Metallic Silver as an Ultimate Standard in Volumetric Analysis C. W. FOULKAND L. A. PAPPENHAGEN Department of Chemistry, The Ohio State University, Columbus, Ohio
T
HE ease with which silver
lamp used for melting the silver This paper presents a precision methodfor the of a high degree of purity was specially cleaned and polished use of pure silver in standardizing a hydrochloric to prevent t h e p o s s i b i l i t y of can be prepared, its stacorroded material falling into the acid solution, a method for preparing silver for bility even if g i v e n n o m o r e silver The actual melting was use as a standard in acidimetry, an analytical protection t h a n a s t o p p e r e d accomplished by d i r e c t i n g the blast flame onto the silver in method for using silt:er as an ultimate standard in bottle, a n d i t s c h e m i c a l bethe crucible. This prevented the havior toward other elements, acidimetry, and a comparison of the analytical occlusion of oxygen by the render it One Of the most molten metal. As a further memethod with the precision method. important standard substances caution the silver was allowed to solidify in a reducing (yellow) in precision chemical measureflame. The resulting buttons were of fine appearance. They ments; and it is obvious that these characteristics should were scrubbed with ground pumice, washed and rinsed with disrender it an ideal ultimate standard in volumetric analysis tilled water, and kept under distilled water till used (4). if a simple and accurate method were at hand for using it in MISCELLANEOUS ANALYTICAL REAGENTS.In the analytical this way. T o provide such a method has been the object of procedures the best analyzed brands of reagents were used. The distilled water was the ordinary kind of the laboratory. All rethis investigation. agents and the water used were tested for chlorides. Specifically, a scheme has been developed for standardizing HYDROCHLORIC ACID. The hydrochloric acid solution was a hydrochloric acid solution by determining the weight (or carefully prepared, because the results of its standardization with volume) of the acid solution which is equivalent to a known analytical silver and the comparison of these results with those weight of pure silver. The procedure consists in dissolving a obtained with atomic weight silver was the major work of the investigation. The best reagent acid was diluted to a density weighed portion of silver in nitric acid and then finding by a of about 1.10, and 600-ml. portions were distilled from a Richard’s Gay-Lussac titration the weight (or volume) of hydrochloric flask ( 3 ) . A Jena glass tube equipped with a water jacket served acid solution which is just equivalent to the silver. Equiva- as a condenser. The first and last portions of the distillate were discarded. A quantity of the acid thus prepared was diluted lence is assumed when two portions of the supernatant liquid with conductivity water to about 0.3 N nnd preserved in a above the precipitated silver chloride give equal opalescence ceresin-lined bottle equipped with a trap so that in-going air on treating one with a n excess of silver and the other with a n passed through soda lime and bubbled through a portion of acid excess of chloride. The precipitated silver chloride is not the same as that in the bottle. filtered or handled in any way. APPARATUS The plan of the investigation was, first to prepare material of “atomic weight” purity and then by “atomic weight” A long-arm Troemner balance and carefully calibrated measurements and manipulation to determine the ratio be- weights were used. Weighings were made with the same detween a hydrochloric acid solution and metallic silver. This gree of accuracy in the analytical as in the precision profurnished a standard b y which the results of the second part cedures in order to keep the comparison of the results with of the investigation could be judged-namely, proposed atomic weight and analytical silver on a n equal basis. analytical procedures for the use of silver in standardizing the A Bausch and Lomb nephelometer was used in the preciacid. sion measurements.
PREPARATION OF MATERIALS The materials fall into two classes: “atomic weight” silver and reagents used in the precision measurements, and “analytical” silver and reagents used in the “analytical” measurements. In the preparation of the “atomic weight” silver and reagents the methods of Richards and his students (5) were followed, including double and sometimes triple precipitations, both chemically and electrolytically, washing with centrifugal drainage, and final fusing on pure lime in a n atmosphere of pure hydrogen. The manipulation employed was also that of modern atomic weight workers-for example, the use of the Richards bottling apparatus (6).
‘l A SILVER.”~ Two samples ~ of analytical ~ silver ~ (A and B, Tables I11 and IV) were prepared as follows: Ordinary silver nitrate was recrystallized once from a saturated solution by the addition of concentrated nitric acid. These crystals were centrifugalized and dissolved in water, and the silver was precipitated hot with ammonium formate made by passing ammonia into freshly distilled formic acid. This silver was then washed and melted in portions in a lime-lined porcelain crucible. The lime lining consisted of four parts of analytical reagent calcium carbonate to one part of powdered calcium nitrate of the same grade. This mixture when packed dry into a crucible sinters together on ignition, making a lining of lime. The blaPt
NORMALITY OF HYDROCHLORIC ACID BY PRECISION
METHOD A button of silver was dissolved in warm dilute nitric acid (water bath) in a glass-stoppered flask equipped with a bead tower for retaining any spray that might otherwise escape. The solution of silver nitrate was then diluted in the flask with two or three volumes of distilled water and boiled quietly t o remove oxides of nitrogen, after which it was carefully transferred to a 500-ml. volumetric flask. diluted to the mark, and mixed. An amount of hydrochloric acid equivalent to the silver solution (by preliminary titration) was next weighed into a 500-ml. flask, diluted to the mark, and mixed. This acid solution was then transferred to a 1500-ml. Pyrex glass flask, exactly 100 ml. of water being used in the transfer. The ~silver solution ~ was added ~ to the hydrochloric ~ ~acid solution in the Pyrex glass flask in a room lighted only by red light. Sufficient shaking was employed to make a uniform mixture. Again exactly 100 ml. of water were used in rinsing the flask. This use of the volumetric flasks and measurement of the rinse water was for the purpose of knowing with close approximation the final volume of the mixture. It was 1200 ml. The flask was finally covered with a black cloth and allowed to stand overnight. On the following day the mixture was stirred rapidly for 15 minutes with a motor stirrer and then allowed t o settle. In nearly all cases 15 minutes’ settling was sufficient to give a supernatant liquid that showed no Tyndall effect. This
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I NDUSTR IAL AN D ENGINEERING CHEMISTRY
solution was then adjusted to equivalence of silver and chloride ions as follows: Two 1.0-ml. portions of the supernatant liquid were placed in test tubes; to one, 1 ml. of 0.01 N silver nitrate solution was added and to the other, 1 ml. of 0.01 N hydrochloric acid. I n nearly all cases this preliminary test showed whether silver or chloride ion was in excess. For example, to determine approximat,ely the extent of an excess of chloride ion, several IO-ml. portions of the supernatant liquid were placed in test tubes and to the first tube 1 drop of 0.001 N silver solution was added, to the second tube 2 drops, etc, (If this method is employed for obtaining the preliminary strength of the acid, it is better to use 0.01 N silver solution in the first tests.) Usually three tubes
FIGURE 1. TEST-TUBE RACKNEPHELOMETER were sufficient. To each of these tubes an excess of chloride ion (1ml. of 0.01 N hydrochloric acid) was added, and the opalescence produced in each case Was compared with that Produced by adding 1ml. of 0.01 N silver solution to a fresh 10-ml. portion of the supernatant solution, designated as tube S. If the tube containing 2 drops of the 0.001 N silver nitrate solution matched tube S in opalescence, it is evident that the excess of chloride ions in a 10-ml. portion of the supernatant liquid was approximately equivalent to 2 drops of 0.001 N silver solution. The above procedure gives only a close approximation to the amount of silver (or hydrochloric acid) to add to the supernatant liquid to produce exact equivalence when portions are compared nephelometrically. Its advantage lies in the fact that it greatly reduces the number of trial and error tests that must be made to obtain the final equivalence. The end point (equivalent concentrations of silver and chloride ions in the supernatant liquid) was finally produced by adding dilute silver (or chloride solution), as determined by the preliminary experiments, stirring the solution, letting settle, and then testing in the nephelometer. A small amount of silver, or chloride, solution was then added as determined by the nephelometric test and the process of stirring, etc., repeated. The adjustment to equal opalescence was not considered final till at least three trials showed equivalent concentrations of silver and chloride ions in the supernatant liquid. I n these nephelometric tests the method of Lamb, Carleton, and Meldrum (8) was used. This consists in heating the tubes in a water bath for 35 minutes a t 45" C. Appropriate calculations were then made in order to arrive a t the normality of the acid solution, the assumption being that the silver used was pure. Weight normality (hydrogen equivalents in 1000 grams of solution) was calculated instead of volume normality. Six determinations of the normality of the acid were made in the above manner, the data of which are presented in Table I.
TABLE I. NORMALITY OF ACID n Y COMPARISON WITH ATOMIC WEIGHTSILT.ERIW A PRECISIONNEPHELOMETER WEIGHT OF
No.
1
2 3 4 5 6
SILVER Grams 2.07253 2.06605 2,35000 2.18111 2.44981 2.36681
WEIGHTOF WEIGHT HCISOLUTION OF HC1 WEIGHTOF SOLUTION SILVER Grams 65.4695 31.5891 31.5864 65.2590 31.5901 74.2370 31.5919 68.9051 31.5874 77.3830 31.5866 74.7600 Av. 31.5886
i2'2ZZHC1 TTy OF
43 1
NORMALITY OF HYDROCHLORIC ACIDBY ANALYTICAL PROCEDURES COMPARI~ONWITH ATOMICWEIGHT SILVER BY A SIMPLE This series Of experiments was made to
see what effect the simplest possible nephelometric procedure would have. The conditions of the precision method were therefore observed in all particulars excepting that the final adjustment to equivalence was made in the test-tube rack nephelometer shown in Figure 1. The essential features of this arrangement are two. The lower ends of the test tubes rest in holes in the base of the rack and the upper part of the column of liquid in the tubes is covered by a strip of wood. The rack is painted a dull black. The procedure in using it was as follows: Two IO-ml. portions of the supernatant liquid to be tested were placed in test tubes in the rack and t o one of them 1 ml. of 0.01 N silver solution was added and to the other 1 ml. of 0.01 N hydrochloric acid. If on looking down into the tubes they appeared equally bright, equivalence was assumed. Six determinations (Table 11) of the normality of the acid were made with this test-tube rack nephelometer. In all other respectsthe experiments were like those of Table 1.
TABLE 11. NORMALITY O F ACID BY COMPARISON WITH ATOMIC WEIGHT SILVER IN A TEST-TUBE RACK NEPHELOMETER WEIGHTOF HC1 SOLUTION WEIGHTOF SILVER 31.5891 31.5853 31.5901 31.5919 31.5887 31.5828 Av 31.5880
No. 1 2 3 4 5 6
WEIGHT NORMALITY OF
ACID 0.29344 0.29348 0.29343 0.29341 0.29344 0.29350 0.29345
Exactly the same normality Was obtained as when the nephelometric comparisons were made in the more elaborate instrument. This agreement is, howexrer, accidental, as insDection shows that the results are not as consistent among themselves. The maximum deviation from the average i i 0.00005 AT or 0.017 per cent. This is nevertheless very good analytical work, and shows that the test-tube rack nephelometer is a satisfactory analytical instrument. COMPARISON WITH AKALYTICAL SILVER BY PRECISION METHOD. The object here was in effect to determine the purity of the analytical silver or, better stated, to compare it with the atomic weight silver. The precision method as described above was used throughout. Results are given in Table 111. TABLE111. NORMALITY OF ACID BY COMPARISON WITH ANALYTICALSILVERIU A PRECISIONNEPHELOMETER
Nn.
1 2 3 Av.
SILVER A A A A
WEIGHTOF HCI SOLUTION WEIGHTOF SILVER 31.5896 31.5855 31.5870 31.5874
WEIGHT
NORMALITY OF HC1 SOLUTION 0.29344 0.29347 0.29346 0.29346
SOLUTION 0.29344 0.29347 0.29343 0,29341 0.29346 0.29846 0.29345
An inspection of Table I shows that the maximum deviation from the average is only 0.00004 N or 0.013 per cent. The average value, 0.29345 N , was therefore accepted as the strength of the acid and used as the basis of comparison for determining the value of the analytical procedures given below.
The results with the analytical silver differed from those obtained with atomic weight silver by only 0,00001 N , and their agreement among themselves was even better. Yothing more is claimed, however, than that they are excellent from a n analytical point of view and show that the analytical silver is a satisfactory standard. COMPARISON WITH ANALYTICAL SILVERBY AN ALL-ANALYTICAL PROCEDURE. Since the results in Tables I1 and 111,obtained with the simple test-tube rack nephelometer and
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ANALYTICAL EDITION
the analytical silver, compared favorably with those in Table I by the more laborious methods of precision measure-
and hydrochloric acid should also be prepared for the final adjustment to equal opalescence.
ment, the next logical step was the development of a n analytical procedure that can be carried out in any laboratory. This involved the design of a simpler a p p a r a t u s for dissolving the silver (Figure 2), which consists of a 100-ml. t u b e c l o s e d w i t h a paraffined cork As*Bmm carrying a short tube (a Gooch crucible holder was used) partly filled with glass beads to prevent the escape of spray carried up by the gas generated during the solution of the silver. The upper part of the large tube was equipped with a water jacket to protect the paraffin against melting. I n all strictness a special series of measurements should have been made with atomic weight silver for testing the effect of this apparatus. It was not done, however, because i t seemed unlikelv that the good results shown in Tabie IV were &e to compensating FIGURE2. APPARATUS FOR DISerrors* The results in Table IV were obSOLVING SILVER tained with the use of analytical silver dissolved in the simple apparatus (Figure 2) and by nephelometric comparisons made with the test-tube rack nephelometer.
DILUTESILVERNITRATE. A convenient strength is 0.0010 gram of silver per ml. It should be made by dissolving an amount of the silver to make about 250 ml. of solution. DILUTE HYDROCHLORIC ACID. The most convenient solution is one made by weighing 50.0 ml. (call the weight P ) of the acid to be standardized, transferring it to a 500-ml. flask, diluting to the mark, and mixing. One milliliter of this dilute solution will contain 0.002 X P grams of the original acid solution. RATIO OF HYDROCHLORIC ACID TO SILVER. A convenient technic is to use in each experiment sufficient silver to react with from 60 to 70 grams of the acid solution to.be standardized. This silver is accurately weighed and dissolved in the apparatus shown in Figure 2. Oxides of nitrogen are boiled out, a little distilled water is poured through the beads to wash back any droplets that may have been carried u and finally the solution is transferred to a 500-ml. flask in wtich it is diluted t o the mark and mixed. The calculated amount of the acid solution to react with the silver is then weighed from a wei ht buret into a 500-ml. flask where it is also diluted to the marf and mixed. This acid solution is then transferred to a 1500-ml. flask with exactly 100 ml. of water. The 500 ml. of silver nitrate solution are next poured slowly into the acid (red light in dark room) in the large flask with rotation of the liquid to insure mixing, after which the flask is rinsed with 100 ml. of water used in several portions and the rinsings are added to the large flask, which will now contain 1200 ml. The mixture should then be motor-stirred for 30 to 40 minutes or until the supernatant liquid becomes clear after 15 to 20 minutes’ settling. The next step is the final adjustment to exact equivalence of silver and chloride ions in the supernatant liquid, which is done as in the precision method for obtaining the normality of the acid-that is, first determining the ion in excess, and then by TABLEIV. NORMALITY OF ACID BY AN ALL-ANALYTICAL treating three or four 10-ml. portions of the clear liquid in the PROCEDURE test-tube rack nephelometer with 1, 2, 3, etc., drops of dilute WEIQHT WEIQHTOF NORMALITY or acid solution (as determined by the first test), findsilver HCI ing the amount of silver or acid solution to add to the large SOLUTION %I flask to produce a nearer approach to exact equivalence. For WEIQHTOB SOLUTION the above drop testing, 10-ml. portions of the dilute solutions No SILVER TIME SILVER should be diluted to 100 ml. Hours The final adjustment to exact equivalence must always be 31.5879 4 0.29345 made by trial-and-error additions of small amounts of dilute 31.5901 17 0.29343 31.5897 3 0.29343 silver and acid solutions with intermediate motor-stirring for 31.5903 5 0.29343 5 or 10 minutes and testing in the nephelometer. The large 0.29346 31.5869 18 flask should be kept covered with a black cloth or other suitable 0.29341 31.5928 5 0.29340 31.5933 7 means for excludin light, so that an ordinary electric light bulb 31.5892 0.29344 4 may be turned on for making the observations in the nephelomeAv. 31.5900 0.29343 ter.
The average of the eight results in Table IV differed from the average of the six results by the precision method (Table I) by only 0.00002 N and the agreement among themselves is as good as that by the precision method, the maximum deviation from the average being only 0.00003 N or 0.01 per cent. This close agreement with the precision method is, of course, accidental. The details followed in obtaining the results in Table IV were those of the suggested analytical method below. Experiments 2 and 5 show t h a t allowing the solution to stand overnight after mixing the silver and acid solutions has no ill effect. There was considerable variation in the time necessary for arriving at equal opalescence in the final adjustment of the solutions. It should perhaps be stated that these results were all carried out one a t a time. SUGQESTED ANALYTICAL PROCEDURE
It is assumed that a supply of silver is a t hand and that the test-tube rack nephelometer and apparatus for dissolving the silver have been made. The approximate strength of the hydrochloric acid to be standardized (distilled to insure the absence of metallic chlorides) should be known either by titration with a convenient alkaline solution or the acid can be made by diluting concentrated acid on the basis of its density and composition. Dilute solutions of silver nitrate
CALCULATION The last step is the calculation of the amounts of silver and acid used. This is done on the basis of the original weights of silver and acid, on the volume of supernatant liquid taken out for testing, and the amounts of silver and acid added for producing equivalence. The simplest way of showing how this is done is to present a typical calculation. Weight of silver = 2,36636 grams in 600 ml. of solution Weight of acid = 74.7501 grams in 600 ml. of solution Total solution = 1200 ml. 1. Two IO-ml. ortions of supernatant solution were removed for determining wlich ion was in excess. Test showed chloride in excess. 2. Four 10-ml. portions were removed for determining approximate volume of dilute silver nitrate solution to add to react with excess of chloride left in lar e flask. Amount found 1.5 ml. This was added to the 1140-mf solution in flask. Calculated amount for original volume of 1200 ml., 1.58 ml. 3. Six 10-ml. portions were removed for three nephelometric tests for equivalence after addition of the 1.5 ml. of silver solution. Test showed slight excess of silver ions, and 0.5 ml. of the dilute acid was added. Calculated amount for 1140 ml., 0.53 ml. 4. Six 10-ml. portions were removed for nephelometric teste. Slight excess of chloride found, and 0.2 ml. of dilute silver solution added to the 1020 ml. in flask. Calculated amount for 1080 ml., 0.21 ml. Tests showed equivalence after this addition of silver.
I N D USTR I A L A N D EN GI N E E R I N G C H E M I ST R Y
November 15,1934
The total weights of silver and hydrochloric acid solution are found by simple addition as follows: Grams 2.36636 0.00158 0.00021
Original weight of silver 1.5s ml. of dilute silver solution 0.21 ml. of dilute silver solution Original weight of acid 0.5:1 ml. of dilute acid
Total 2.36815 74.7501 0.053 gram of original acid 0.0530
-
74.8031
The weight normality is then 1000 X 2.36815 74.8031 X 107.88 = 0'29346
The above specimen calculation presupposed three trialand-error additions to bring about equivalence. I n none of the actual experiments, however, was this necessary. The preliminary test described above was so effective that two additions always gave the end point.
CONCLUSION Readers are likely to assume that this silver method for standardizing an acid is too cumbersome to employ. Trial will show, however, that this is not the case. The procedure appears cumbersome because it is unfamiliar, but it is, for example, no more difficult or time-consuming than the con-
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stant-boiling acid method (1) so often employed. Also, it is not offered as a rapid method of standardizing hydrochloric acid but rather as a reliable one. Nothing has been said about applying vacuum corrections to the weighings. Such corrections are not necessary, since the paper deals with a comparison of two procedures, the atomic weight or precision method of standardizing and the analytical method. Owing to the small difference in density between silver and the brass of the weights, a vacuum correction applied to the weight of silver would affect the normality of the acid somewhat less than one unit in the fifth decimal place. The vacuum correction in weighing the hydrochloric acid solution would be large, but it can be avoided by the simple expedient of defining a weight normal solution as one that contains a hydrogen equivalent in 1000 grams of solution as weighed with brass weights in air.
LITERATURE CITED (1) Foulk and Hollingsworth, J. Am. Chem. Soc., 45, 1222 (1923), (2) Lamb, Carleton, and Meldrum, Ibid., 42, 251 (1920). (3) Richards, Proc. Am. Acad. Arts Sci., 30, 380 (1894). (4) Richards and Archibold, Ibid., 38, 441 (1902). (5) Richards and Wells, J. Am. Chem. Soc., 27,475 (1905). (6) Richards and Willard, Ibid., 32, 4 (1910). RECEIYBDJuly 27, 1934. Presented before the Division of Physical and Inorganic Chemistry at the 88th Meeting of the American Chemioal Society, Cleveland, Ohio, September 10 to 14, 1934.
Estimation of Aldehydes by the Bisulfite Method An Improved Procedure A. ERICPARKINSON AND E. C. WAGNER John Harrison Laboratory of Chemistry, University of Pennsylvania, Philadelphia, Pa.
T
HE bisulfite method of Ripper (27) was initially tested
only with formaldehyde, acetaldehyde, benzaldehyde, and vanillin. A study by Feinberg (7) included also salicylaldehyde, p-hydroxybenzaldehyde, and anisaldehyde, results for these being only semi-quantitative. Jolles determined furfural and pentoses (IS)and acetone (12,29); Meyer (26, 26) recommends the method for several aldehydes in addition to those mentioned, but without evidence that its applicability has been tested. The bisulfite method has been used for estimation of lactic acid via acetaldehyde (2,3,9, IO), and of some unsaturated aldehydes (11, 15, dG). Reports as to the accuracy of the method are conflicting, in some cases because of failure to appreciate the nature of the reactions involved and the conditions essential to accuracy. The reaction between carbonyl compounds and bisulfite is reversible :
R.CO
I
+
-HSOs
R.C(OH)SOs-
I
The distribution a t equilibrium varies with the identity of the carbonyl compound, the pH, temperature and concentration of the solution, and the excess of bisulfite. The results of analysis depend further upon the specific rates of the addition and dissociation reactions, these also being affected by the conditions mentioned. Kerp and collaborators (14) made equilibrium studies of the bisulfite compounds of formaldehyde (6),acetaldehyde, benzaldehyde, furfural, acetone, and glucose, and Stewart and Donnally (31) reported a more
elaborate study of benzaldehyde-bisulfite. Using Kerp's data, Kolthoff (19) calculated the inherent error in analysis due to dissociation. A consideration of available evidence permits the following conclusions: 1. Thc accuracy of analysis is determined primarily by the value of the equilibrium constant for the dissociation. When this is of the order of IO-* or less, accurate analysis is feasible (formaldehyde, acetaldehyde, benzaldehyde, furfural). When K equals 10-3 (acetone), analysis is posgible if bisulfite is used in luge excess. When K is greater than 10-8 (glucose), the results are too low. 2. The accuracy of the method is increased, especially when K is unfavorably high, by excess of bisulfite and by increase in the concentrations of both reactants. When K e uals 10-7 (formaldehyde) or IO+ (acetaldehyde), even dilute so?utions can be analyzed using only a moderate excess of bisulfite. 3. The accuracy of analysis is affected by temperature, but here two conflicting effects are to be noted. The error due to dissociation can be decreased by working at low temperature, as the value of K decreases with decreasin temperature (14, SI), but the rate of the addition reaction is &ereby decreased, and the time necessary for attainment of e uilibrium ma exceed the 15 to 60 minutes usually specified. ?he analysis orsome aldehydes requires a low temperature at the time of titration; in such cases the reaction liquid may be allowed to stand a suitable interval at room temperature and then chilled for a time before and during titration. 4. The rates of addition and dissociation are affected by the hydrogen-ion concentration, the former decreasing with increasing acidity, while the latter and the dissociation constant are at a minimum somewhat in the acid region (at about pH = 1.8 for
