October 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
0.1 N sulfuric acid, heated to boiling, and the mercury precipitated by adding slowly, with constant stirring, 2 grams of sodium or potassium periodate dissolved in 50 cc. of water. It was cooled, filtered through a filtering crucible with a sintered-glass or porous porcelain bottom, washed with warm water, dried at 100” C. for 2 or 3 hours, and weighed as Hg,(IO&. The results are shown in Table I. Volumetric Determination
Method A-The filtered and washed precipitate of mercuric periodate in the crucible was treated with 2 or 3 grams of solid potassium iodide and 10 to 15 cc. of water, stirring until all the periodate had dissolved. The solution was washed into a 150-cc. conical flask (most conveniently a suction flask), acidified with 10 cc. of 2 N hydrochloric acid, and the liberated iodine titrated with 0.1 N sodium thiosulfate, using starch as indicator. Method B-The precipitate in the crucible was treated with an excess of standard arsenite and then concentrated hydrochloric acid added until it dissolved, about 35 cc. being required if the volume at the end of the titration is about 100 cc. The solution was washed into a 150-cc. glassstoppered conical flask, and titrated with 0.1 N potassium iodate until it had a light brown color. Four or five cubic centimeters of chloroform were then added and the titration continued, shaking after each addition until the purple color of the iodine just disappeared (1, 6). The solution a t the
399
end point should contain between 28 and 45 cc. of hydrochloric acid, sp. gr. 1.18, per 100 cc. The results, both with pure mercury and in mixtures with other metals, are shown in Table 11. I n the first three experiments method B was used; in all others, method A . The mercuric salt was precipitated from a nitric acid solution. Table 11-Volumetric D e t e r m i n a t i o n of Mercury as Hgs(I0a)Z MERCURY OTHERMETALS Taken Found PRESENT ERROR
Gram 0.3645 0.2071 0.0513 0.2556 0.1327 0.1903 0.1064
Gram 0.3646 0.2070 0.0513 0.2558 0.1327 0.1902 0.1065
Mi?.
Gram 0.20N i 0.15 AI 0.18 Cd 0.17 Zn 0.13 Cu
+o. 1
,
-0.1 0 +0.2 0 -0.1 +o. 1
If a chloride solution is to be analyzed, the mercury may first be precipitated as metal or sulfide and then converted into nitrate or sulfate. Literature Cited (1) Andrews, J . Am. Chem. Soc., 25,756(1903). (2) Bengieser, Ann., 17,259 (1834). (3) Hillebrand and Lundell, “Applied Inorganic Analysis,” p. 170,Wiley, 1929. (4) Hillehrand and Lundell, I b i d . , p. 173. (5) Jamieson, “Volumetric Iodate Methods,” p. 18, Chemical Catalog, 1926. (6) Lautsch, J. prakt. Chem., 100,86 (1867). (7) Rammelsberg, Ann. Physik, 134, 524 (1868).
Volumetric Determination of Manganese after Oxidation by Periodate Hobart H. Willard and J. J. Thompson CEIEMICAL’LABORATORY, UNIVERSITY OF MICHIGAN, ANN ARBOR,MICH
vv
Manganese up to 30 mg. can be determined by oxidaILLARD and the excess periodate was pretion to permanganate with periodate in either phoscipitated by the addition of G r e a t h o u s e (4) have shown that phoric or sulfuric acid solution, removal Of excess b i s m u t h perchlorate, as a manganese can be determined periodate by Precipitation as mercuric Salt,. and rewhite, voluminous precipitate colorimetrically by oxidation duction of the permanganate by S t t ~ ~ d a r fd m ~ of bismuth ~ periodate. The sulfate. The reaction goes best in a phosphoric acid to permanganate with a small solution was i m m e d i a t e l y excess of periodate. The SOsolution, in which case less than a milligram of chrotitrated with standard ferrous mium does not interfere. Cobalt, cerium, and chloride sulfate. To make the end ]ution thus obtained is stable for weeks, a fact confirmed by must be absent. point more d i s t i n c t in the presence of the precipitate, others ( 3 ) . It seemed desirable, therefore, to make this the basis of a volumetric method alphazurine (Erioglaucine A) (2) was added just before the end by removing the excess of periodate. This was accomplished point. Too little ferrous sulfate was always used, showing that by precipitation as mercuric periodate, after which the per- some permanganate had been reduced, because other experimanganate was titrated by adding excess of standard ferrous ments proved that oxidation was complete. The surprising sulfate and back titrating with permanganate. Although fact was that in the presence of iodate some of the permangabismuth periodate is the least soluble of all, its presence nate was used in the oxidation of bismuth. In the presence of caused certain errors which made its use for this purpose bismuth periodate only, no such error occurred. Both bisimaossible. muth and iodate are necessarily aresent because the latter is formed by reduction of the per”io4ateby manganous ion (4). Precipitation of Periodate as Bismuth Salt It could not be completely removed by precipitation as silver A known volume of standard permanganate was reduced iodate. If standard permanganate to which as much as with ferrous sulfate in a dilute sulfuric acid solution, and the 0.6 gram of sodium periodate had been added was treated manganous salt was then oxidized to permanganate by add- with a bismuth salt, and, without filtering, titrated with ing 0.3 gram of sodium metaperiodate, NaI04, and boiling ferrous sulfate, accurate results were obtained. If, howgently for 15 minutes. After cooling to room temperature, ever, excess of the latter was added and back titrated with permanganate, the results indicated that the excess of ferrous 1 Received August 12, 1931 Presented before the Division of Physical and Inorganic Chemistry a t the 81st Meeting of the American Chemical salt was slowly oxidized by the precipitate of bismuth perioSociety, Indianapolis, Ind., March 30 to April 3, 1931. date even in cold solution. * From a dissertation submitted by J. J. Thompson to the Graduate There are, therefore, two errors to be considered: oxiSchool of the University of Michigan in partial fulfilment of the requiredation of the excess reducing agent by iodate or insoluble ments for the degree of doctor of philosophy.
400
ANALYTICAL EDITION
bismuth periodate, and oxidation of bismuth. Of the possible volumetric reducing agents, vanadyl sulfate and ferrous sulfate were slowly oxidized by bismuth periodate, but oxalate and hydrogen peroxide were not affected. On the other hand, the former were not oxidized by the low concentration of iodate, but the two latter were oxidized. The use of bismuth as a precipitant, therefore, had to be abandoned.
VOl. 3, s o . 4
only 0.3 gram of sodium or potassium periodate should be used for 15 mg. or less of manganese. Accurate results can be obtained, however, with as much as 30 mg. of manganese, using, in this case, 0.5 gram of periodate. With larger amounts such voluminous precipitates are formed that all the permanganic acid cannot be removed by washing. Instead of the metaperiodate, the paraperiodate, NarHJOe, may be used. Manganese is more readily oxidized and the permanganic acid formed is more stable in a phosPrecipitation of Periodate as Mercuric Salt phoric acid solution, so that tliis acid is desirable when conIt was found that periodate could be completely precipi- siderable amounts of manganese are present. tated as the mercuric salt, Hg,(IO& (*5). Although more The concentration of acid may vary within wide limits soluble than the bismuth salt, it is still sufficiently insoluble as far as oxidation of manganese is concerned, although even in the concentration of acid used for the oxidation of with large amounts of the latter, manganese dioxide may manganese, provided a large excess of mercuric ion is present. form unless phosphoric acid is present. A fairly high conThe mercuric salt has a very decided advantage in that it centration of acid increases the rate of oxidation. The is much more easily filtered. There is also no possibility of solubility of mercuric periodate, however, is increased under further oxidation of the mercury by permanganate. Experi- these conditions, so that it is advisable to dilute the soluments showed that amounts of manganese up to 30 mg. tion before removing the excess of periodate. Concencould be accurately determined by oxidation to permanganate trations not greater than 5 cc. of sulfuric acid, sp. gr. 1.83, in a solution containing either phosphoric, sulfuric, or phos- or 15 cc. of 85 per cent phosphoric acid per 100 cc. are phoric and sulfuric acids. There was no blank correctrion recommended. Mixtures of the two acids are desirable, to be applied. In this respect there is a slight advantage because the mercuric periodate is more crystalline in the over the bismuthate method (1). Iron ores can be quickly presence of sulfuric acid, and without some phosphoric acid, dissolved in phosphoric acid3 and this method applied di- manganese dioxide may form. rectly to the solution, but this is not possible with bismuthate, The weight of mercuric nitrate required for complete precipitation of periodate varies from 5 grams with 5 cc. Table I-Effect of Chromium o n Determination of Manganese of sulfuric acid to 2 grams with 15 cc. of phosphoric acid per AD_-PROX. CHRO100 cc. ACID WT.OF MIUM MANGANESE Titration of the permanganic acid with arsenite instead MATERIAL USED PRESENT SAMPLE PRESENTPresent Found Grams Ma. % % of ferrous sulfate is impossible because of an indefinite end ,,B. s. Steel No. 1Oc Phosphoric 1.1 4.0 1 . 1 3 1.160 point. 3.0 Phosphoric 1.0 1 . 1 3 1.153 B. S. Steel: NO. 10c 2.0 Phosphoric 1.0 1.13 1.148 B. S. Steel, No. 1Oc Chloride must be absent. Cobalt interferes as in the 1.232 2.0 Sulfuric 1.3 B. S. Steel, No. 1Oc 1.13 bismuthate method (1). When periodate is added to a solu1,143 1.5 Phosphoric 1.0 B. S. Steel No 1Oc 1.13 1.138 1.0 Phosphoric 1.0 B. S. Steel: NO: 10c 1.13 tion of cobaltous sulfate, the solution becomes dark brown, 0.5 1 . 1 3 1.139 Phosphoric 1.0 B. S. 1.14 7.3 0.624 0.661 Phosphoric B. S. owing to oxidation of the cobaltous salt. In steel contain0 . 6 2 4 0.664 6.6 1.04 Phosphoric B. S. ing 1.13 per cent of manganese and 0.5 per cent of cobalt, 0.624 0.740 7.6 1.18 Phosphoric B. S. 0.624 0.686 7.7 1.20 Phosphoric B. S. 1.32 per cent of manganese was obtained. Cerium also interferes because it is oxidized to a ceric salt. Experiments using standard permanganate were carried out to determine the accuracy of the method. Samples of Procedure for Steel or Iron Free from Chromium 25 to 40 cc. of 0.05 N permanganate were reduced with Dissolve 1 gram of steel containing not more than 0.15 ferrous sulfate in a solution containing, in a volume of 100 cc., about 10 to 15 cc. of 85 per cent phosphoric acid, or mg. of chromium in 4 cc. of concentrated sulfuric acid and mixtures of 3 or 4 cc. of phosphoric acid with 5 or 4 cc. of 25 cc. of water. To the hot solution, add, cautiously, 1 cc. concentrated sulfuric acid, and oxidized by adding 0.3 to or more of concentrated nitric acid to oxidize the ferrous 0.5 gram of sodium or potassium periodate and heating just iron and carbonaceous matter, and boil to remove nitrous to boiling for 15 minutes. The solution was cooled to room fumes. Any graphite present will do no harm and will be temperature, diluted to 150 cc., and 2 to 5 grams of mercuric filtered out later. Dilute to 50 or 75 cc., add 3 cc. of 85 nitrate, Hg(N0&.2H20, dissolved in a little water, were per cent phosphoric acid (or an equivalent amount of more added to precipitate the excess periodate and nearly all of dilute acid) and 0.3 gram of sodium or potassium periodate. the iodate. The solution was filtered into an excess of 0.05 Boil gently for 15 minutes to oxidize the manganese, dilute N ferrous sulfate through an asbestos mat 3 to 4 mm. thick to 150 cc., cool to room temperature, and add slowly, with on a 9-cm. No. 17G sintered-glass filtering funnel. A Buch- constant stirring, 4 or 5 grams of mercuric nitrate (Hgner funnel would doubtless be almost as satisfactory. The (N03)*.2H20)dissolved in a little water. Filter immediately precipitate of mercuric periodate was washed four times through a fairly large asbestos filter into an excess of standard with water, and the excess of ferrous sulfate was titrated ferrous sulfate, wash with cold water four or five times, back with standard permanganate. The results obtained and titrate back with standard permanganate. were the same within 0.01 cc. as in the direct titration of or Iron Containing Not Over 0.1 Per the ferrous sulfate with permanganate. When chromium Procedure for SteelCent of Chromium was present, it was found that in the presence of sulfuric acid about 60 per cent of it was oxidized when the content Dissolve 1 gram in a mixture of 15 cc. of water and 15 was about 1.5 mg., thereby giving high results for manganese. cc. of 85 per cent phosphoric acid. Oxidize the ferrous However, this tendency is very much minimized in phosphoric iron and carbonaceous matter by adding carefully to the acid solutions, so that an accurate determination of man- hot solution 1 or 2 cc. of concentrated nitric acid, and boil ganese can be made in the presence of 1 mg. of chromium. to remove nitrous fumes, Dilute to 100 cc., add 0.3 gram Because of the difficulty in washing out all the permanganic of sodium or potassium periodate, boil gently 15 minutes acid from very large precipitates of mercuric periodate, to oxidize the manganese, dilute to 150 cc., and cool to room temperature. Precipitate the periodate by adding slowly, 8 Unpublished work by one of the authors.
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 15, 1931
with constant stirring, 2 to 3 grams of mercuric nitrate dissolved in a little water, and filter immediately through a fairly large asbestos filter into excess of standard ferrous sulfate containing 10 cc. of 50 per cent sulfuric acid to prevent precipitation of mercuric phosphate. Wash with cold water four or five times and titrate back the filtrate and washings with standard permanganate. The filtration will be slower than when sulfuric acid is present during precipitation, but the time required, including washing, should not be more than 3 minutes. The effect of chromium with and without phosphoric acid is shown in Table I.
401
Note-If the amount of manganese is between 15 and 30 mg., the amount of periodate in the above procedures should be increased to 0.5 gram.
Results obtained by this method with different materials are shown in Table 11. I n all cases phosphoric acid was used with the sulfuric to prevent formation of manganese dioxide. Table 11-Determination
of Manganese in Various Materials AP-
PRO~.CHRO-
MATERIAL ANALYZED
ACID USED
WT. MIUM OF PRES- MANGANESE SAMPLEENT Present Found Grams % % (r, ," ,I
Phosphoric Phosphoric Sulfuric Sulfuric Phosphoric Phosphoric Sulfuric
Procedure for Iron Ore and Other Oxide Ores
Place 1 gram in a 250-cc. Pyrex beaker, and add 15 cc. of 85 per cent phosphoric acid and a few drops of concentrated sulfuric acid. Stir until all the ore is free from the bottom of the beaker, cover, and heat until fumes of sulfuric acid are given off, taking care that it does not froth over or cake on the bottom. If the ore is not dissolved by this time, keep it hot for a longer time, stirring occasionally, but not allowing the temperature to rise, because the beaker would be attacked. Cool until the mass begins to be viscous, then add quickly 100 cc. of water, and heat. Everything should dissolve except some gelatinous silica. From here on the procedure is the same as for steel containing 0.1 per cent of chsoxnium. Procedure for Bronze
Dissolve 1 gram in a mixture of 3 cc. of concentrated nitric acid, 10 cc. of 85 per cent phosphoric acid, and about 7 or 8 cc. of water. Then dilute to 50 or 75 cc. and proceed as above. Owing to the deep blue color of the copper salt, the back titration with permanganate is conveniently carried out electrometrically or by use of alphazurine indicator ( 2 ) .
Sulfuric
1 0 6 0.01 1.03 0.01 1 2 9 0.01 1.01 0.01 1.04 0.016 1.18 0.016 1.14 0.016 1.13 0.016 1.26 0 021 1.12 0.021 1.10 0.007 1.24 0.007 1.02 0.007 1.22 0.007 1.47a 0.065 1.46'" 0,065 0.49 0.065 0.43 0.065 0.69 , . 0.62 1.00 1.09 , 1.27 1.26 ,
Phosphoric Phosphoric Phosphoric Phosphoric Sulfuric Sulfuric Phosphoric Phosphoric Sulfuric Steel A Sulfuric B. S. Mn Bronze No. 62 Phosphoric B. S. Mn Bronze' No. 62 Phosphoric B. S. Norric Iron) Ore No. 28 Phosphoric B. S. Norric Iron Ore: No. 28 Phosphoric B. S. Bauxite, No. 69 Phosphoric B. S. Bauxite, No. 69 Phosphoric a These samples contain nearly 30 mg. of manganese.
....... . .. .. ..
1.13 1.13 1.13 1.13 0.897 0.897 0.897 0.897 0.564 0.564 0.381 0.381 0.381 0.381 1.99 1.99 1.99 1.99 1.59 1.59 0.465 0.465 0.426 0.426
1.136 1.132 1.138 1.139 0.890 0.894 0.898 0.912 0.570 0.562 0.389 0.388 0.390 0.392 1.99 1.9s
2.02 2.00 1.61 1.64 0.446 0.447 0.421 0.420
Literature Cited (1) Hillebrand and Lundell, "Applied Inorganic Analysis," p. 346, Wiley, 1929. (2) Knop, Z.anal. Chem., 77, 125 (1929). (3) Richards, Analyst, 55, 554 (1930). (4) Willard and Greathouse, J . A m . Chem. Soc., 39, 2366 (1917). ( 5 ) Willard and Thompson, IND. ENG.CHEM.,Anal. Ed., 3, 398 (1931).
Use of Selenium as Catalyst in Determination of Nitrogen by Kjeldahl Method' Michael F. Lauro2 2 BROADWAY, NEW YORK,N. Y .
IME is always a factor in commercial analysis. Experiments are forever being carried on to find short cuts without sacrificing accuracy. In cereal work this is especially true. While on work of an entirely different nature, the analysis of rare earths, the author hit upon a solution of the problem of shortening the time in Kjeldahl digestions. The general method for the determination of nitrogen (organic and ammoniacal) by the Kjeldahl process of oxidation with concentrated sulfuric acid is outlined in the methods of analysis of the Association of Official AgTicultural Chemists ( 3 ) . From 0.7 to 3.5 grams of the substance to be analyzed are placed in the special digestion flask with 0.7 gram of mercuric oxide or its equivalent in metallic mercury, and 20 to 30 cc. of concentrated sulfuric acid. From 0.1 to 0.3 gram of crystallized copper sulfate may be substituted for or added to the mercury. The digestion is carried on until the mixture is colorless or nearly so, when oxidation is presumed complete. This usually requires 2 hours. The Gunning modification of this method makes use of 10 grams of powdered sulfate or anhydrous sodium sulfate and suggests the use of 0.1 to 0.3 gram of copper sulfate in place of the mercury.
T
1
Received April 21, 1931.
* Consulting chemist.
A further modification, Kjeldahl-Gunning-Arnold,specifies 15 to 18 grams of sodium or potassium sulfate, 1 gram of copper sulfate or 0.7 gram of mercuric oxide in 25 cc. of sulfuric acid. The American Association of Cereal Chemists ( 1 ) specifies a 1-gram sample, 10 grams of a mixture of potassium and sodium sulfates, 20 cc. of sulfuric acid, and 0.5 gram of mercuric oxide. The American Oil Chemists Society (%')recommends 1.7304 grams (the ammonia equivalent) of substance to be analyzed, 10 grams of potassium or sodium sulfate, 25 cc. of concentrated acid, and 0.5 gram of mercury or 0.7 gram of mercuric oxide. As can be seen, the catalyst used to hasten the process of breaking down of the nitrogenous material and its conversion into ammonium sulfate may be either copper as sulfate or mercury as oxide or as the metal. Where mercury is used, this must be precipitated out with sodium or potassium sulfide before subsequent distillation with lye. With copper, however, this step is unnecessary. The time of digestion is, of course, affected by the amounts used, but as the method is now standardized, about the only remaining factor arises from the kind of flame used in heating. A hotter flame naturally shortens the time materially. Nevertheless, allowing for this with the type of burners a t
