Microdetermination of nitrates and nitrites—III1

Talmta, 1971,Vol. 18,pp. 219to 224. PersammtPrcls. Printedin Northernlnlqnd

MICRODETERMINATION OF NITRATES AND NITRITES-III* GASOMETRIC AND GRAVIMETRIC METHODS BASED ON REDUCTION WITH FORMIC ACID W. I. AWAD, S. S. M. HAWAN and M. T. M. ZAKI Research Microanalytical Laboratories, Chemistry Department, Faculty of Science, Ain Shams University, Cairo, U.A.R. (Received 15 May 1970. Accepted 16 June 1970)

Summa~--Simple nncrogasometric and gravunetric methods for the determination of the nitrate and nitrite groups are descrmed. These are based on reduction with formic acid whereby one mole of mtrous oxrde and four moles of carbon dioxide are simultaneously liberated per two moles of mtrate; two moles of mtrous oxrde and six moles of carbon dioxide are liberated per seven moles of nitrite. Nitrous oxide is measured gasometrically and carbon dioxrde grawmetrically. Results accurate to &0*2% absolute are obtained for both nitrate and nitrite.

Lrrr~~ work has been reported on the use of organic reagents for gasometric and gravimetnc determination of the nitrate group, apart from salicylic acidi and hydroquinone.= Organic compounds such as nitron ,3 di(a-naphthylmethyl) amine,* a-phenyi-pdlethylaminoethyl-p-nitrobenzoate,6 &liethylbenzhydrylamme,8 cinchonamine, diphenylguanidine,8p-tolyl isothiourea! 5-mtro-6-ethoxyquinoline,s and iV- substituted a-naphthyl-methylamine’ have been used to determine the nitrate group gravimetrically, but all have their limitations and give less accurated results when used on the microscale. In the present work formic acid is used as an organic reductant for both the nitrate and nitrite groups. The nature of the reaction, advantages, limitations, and some possible interferences have been studied and microgasometric and gravimetric procedures are described. EXPERIMENTAL Apparatus The gasometric measurements were made with the apparatus previously described.’ The gravi-

metrrc measurements were made by usin the apparatus shown m Fig. 1. It conststs of a 25-ml reaction vessel (A), a 100 x 20mm trap Por moisture condensation (IQ, a 100 x 20mm trap iilled with 14-22 mesh anhydrone for moisture. absorption (C), a Pregl absorption tube fYled with a layer of 10-20 mesh manganese dioxide and a layer of anhydrone for nitric oxide absorption @), a Pregl absorption tube filled with a layer of 14-22 mesh soda asbestos and a layer of anhydrone for carbon dioxrde absorption (E), a guard-tube filled with alternate layers of soda asbestos and anhydrone (G), a small funnel (F), a Mariotte bottle, a stop-watch and a microburner. The mtrogen used is purified by passage through a bell-chamber immersed in concentrated sulphuric acid contained in a cylindrical jar, and then throughtwo U-tubes filled with anhydrone and soda asbestos respectively. Reagents All reagents were of analytical grade unless otherwise speciikd. The organic nitrates used were prepared acwrdmg to standard methods. Cadmmm nitrate tetrahydrate, bismuth nitrate pentahydrate, and lanthanum nitrate hexahydrate (commercial grade) were puri6ed b several crystallirations. All the nitrate samples were >99*5 % pure (standard nitrometer method r . + Part II, Talanta, 1969,16,1393. 219

220

W. I AWAD, S. S. M.

HASSAN and

M. T. M. ZAKI

Gasometric d&ermination ofnitrates andnitrites, Introduce 3-5 mg of the nitrate into the reaction vessel, displace the air with carbon dioxide for 5 min till no air bubbles are collected in the nitrometer. Introduce 3 mi of formic acrd through the funnel. Heat gently (-5 min), usmg a microburner, till no more gas bubbles are collected. Leave for 5 min and record the volume of nitrous oxide liberated. Carry out a blank experiment. For nitrite determination, use the same procedure but msert a Pregl absorption tube filled with manganese dioxide between the reactron vessel and the nitrometer. Calculate the nitrate and nitrite nitrogen-content according to the equation

%Nitmte or mtrite nitrogen = F( Vi - V&P - ~)I(273 + T)W where V, ml is the volume of nitrous oxide, V, is the blank, P is the observed atmospheric pressure, p the vapour pressure of potassium hydroxide soIution at To (the average temperature of the room and of the solution m the nitrometer), Wmg is the sampIe weight, and F equals 4502 and 78.61 for nitrate and nitrite respectively if the pmssures are measured in mmHg (the practical measurement) and the corresponding factors am 33-77 and 5646 for pressures measumd in mbar. Grauimetric determination of nitrates and nitrites. Weigh 3-8 mg of nitrate or nitrite sample into the reaction vessel (A) (Fig. 1). tiMed the vessel to traps (B), (C) and tube (D). Adjust the rate of nitrogen flow to 20-25 mi/min and sweep for 10 min. Wipe and weigh the carbon dioxide absorption tube (E) and corm@ it to tube 0). Add 5 mi of formic acid through the funnel (p). Heat gently for 3 min. Sweep out the gaseousreaction products for 10 min. Disconnect the carbon dioxide absorptron tube (E) and close it tightly with its cone and socket. Leave the tube in the balance room for 5 min; meanwhile weigh the next sample. Rewipe and reweigh the tube, using the same weighing schedule. Carry out a blank experiment. Calculate the nitrate and nitrite nitrogen-content from the equation Nitrate or nitrite nitrogen = FW,/w% where W, mg is the weight of carbon dioxide, Wmg is the sample weight and FequaIs 15.91 and 37.12 for nitrate and nitrite respectively. RESULTS Gasometric

determi~tio~

AND

DISCUSSION

ofnitrates and nitrites

Determination ofthe nitrate group. Formic acid reduces the nitrate group to nitrous oxtde according to the equation l1 6HCOOH + 2KN0, -+ 2HCOOK + NaO + 4C0, + 2H,O The reduction product of the nitrate group is proved to be pure nitrous oxide, since it is still recovered quantitatively when a trap of iron solution or manganese dioxide is inserted between the reaction vessel and the mtrometer, and is therefore not mtric oxide or nitrogen dioxide. That it is not nitrogen is shown by passing the gaseous reduction products through a trap containing hydroiodic acid; iodine is liberated.12

Marlotte bottle

FIG.

I.-Apparatus

for gravimetric devotion of nitrates and nitrites (for description, see text).

~r~te~ation

of nitrates and nitrites-III

221

Since nitrous oxide shows no solubility in alkaline sohrtions and known volume of the gas was unaltered when kept overnight over 50% potassium hydroxide solution, the reaction is used for the gasometric microdetermination of nitrates. The action of formic acid on other nitrogen groups (e.g., amide, azo, nitro, amine, oxime, and ammonium ions) gives no gaseous products. The presence of acetamide, Methyl Red, nitrobenzoic acid, p-aminophenol, a-benzoinoxime, and ammonium acetate does not affect the results for potassium n&ate, and the error was &-0.2x absolute. Inorganic oxidants such as potassium &chromate, permanganate, or periodate, ammonium metavanadate, sodium tungstate and potassium perchlorate do not interfere with the reaction. Some nitrate samples are satisfactorily analysed by reduction with a~y~ous formic acid (c$ Table I). The mean absolute error is f0.1 %. Similar results are also obtained by using acid of 70 % concentration, but less accurate results are obtained by using more dilute acid. Nrtrate esters (e.g., P.E.T.N.) and nitramines (e.g., nitroguanidine) are not reduced under these conditions. TAFILEI.-MICROGASOMETRIC

DETERMINATION OF SOME NITRATE SAMPLFS BY XU3DUCTION WITH PORMIC ACID

Nitrate-mtrogen, % Sample Calculated

Found

Potassium mtrate

13*85

Barium nitrate

10.71

13-S 13.8 13.9 10.6 10.7 10.8 8.2 8.2

Silver nitrate

8%

Bismuth nitrate pentahydrate

8.67

;:; 8.6

Cadmium nitrate tetrahydrate

9.08

Lanthanum nitrate hexahydrate

9-70

Guard&e nitrate

11‘47

9”:; 9.1 9.1 9.7 P-6 96 11.4 11.4 11.5

Other reducing acids such as oxalic, tartaric and citrrc acids were also tried both in the presence of hydrochloric acid and as saturated solutions. No gaseous products were obtained, and the nitrate-nitrogen was quantitatively recovered as nitrous oxide after addition of formic acid, indicating that the nitrate group was unaffected by these acids. However, ascorbic acid in 6M! hydrochloric acid reduced potassium nitrate to nitric oxide; the average recovery was only 70 %. De~e~in~~~o~of the nitrite group. Reduction of sodium nitrite in acidic tnedia with mercury, iron(n) and hy~oq~none was tried. The mean average recoveries obtained were 100, 86 and 93% respectively with these reagents (Table II). Although satisfactory results were obtained with the mercury-sulphuric acid procedure, this method 7

222

W. I. AWAD, S. S. ha.

-N

and M. T. M. !&a

has its limitations in presence of aromatic substances, because of mtrosation of the aromatic moiety in preference to reduction of the nitrate group.2 It was found that sodium nitrite reacts with anhydrous formic acid according to the equation 7NaN02 + 13HCOOH -+ 7HCOONa + 6C02 + 9HaO + 3N0 + 2N,O Compounds containing an aromatic moiety do not interfere. The presence of nitric oxide is confirmed by the brown fumes formed on exposure of the gaseous reduction products to air and by the reaction with iron(R) solution. The nitrous oxide is indicated by the hberation of iodine from hydrrodic acid after the nitric oxide has been absorbed in a trap of granuIar mang~ese dioxide. Reduction of sodium rut&e with anhydrous formic acid and collection of the liberated gas over 50% po~ssium hydroxide solution, with a manganese dioxide trap mserted between the reaction vessel and the mtrometer, showed that the volume of gas obtained was two-fifths of that obtained without the trap. This indicates that nitrous and nitric oxide are liberated in ratio of 2:3. This can be explained on the basis that two reactions proceed simultaneously: 4NaN0,

+ 8HCOOH -+ 4HCOONa + 4COa + 6Ha0 + 2N,O

3NaN02 + SHCOOH -+ 3HCOONa + 2C02 + 3Hz0 + 3N0 The recovery of two moles of nitrous oxide per seven moles of the nitrite, after abso~tion of the nitric oxide, constitutes the basis of the geometric procedure. The results obtained by reduction of sodium nitrite with anhydrous formic acid show a mean absolute error of &@I % (Table II). Reduction with diIute formic acid (70% v/v) IS not quantitative and proceeds according to the first route only, since the gas hberated is pure mtrous oxide. Gravimetricdeterminationof nitrates and nitrites The use of nitron. Gravimetric mrcrodeterminatlon of nitrate (e.g., potassium nitrate) by precipitation with nitron proved to be unsuccessful. The average recovery is 81%. The solubility product of nitron nitrate was found to be 1.2 x lo-’ at 25”. The accuracy is independent of pH over the range 0*5-K ‘bite

~.-MI~~~~c DE~~NA~ON OF SODIulw NITRITE (20.29% N) REDUCTION WITH IFtON( HYDROQUINONE, MERCURY AND FORhfIC ACID

Reductant Iron(hydrochlonc acid Hydroquinone-hydrochlom acid Mercury-sulphuric aad Formic acid

Nrtntemtrogen, found, % 17.7 17.4 17.0 18.7 18.9 18.8 20*3 20.3 20.3 20-2 20-2 20.2

Recovery % 87 86 84 92 93 93

loo 100 100 100 100 loo

BY

Micr~t~ation

of nitrated and &rites-III

223

Precipitation from solutions containing 5 mg of potassium nitrate per ml, in a fiber beaker,Is cooling, filtration and washing with 5 ml of cold water (5“) gave results correct within -&0*7x absolute but the use of this precipitant on the micro scale suffers from many ditlkulties; (i) several carefully controlled operations are needed, including precipitation, filtration washing and drying; (ii) solubility of the precipitate; (iii) poor precision and accuracy. The use offormic acid. Carbon dioxide is an oxidation product of formic acid with both nitrates and nitrites. Four and six moles of carbon dioxide are liberated per two moles of nitrate and seven moles of nitrite respectively. The carbon dioxide is dned over anhydrone then swept with pure nitrogen gas into a previously weighed absorption tube ~ontain~g soda asbestos. In the nitrite destination an ad~tional trap of granular manganese dioxide is used to absorb the nitric oxide gas liberated. TABLE1II.-M1c~~~~v&nrnuc AND

NITRITE

Sample Barmm nitrate

SAMPLES

DE -TBRMINATION UsIN

FORMIC

OF SOME NM’RATB ACID

Nitrate or nitnte nitrogen, % Calculated IO.71

Found 10*7 10.7 10.7 8.3 8.3

Silver nitrate

8.24

Cadmium nitrate tetrahydrate

9.08

;::

Bismuth nitrate pentahydrate

8 67

;:; 86 8.8

Lanthanum nitrate hexahydrate

9.70

z:‘B 9.7

Urea nitrate

11.38

Sodium nitrite

20.29

17.: 11.4 11.3 20.3 20~3 20.2

Gravimetric analysis of different nitrate samples shows a mean absolute error of 034%. For sodium nitrite the mean absolute error is fO*1 % (Table III). However, this method is not applicabIe to nitrite or nitrate samples in presence of inorganic oxidants such as potassium dichromate, permanganate, periodate or perchlorate, ammonium metavanadate and sodium tungstate. Organic compounds containmg a nitrogen function (e.g., acetamide, Methyl Red, nitrobenzoic acid, p-aminophenol, a-benzoinoxime and ammonium acetate) do not interfere. Zusammenfassung-Einfache mikrogasometrrsche und gravimetnsche Methoden zur Bestunmung von Nttrat- und Nuritgruppen werden beschrieben. Sie beruhen auf Reduktion mit Ameisens&re, wobei ftir zwei Moi Nitrat ein Mol Distickstoffmonoxid und vier Mol Kohlendioxid zugleich frei werden; auf sieben Mot Nitrrt entstehen xwei Mol Distic~to~ono~d und se&s Mol Ko~~dioxid. Distrckstoffmonoxid wud gasometrisch. Kohlendroxid gravhnetrisch gemessen. Sowohl ftir Nitrat ais such fitr Nitnt werden auf f0,2% absolut genaue Brgebnisse erhalten.

224

W. I. AWAD, S. S. M. HA&UNand M. T. M. ZAKI R&nm&On d&it des m&odes microgazom&rtque et gravtmdtrique simples pour la determmation des groupes mtrate et nitrite. Elles sont ba&s sur la reduction au moyen d’acrde formique, par laquelle une mole de protoxyde d’azote et quatre moles de gaz carbonique sont lib&es simultanement pour deux moles de nitrate; deux moles de protoxyde d’azote et six moles de gaz carbonique sent lib&es pour sept moles de nitrite. On mesure le protoxyde d’azote gazometnquement et le gaz carbomque gravim&riquement. On obtient des &ultats exacts a *0,2 % absolu tant pour le nitrate que pour le nitrite.

REFERENCES 1. S. Ohashr and H. Makishtma, Bull. Chem. SW. Japan, 1956,29, 700 2. W. I. Awad and S. S. M. Hassan, Talanta, 1969,16,1393. 3. M. Busch, Ber., 1905,38, 861. 4. H. Rupe and F. Becherer, Heiv. Chim. Acta., 1923, 6,674. 5. C. S. Marvel and V. Du Vtgneaud, J. Am. Chem. Sot., 1924,46,2661. 6. A. Ogata and M. Konishr, J. Pharm. Sot. Japan, 1934,54,546. 7. M. Amaud, Compt. Rend., 1884,97,174. 8. F. Amdt, Annalen, 1911,384,322. 9. M. C. Grimaux, Compt. Rend., l&395,121,749. 10. R. Hutton, S. Salam and W. Stephen, J. Chem. Sot., A 1966, 1573. 11. J. Mellor, A Comprehensive neatise on Inorgam and beoretlcal Chemistry, Vol VIII, p, 397. Longmans, London, 1947. 12. A. Quartaroli, Gaze. Chim. Ital., 1911,41,53, 64. 13. A. Vogel, A Text Book of Quantitative Inorganic Analysis, 2nd Ed., p 826. Longmans, London, 1957.