Oxidimetric Nitrate Analysis of Fertilizers and Other Commercial Products WOLFGANG LE ITIIE Laboratory os the Oesterreichische Stickstofftcerke AG, Linz, .lustria
Oxidimetric procedures for the routine analysis of fertilizers, meat pickle salts (mixtures of nitrates and nitrites), and spent mixed acids from organic nitrations are outlined. Time, effort, and expense have been reduced to a minimum. The procedures involve a 3- to 4-minute reaction of the sample solution with a strongly acidic ferrous sulfate solution and a subsequent titration. Nitrites may be decomposed with urea. An accuracy of 0.3% of the quantity measured may be attained.
0
XIDIMETRIC nitrate determinations were first carried out by Pelouze and Fresenius (6)90 years ago. The method was considerably improved by Kolthoff, Sandell, and Moskovits ( I ) , who added ammonium molybdate as a catalyst, and thus shortened the time of reaction "OB
.
+ 3FeClz + 3HC1 = NO + 3FeC13 f 2H20
to 10 minutes' boiling. An atmosphere of carbon dioxide was still considered necessary. The author of this paper has simplified this method ( 2 , and outlined a procedure that requires minimum time and apparatus. The time of reaction was reduced to 3 minutes of boiling by raising the acidity of the mixture to about 65 grams of sulfuric acid in 75 ml. of the mixture-i.e., a 6 to 8 molar solution of sulfuric acidand the exclusion of air proved unnecessary. I n a subsequent paper (3) it was shown that by adding sodium chloride to the sulfuric acid mixture the delicacy of this reaction is raised to such an extent that it may serve for the determination of amounts of nitrate down to 0.002 mg. Thus, a very simple, rapid, and accurate method of nitrate determination in .well water or soil extracts was outlined (4). This paper deals with the adaptation of this method t o the routine analysis of commercial products such as fertilizers (commercial nitrates), meat pickle salts (mixtures of nitrate and nitrites), and spent mixed acids from nitrobenzene preparations. Special stress was laid on reducing the time, effort, and costs of these procedures as much as possible. KVALYSIS OF FERTILIZERS AND COMMERCIAL NITRATES
I. Titration with 0.5 S Potassium Permanganate Solution. I n routine analysis larger samples are preferred in order to aroid errors caused by an unequal composition of the sample. This procedure makes use of a 0.5 S potassium permanganate solution, which provides a very simple and cheap titration technique. Special experiments have proved that an excess of the following common ions does not interfere with this test: potassium, sodium, ammonium, calcium, ferric, manganese, cupric, zinc, chloride, sulfate, and phosphate. Carbonates should be decomposed with hydrochloric acid before the ferrous sulfate solution is added. If appreciable amounts of urea are present, 5 grams of sodium chloride must be added and the procedure given below applied. Substances that affect the stability of the potassium permanganate or potassium bichromate (Procedure 11) solution or oxidize the ferrous sulfate should be absent, or they may somrtimcs be checked bj the blank test or a special test. A- FERROUS SULFATE SOLUTION.Place 270 grams of crystallized ferrous sulfate (FeSOd 7 HzO) in a, 1-liter volumetric flask, diqsolve in water after adding a little dilute sulfuric acid, add 100
ml. of concentrated sulfuric acid, cool to room temperature, and dilute with water up to the mark. If the sample is dissolved in hydrochloric acid corresponding to about 0.5 to 0.7 gram of hydrochloric acid for each titration, no further addition of sodium chloride is necessary. If the sample is to be dissolved in water and no chlorides are present, an addition of about 1 gram of sodium chloride for each titration is necessary. This amount may be added beforehand in the preparation of the ferrous sulfate solution; 40 grams of pure sodium chloride are added to the ferrous sulfate solution before it is made up to the mark of 1liter. Standardization against the 0.5 A' potassium permanganate solution (blank test) is carried out in precisely the same manner as in the titration of nitrates. In place of the sample solution 25 ml. 'of pure ITater are mixed iyith 25.00 ml. of the ferrous sulfate solution and 20 ml. of concentrated sulfuric acid, boiled for 4 minutes, and titrated with the permanganate solution in the same manner as with the sample. This blank test holds for a t least one day, but frequently for several days. PROCEDCRE. Weigh out 10 to 20 grams of sample, transfer it into a 1-liter volumetric flaqk. dissolve in water, add dilute hydrochloric acid if necessary, fill up to the mark, and filter if necessary. (If about 20% of nitrate nitrogen is present, weigh out 10 grams; if only lo%, weigh out 20 grams of sample. If carbonates such as limestone are present, suspend the sample in about 100 ml. of water, and slowly add 50 nil. of hydrochloric acid (1 to 1). Insoluble constituents such as clay and similar silicates evhibit an interfering catalytic influence on the decomposition of the nitrates, causing the values to be high. It is therefore necessary to use clear solutions, either by filtration or by settling.) Tranbfer 25.00 ml. of solution with a pipet to a 200-ml. Erlenmeyer flask, add 25.00 nil. of 1S ferrous sulfate solution w-ith a pipet, and finally add 20 ml. of concentrated sulfuric acid slowly with a pipet, shaking the flask well. Place the flask on a mesh square without asbestos, and heat to moderate boiling over a medium flame for 4 minutes. The dark color fades after 1.5 to 2 minutes; a pure orange color persists during the last minute's boiling. After this, pour the liquid immediately into a 1.5-liter Erlenmeyer flask containing about 1 liter of cold tap water, which must be stable against potassium permanganate, and rinse twice with a little water. Add slo~vly0.5 N potassium permanganate solution from a buret with continuous good shaking, until a marked reddish color persists during a t least 10 seconds. Cu,cur,aTros. One milliliter of the difference between the volume of 0.5 S potassium permanganate solution consumed by the blank test (standardization of the 1N ferrous sulfate solution) and the corresponding volume consumed by the sample titration = 0.01417 gram of SaXOB = 0.010335 gram of NOS = 0.002335 gram of N. Tests KNOa, analytically pure Present, mg. Found, mg. Devirttion, yo
1082
400 402, 400, 402, 401, 401, 401, 402 +0.1 - +0.6
300 301
+0.3
200 199 $0.5
V O L U M E 2 0 , N'O. 11, N O V E M B E R 1 9 4 8
1083
C o m m e r c i a l Fertilizers % Piitrate Xitrogen Found Ammonium nitrate-limestone (4 samples) Nitrophoslar Commercial calcium nitrate Leuna saltpeter (ammonium sulfate nitrate)
This method 10.02,10.6:, 10.14,10.46 .5,19 13,lO
Ulsch's Arnd's method method 10.02, 10.59, ... 10.08, 10.40 5.25 5.25 13.15 13,OQ
6.60
6.55
6.65
ERRORS.On an average, the values of the osidimetrir test determinations are about 0.3c0 high. For practical use a correction of this magnitude may be justified--e.g., lO.OOs$ nitrogen instead of 1 0 . 0 3 ~ ; .The deviations within a series of oxidimetric determinations art: n-ithin the usual titration errors. 11. Nitrate Determinations with 0.1 LYPotassium Dichromate Solution. This procedure corresponds to the normal analytical technique with 0.1 &Ystandard solutions; it is still less susceptible to impurities such as clay than Procedure I. The color change rvith 0.1 potassium dichromate solution and fe~roinas an oxidation-reduction indicator is extraordinarily sharp and delicate; but good results have also been obtained by titrating with 0.1 A- potassium permanganate solution, if the titration of sample and blank test are carried out in strictly the same manner. 0.2 X FERROUS SULFATE SOLUTIOX.This solution is a trace weaker than 0.2 S,so that 50 ml. of 0.1 -V patassium dichromate solution (one buret) may suffice in the blank test. Place 55 grams of pure ferrous sulfate and 20 grams of pure sodium chloride in a 1-liter volumetric flask; dissolve completely in 100 nil. of water, after adding a few drops of dilute sulfuric acid, and fill up to ,the mark with sulfuric acid of 5076 sulfuric acid (specific weight 1.40). Carry out the blank test in exactly the same manner as the titration of the sample; but instead of the sample solution apply 25 ml. of water. The sqlution is stable for at least one day, and frequently over a longer time. PROCEDURE. Weigh out such an amount of the sample that 25 ml. of the solution contain 25 to 80 mg. of nitrate. The necessary excess of 0.2 X ferrous sulfate solution is slight; onl?- about one fifth to one tenth of the original amount of ferrous sulfate may remain unchanged after reaction with the nitrate, but a larger excess of ferrous sulfate does not interfere. If conlnlercial nitrates are to be analyzed, dissolve 2 grams in a 500-ml. volumetric flask. Turbid solutions have to be filtered. Transfer 25.00 ml. into a 200-m!. Erlenmeyer flask with a pipet, add 25.00 ml. of 0.2 ferrous sulfate solution with a pipet and finally 20 ml. of concentrated pure sulfuric acid also with a pipet, shaking the flask frequently. Heat to boiling on a mesh square Tvithout asbestos and boil moderately for 3 minutes over a medium flame. Take from the flame, cool by shaking under the tap until lukewarm, add 50 ml. of cold water and 1 ml. of 0.0025 molar ferroin (o-phenanthroline ferrous sulfate) solution (the usual 0.025 molar solution diluted 1 to IO), and titrate with 0.1 N potassium bichromate solution, until the color changes from orange over brown to blue-green. The end point is very delicate a,nd may be recognized within a few hundredths of a milliliter. C-~LCCLATIOS. One milliliter between the volume of the blank test' and the sample titration = 2.833 mg. of S a X O d = 2.067 mg. of SO1 = 0.4670 mg of N. T e s t s w i t h Analytically P u r e N i t r a t e s S H I S O~. I
Present, mg. Found, mg. I) i,i-iation,
20.0
20.1
(,z
+0.5
40.0 40.0
80.0 80.1 +O.l
*o.o
100.0 99.7 -0.3
SaS03
Present, mg. Found, mg.
Ileviation, ?& KSOa Present, ing. Foiind, ing. Ilmiation, C;
40.0 5 0 . 0 50.0 50.0 50.0 50.0 50.0 40.1 40.15 49.9 49.8 50.0 49.9 50.2 50.1 4-0.2 +0.3 -0.2 -0.4 * O . O -0.2 +0.4 f 0 . 2 40.0
40.0 40.1 +0.2
60.0 60.2
+0.3
100.0 100.2 +0.2
100.0 100.0 -0.0
100.0 100.2 +0.2
ERRCIR\ The deviations average about * O 2 5 from the
t IlCYJl ?tlCd \du?S.
Analysis of Nitrate-Nitrite Mixtures (Meat Pickle Salts). Sitiites decompose with ferrous salts as do nitrates; one mole of nitrous acid oxidizes one mole of ferrous sulfate. Under the conditions of this procedure the reaction does not run strictly in this way; th6efore it is necessary to destroy the nitrites before carrying through the nitrate determination. Sitrites are easily de-
composed by allowing the acidified solution to stand for some minutes with an excess of urea. If fairly dilute solutions such as 1 gram per 500 ml. are applied, no marked amount of nitrate is formed in the course of this reaction. To avoid a harmful effect of the urea excess upon the subsequent decomposition of the nitrate, it is necessary to increase the addition of sodium chloride up to 5 grams. As the values tcnd to become somewhat low, this effect may be compensated for by the addition of small pieces of porcelain. The concentration of the sample should not be raised beyond 1 gram per 500 ml. IF only small amounts of nitrates are to be determined accurately in thwe mixtures, a semimicro or a micromethod is advisable ( 4 ) . Procedures for the determination of nitrates along with nitrites in well water and in soil extracts are also outlined (4).
PROCEDURE. Dissolve 1 gram of the nitrate-nitrite mixture in 500 ml. of water. Place 25.00 ml. of this solution in a 200-ml. Erlenmeyer flask, dissolve 2 grams o€ pure urea in it, and add 1 ml. of a 50n0 sulfuric acid in small drops with vigorous shaking. .Ulow the mixtures to stand for 5 minutes, and add 5 grams of pure sodium chloride, 25.00 ml. of 0.2 S ferrous sulfate solution, and 20 ml. of concentrated sulfuric acid with a pipet, shaking well. ,Idd three small pieces (about 5 mm. in diameter) of unglazed porcelain and treat the mixture in the manner directed above. T e s t s of NaNOs-SaNOz Mixtures of P u r e Salts SaNOa Present, 70 Found, yo
100.0 100.3
99.7 99.5
96.3 96.0
67.0 66.8
57.4 57.3
40.5 40.0
3.9 3.9
Spent Mixed Acids from Benzene Nitration. The nitrate determination in spent mixed acids is an important control test in nitration plants-e.g., benzene nitration. This test is usually carried out by the Lunge nitrometer method. However, the use of mercury may be avoided bl- employing the oxidimetric procedure: the time required does not appreciably exceed that of the Lunge method. The Lunge method is sometimes misleading. If considerable amounts of nitrous acid in a combined state are present, these arr indicated in the same manner as nitric acid in the nit,rometer, though nitrous acid does not exhibit a nitrating action as does nitric acid. The true nitrate contents of the spent acid may be found with but little delay, if the nitrous acid is destroyed with urea, as described above, and the residue is tested for nitrates in the usual manner. Usually 0.2 N ferrous sulfate and 0.1 S potassium dichromate solutions are adequate, if only small amounts of nitric acid (less than 27,) are to be expected. Larger amounts may be determined with N ferrous sulfate and 0.5 N potassium permanganate solutions. Unfortunately, no way has been found for determining the nitrate groups in esters such M nitrated glycerol or nitrocelluloses. The alcoholic residues are not stable against oxidizing agents such as potassium dichromate after the decomposition of the released nitric acid, and no distinct end point is visible during the titration.
PROCEDURE FOR SPENTACIDS (0 to 2% Nitric Acid). A. Without Regard to Nitrous Acid. Place 25.00 ml. of the 0.2 N ferrous sulfate solution in a 100-ml. Erlenmeyer flask, and add 5.0 ml. of the acidic sample with a pipet in a slow run with frequent shaking, and 10 ml. of a 50% sulfuric acid. Treat the mixture in the manner described above. B. -4fter Destroying Nitrous Acid. Dissolve 2 grams of urea in 5 ml. of water, run in slowly and with cooling 5.0 ml. of the sample with a pipet, allow the mixture to stand for a few minutes. and add 2 grams of sodium chloride, 25.00 ml. of the 0.2 .V ferrous sulfate solution, and 5 ml. of concentrated sulfuric acid. Treat the mixture in the manner described. TESTS. A spent mixed acid showed an apparent amount of 1.2070 nitric acid in the Lunge nitrometer. By the oxidimetric method (A) without destroying the nitrous acid an apparent amount of 0.517, nitric acid was found. When the nitrous acid was destroyed with urea (B), the oxidimetric determination showed only O . O . ? I ~ ~nitric acid.
1084
A N A L Y T I C A‘L C H E M IS T R Y
To the original spent acid sample 1.570 nitric acid was added. After the nitrous acid was destroyed 1.4570 nitric acid was found by Procedure B.
LITERATURE CITED
(1)
ACKNOWLEDGMENTS
(2)
The author wishes t o express his gratitude to H. Bray for assistance in the experimental work. Appreciation is expressed to the Oesterreichische Stickstoffwerke AG, Linz, for permission to publish this work.
(4) (5)
(3)
Kolthoff, I. M., Sandell, E. B., a n d Moskovits, B., J . Am. Chem. SOC.,55, 1454 (1933). Leithe, W., Mikrochemie, 33, 48 (1946). I b i d . , 3 3 , 8 5 (1947). Ibid., 3 3 , 2 1 0 (1947). Peloure a n d Fresenius, Ann., 106, 217 (1857).
RECEIVED January 13, 1948.
Determination of Organic Isocyanates Is0thiocyanates SIDNEY SIGbIA AND J. GORDON HANNA, General Aniline & Film Corporation, Easton, P a .
A method has been developed for determining isocyanates and isothiocyanates by reaction with excess butylamine and acidimetric titration of the excess amine. The procedure has an average precision of better than ”0.47‘.
I
SOCYAKATES and isothiocyanates, both aliphatic and aromatic, react with amines to yield substituted ureas and R’NH2+ fhioureas according to the formulas: RS=C=O RHNCONHR’ and RN=C=S R’KH2 -+ RHNCSNHR’. As amines form strongly basic solutions, they can be titrated with standard acids. By adding a known amount of amine and titrating the excess after reaction with the sample, the amount of isocyanate or isothiocyanate present in the sample can be determined.
+
+
Table I.
Analytical Results %
98.6 98.7 98.2 98.7 98.6 98.8 99.0 99.6 100.2 100.9 100.0 98.6 98.7 98.4 98.5 98.1
cr-xaphthyl isocyanate Phenyl isocyanate Phenyl isothiocyanatea Ethyl i s ~ t h i o c y a n a t e ~ (45 minutes a t room temperature) Methyl isothiocyanate (45 minutes a t room temperature)” Ethyl Isocyanate Phenyl Isocyanate 70 calcd. % found found 41.4 41.42 62.35 C 62.3 5.8 5.59 3.60 H 3.7 15.83 10.22 16.1 K 10.4 36.90 23.42 36.8 S 23.7 a Analyses for C, H, N, and S check figures shown.
% calcd.
Methyl Isocyanate
% calcd. % ’ found 32.9 4.1 19.2 43.8
32.78 4.18 19.30 43.22
,
Acidic or basic impurites in the samples act as interferences, since the final analysis is a neutralization. The free acid or base in the samples should be determined, and the final analysis should be corrected t o obtain the correct values for isocyanate or isothiocyanate. REAGENTS
Butylamine Solution. Dilute 25 grams of mono-n-butylamine to 1 liter with dioxane, which has been dried over potassium hydroxide pellets. Standard Acid = 0.1 N sulfuric acid. PROCEDURE
A sample containing approximately 0.002 mole of isocyanate or isothiocyanate is weighed in a small glass-stoppered weighing bot-
tle. Very volatile samples are weighed in sealed glass ampoules, The stopper from the weighing bottle is removed, and the weighing bottle containing the sample is placed in a 250-ml. Erlenmeyer flask. To the flask are added 20 ml. of the butylamine solution, and the flask is swirled to mix the reactants. Alkyl isocyanates and alkyl isothiocyanates are allowed to stand 45 minutes a t room temperature for complete reaction. Aromatic compounds react more rapidly with the butylamine, and the reaction mixture can be titrated immediately after mixing the reactants. Then 25 ml. of distilled water are added, and the solution is titrated to the methyl red end point with 0.1 N sulfuric acid. A blank is run on 20 ml. of the butylamine solution. From the difference between the two titrations, the amount of isocyanate or isothiocyanate in the sample can be calculated (Table I). All the samples were Eastman Kodak White Label chemicals used without further purification. Dioxane was chosen as the solvent because isocyanates and isothiocyanates react significantly with water and alcohols. As dioxane is miscible with water, aqueous standard acid can be used for titration of the excess amine after completion of the reaction. Bn aliphatic amine was chosen as the reagent, because these amines are sufficiently basic to be titrated with standard acid. Butylamine was specifically chosen because its boiling point is high enough to avoid trouble with evaporation. (Ethylamine boils a t 16.6” C., propylamine a t 48.7”, and butylamine at 77.8”.) Aniline, methylaniline, and P-naphthylamine react quantitatively with isocyanates and isothiocyanates, but the reaction is somewhat slower than the reaction with butylamine. These amines are more difficult to determine than aliphatic amines. Other primary aliphatic amines, such as n-amyl and n-pentyl amines, could be used as well as butylamine, but the reactivity decreases slightly as one goes up the series; when octadecylamine is used, the reaction takes about 3 hours for completion. Water and alcohols should be entirely absent in the reagent as these materials react readily with isocyanates and isothiocyanates. Because or the acidimetric nature of the final analysis, any free acidic of basic materials present in the sample should be determined on a separate sample, and the final analysis should be corrected accordingly. RECEIVED February 4, 1948.
COI~RECTIOX. In the article “Nomograph for Particle Size Determination with the Sharples Supercentrifuge” [Saunders, E., 4 i x . 4 ~ .CHEW,20, 379 (1948)], the feed rate is given on page 381, third paragraph under heading “Sample Calculation” a s 100 cc. per minute, whereas it should be 1000 cc. per minute.
