolal organic acids in the precipitate is malic, about fifty precipitates, obtained from a large (unrecorded) number of sirups, mere ground together and transformed with hydrogen sulfide as described above. The solution of the acids (1500 cc.) was concentrated to about 200 cc., boiled with decolorizing carbon, and filtered. Different decolorizing carbons were used in duplicate experiments. Total acids were determined in a measured volume of the filtrate by titration with 0.05 11- sodium hydroxide. Malic acid was determined by polarization of the neutralized solution, with and without saturation with uranyl acetate.18
Seutral lead equivalent of organic acids Neutral lead in excess of organic acids Ratio neutral lead (by acids) t o basic lead Ratio of neutral lead (by difference) t o basic lead Total lead precipitated by 100 grams dry matter of sirup, grams
25.95 8.07 0.77
26.70 5.30 0,69
2S,93 3.21 0.82
0.88
0.83
0.91
1.65
2.24
3.68
Normal lead malate equivalent of 97 per cent oi the organic acids Lead oxide as base Sum
41.23 41.61 82.84
42.40
45.99 38.18 84.17
Carbon of normal lead malate as estimated above Carbon of lead malate, per cent of total carbon
52 5
5.84
41.51 83.91 5.99
51 1
6,5l
53 2
Calculated as malic acid the percentages of organic acids are: ;,irup 36, 16.56; 41, 17.02; 40, 18.46. In the table the equivalent amounts of neutral lead are given. As thus calculated, the neutral lead is lower, the lower the Canadian lead d u e . There is, however, a considerable discrepancy between this estimate of neutral lead and that obtained by taking the difference between total and basic lead, the latter being always the greater. This suggests that some of the neutral lead of the precipitate may be associated with substances which are not determinable as acids by titration. The percentage of neutral lead by difference is nearly equal in the precipitates from the two sirups of higher lead values, but distinctly greater in the other sirup (36). The content of basic lead is decidedly higher than that of neutral lead, indicating the presence of salts more basic than the lead malates mentioned in the introduction. 13 Assocn. Official Agr. Chem., Methods, 1925, p. 213, Dunbar and Bacon, U. S. Dept. Agr., Bur. Chem. Czrc 76 (1911).
Summary
1-The Canadian lead precipitates of three maple sirup samples have been analyzed for t,otal and basic lead, carbon and hydrogen, and total (organic) acids, liberated by hydrogen sulfide. 2-The total lead content is greater, the lower the Canadian lead value, varying from 72.64 t o 67.58 per cent. 3-The basic lead content appears t o be slightly higher, the lower the Canadian lead value. 4-The difference between the tot.al and basic lead considerably exceeds the lead equivalent of the acids liberated by hydrogen sulfide. &-The inalic acid, as determined polarimetrically in a composite precipitate from more than twenty sirups, constitutes 97 per cent of the acids liberated by hydrogen sulfide. 6-The precipitates were found t o contain 11.1t o 12.2 per cent of carbon and 1.46 to 1.15 per cent of hydrogen. This indicates that substances much more carbonaceous than lead malate are present. 7-The flavoring and coloring matters appear to be precipitated by the basic lead acetate. 18 Nelson, J . Am. Chem. Soc., 60, 2006 (1928), finds material quantities of formic, acetic, and citric acid in maple sirup. The last mentioned was discovered by him in a lead subacetate precipitate. Hls estimate makes i t constitute S t o 12 per cent of the sum of the malic and citric in the sirup. Oui analyses would allow of only 2 t o 4 per cent of the acids of the precipitate being other than malic. 20 Hortvet, J . Am. Chem. Soc.. 26, 1523 (1904); Snell, Trans. Roy. SOC.Can., [3] 13, 111, 221 (1919).
