Ion-selective electrodes in organic functional group analysis

738

SHORT

COMMUNlCATIONS

Summary-A new extractive photometric method IS described for estimation of molybdenum with 2-aminobenzenethiol. The green complex in chloroform has its absorbance maximum at 700nm and IS stable for 2 hr when extracted from a solution of optimum pH range 1.4-2.8. The extraction is quantitative. The sensitivity is 0.0075 &cmz. Beer’s law is obeyed over the range 0.25-10 ppm with optimum range 0.54.5 ppm. The molar absorptivity is 7.08 x 10’ 1.mole- I. cm- I. The overall stahlity constant is 2.0 x 10’ at 25 f 0.1”.

Tdanta.

Vol. 23. pp. 73X-740

Pergamon

Press. 1976 Rntcd

m Great Bntam.

ION-SELECTIVE ELECTRODES IN ORGANIC FUNCTIONAL GROUP ANALYSIS MICRODETERMINATION OF NITRATES AND NITRAMINES WITH USE OF THE IODIDE ELECTRODE Research Microanalytical

&AD s. hf. -AN Laboratory, Department of Chemistry, Faculty of Science Ain Shams University, Cairo, Egypt

(Received 20 February 1976. Accepted 12 April 1976) Organic nitrates and nitramines have been determined by titration with various reductants.’ However, these methods suffer from the defect that many nitrogenous and nonnitrogenous compounds interfere and the titrants need special precautions during preparation, storage and use.’ Methods based on spectrophotomet& and gravimetric* procedures are usually time-consuming and unreliable when used on a routine basis. Gasometric reactions using inorganic5-9 and organic’“,” reagents have also been suggested. Recently, the development of the nitrate-responsive electrodes’2-14 has made possible substantial improvement in the analysis of inorganic nitrates. Reduction of the nitrate ion followed by measurement of the liberated ammonia by means of the ammonium-responsive electrode has also been reported. I’ The use of both electrodes for the analysis of the nitrates by direct potential measurements requires careful adjustment of many variables and the resulting precision is not better than + 2%.“-” Potentiometric titration is more accurate provided that the titrant forms either a stable complex or a precipitate, but unfortunately not many such titrants are available for nitrate or ammonia. However, diphenylthallium(II1) sulphate has been applied for the titration of the nitrate ion, on the semi-micro scale only, with use of the nitrate electrode.16 On the other hand, the nitrate electrode is inapplicable to the determination of organic nitrates, and prior conversion of the organic nitrate or nitramine into inorganic nitrate by acid or alkaline hydrolysis is not quantitative.1’.‘8 The organic moiety of these compounds partially reduces the nitrate to various products such as ammonia and nitrogen oxides. The present work describes a new finish to the determination of organic nitrates and nitramines by reaction with mercury-sulphunc acid mixture, the mercurous ions released being titrated with iodide. and an iodide electrode used to detect the end-point. Several compounds used as high explosives, industrial intermediates and vasodilators have been analysed and the results obtained are accurate. EXPERIMENTAL

Reaqmts and materials

All reagents were analytical grade except where stated. Doublv-distilled water was used throughout. The nitrate and &amine samples used were of p&ty not less than 99”, as confirmed by the gasometric method.5

Apparatus

A Pye Unicam 292 MK2 pH-meter. an Orion 94-53 solid-state iodide-selective electrode and an Orion 90-02 double-junction reference electrode were used. Procedure

Weigh accurately 2-5mg of the ground dried nitrate, nitrite or nitramine sample and transfer it to a test-tube (10 x 21 cm). For smaller samples, transfer to the tube a portion of solution containing 0.1-1.0 mg of the sample and evaporate to complete dryness. Add 2-3 ml of 96% sulphuric acid and displace the air in the tube with pure nitrogen. Add 3 drops of mercury and shake the tube for 5-7min at room temperature, with a continuous flow of nitrogen. Transfer the contents of the tube to a 250-ml beaker, rinsing with co. SOml of doubly-distilled water, and stir. Insert the iodide and reference electrodes, titrate with 0.02M potassium iodide for sample sizes above 2 mg and with 0.002M solution for sample sizes below 2 mg and monitor the e.m.f. As the end-point is approached add the titrant in O.Ol-ml increments. For sample sizes above 5 mg, the titration has to be conducted slowly, with efficient stirnng from the beginning of the titration, since the equilibrium is reached slowly. Run a blank in the same manner. RESULTS

AND

DIBCUSSION

Nature of the reaction

Mercury in presence of concentrated sulphuric acid quantitatively reduces nitrates to nitric oxide,s-9 and is itself converted into mercurous and/or mercuric ions. It is found experimentally that three moles of potassium

Table 1. Effect of temperature on the reaction of mercury with 96% sulphuric acid (reaction time 15 min) Temperature, 20 30 40 60 80 100

‘C

Dissolved mercury, peel 0.4 0.6 1.6 3.6 6.4 90.0

iodide are required to titrate quantitatively the mercury ions produced by reaction of one mole of the nitrate or nitramine group. Consequently. the reactions of nitrates and nitramines with mercury may be 2 R-O-NO2 + 3H&& + 6Hg = 3Hg,SU, + 2NO + 2R-UH 2 R-NH-NO2 + 3H,S04 + 6Hg = 3Hg#O, + 2N0 + 2R-NH2

+ 2H10 + 2H20

(1) (2)

The possibility of formation of mercuric ions as shown by quation (3) leads to the same overall stoichiometry 2NO; =3Hgi* r3Hgzf =6I-. 2 R-O-NO,

+ 3H,SO, + 3Hg 3: 3HgS0, + 2NO + 2R-OH

+ 2H,O

Table 2. Effect of sulphuric acid concentration on the solubility of mercurous sulphate at 25°C

EWfd,

Solubility, g/l.

CH$OA

Solubility. g/t.

Cl.1 0.5 1.0

0.4983 0.4508* 0.5457 0.5933

2s 3.0 3.5 4.0

0.6644 0.6406 0.6169 a.5695

:::

0*6644 0.6406

4.5 5.0

0.4983 0.5220

0

*The literature value is 0.44 g,/Li9

(3)

However, qualitative identification of the inorganic reacm tion products, by addition of dilute hydrochloric acid and enough ethanol to give a concentration of 75% v/v, removat of H&l, by filtration. then Passage of hydrogen sub phide, showed that mercurous ions were the sole pmdmt. Reaction conditions Reaction of organic nitrates and nitramines with mercury and sulphuric acid is fast enough to ensure quantitative reduction within 5 min at 20”. Table 1 shows the blank values obtained by reaction of mercury with %% sulphuric acid for IS min at temperatures ranging from 20” up to 1009 Reaction at temperatures above 40” is not recommended. since the blank values tend to increase and affect the results, especially with sample sizes in the range between 100 fig and 1 mg. Study of the effect of the sulphuric acid concentration shows that g5% v/v is the minimum concentration pcrmissible for quantitative reduction (Fig. 1). The amount of acid actual& required is determined not only by the amount needed for the reduction but also by the amount needed to give maximum solubility of mercurous sulphate during the titration. Measurements on mercurous sulphate in various concentrations of sulphuric acid show that the solubility of mercnrous sulpbate is maximal in the 1.5-3 N acid (Table 2). Consequently. the optimum volume of 96% sulphuric acid rquired for the reaction is 2-3 ml, which on dilution with water to cc. 50 ml glues a 1.5-3 N soiution which will completefy dissolve up to 33 mg of mercurous sulphate (equivalent to about 4 mg of nitrate sample containing 15% nitrogen). It is necessary to carry out the reaction under a flow of nitrogen to remove the nitric oxide produced. because in presence of air or oxygen some nitric oxide will be ox& dized. dissolve in the acid and cause oxidation of some OF the mercurous ions, and possibly dissolve a little additional mercury, resulting in erratic positive errors. Determination of mercurous ions, by using the ion-selective electrode It has been reported that the solid-state iodidusensitive e&t&e {Orion V4-53) responds to mercuric ions a0 down to 10-s M and is useful as end-point detector in the titration of mercuric ions with potassium iodide. *t The behaviour of this electrode towards mercurous ions has not been explored. In the present work, attempts were made to use it to determine mercurous ions by both direct potenttometry and potentiometric titration. A test of the potential response of the iodide electrode (Orion 94-53) in conpmction with a double-junction reference electrode (Orion 90-02) at pH 2-4 in 0.1 N sodium perchlorate shows a more or less lmear relation between the logarithm of mercurous ion concentration in the range 10-2-10-6 M and the potential, but the slope is less thaa Nernstian. being about 45 mV/decade. Direct measurement

Table 3. Microdetermination of some organic nitrates and nitramines by using the iodide electrode

Sampie

Nitrate- or nit~~ne~ni~~~, % Calcuhted Found*

Recovery %

Pentaerythritol tetranitrate

11.12

17.6 17.6 17.5

99.3 99.3 98.8

Celhdose nitrate

14.14

14.0 14.0 139

g; 98.3

11.2 11.2 11.3 1f.3 11.3 11.3

98.4 98.4 99.3 98.5 98.5 98.5 lot.l.2 101.0 101.0 98.3 99.4 99.4 99.4 98.8 9g.g

Urea nitrate

11.38

Guanidine nitrate

11.47

Nitroguamdine

13.46

13.5 13.6 13.6

9.46

9.3 9.4 9.4 18.8 18.7 18.7

Nitrobiuret

Hexogen (RDX)

I g.92

l Based on the consumption of 3 equivalents of potassium iodide per mole of nitrate or nhramine group.

Sulohuric acid concentration,

% v/v

Fig. 1. Effect of sulphmic acid concentration on the reduction of pentaerythritol tetranitrate with mercury

740

SHORT

COMM~NlCATlONS

Table 4. Microdetermmation of some inorgamc nitrates and nitrates by using the iodide electrode Nitrate- or nitrite-mtrogen. 0

-

Sample

Calculated0

~~

Found*

Potassium nitrate

13.85

13.8 13.7 13.7

Barium mtrate

10.71

10.6

Recovery, Go

10.5

16.41

16.0 16.1 16.2

Sodium mtrite

20.29

19.6 19.9 19.7

99.6 99.3 99.3 99.0 98.0 98.0 97. I 97.8 98.4 96.6 98.1 97.1

* Based on the consumption of one and three equivalents of potassium iodide per group of nitrite and nitrate, respectively. of the mercurous ion concentration by using a calibration graph gives unsatisfactory results. The use of the iodide electrode as indicator for the potentiomet~c titration was therefore tried. The titration curves show a sharp inflection ( -4OOmV) at the equivalence point. The relative standard deviation for I mg of mercurous ion is 0.24; and the reaction follows a 1:2 (Hgj* :I-) stoichiometry down to mercurous ion concentrations of lo-* M. ~et~mination

of argumc nitrates and ~ttralnines

The results obtained (Table 3) for the analysis of some nitrate esters, nitrate salts of organic bases and nitramines in the range 0.1-50 Imole show an average recovery of 99.0% and a mean relative standard deviation of O.Z?/,.A series of 10 replicate analysis of pentaerythritol tetranitrate and of nitroguanidine in the range If@-lOOOpg showed a mean relative standard deviation of 0.39&and an average recovery of 98.302. The response of many nitrogenous groups to the reaction was tested Mono-, di- and tri-nitro compounds (e.g., p-nitrophenol, m-dinitrobenzene, 2.4.6trinitrobenzoic acid and 2,4.6-trinitrophenol) as well as amides, anilides, oximes and hydrazides (e.g.. benzamide. benzanilide, dimethylgiyoxime. sulphanilic acid and hydrazobenzene) were checked and none of these compounds responded to the reaction. Determination

+ 4H,SOp + 6Hg = 3Hg,SO, + 4H,O + 2N0 + SO:-

2NO;

10.5 Potassium nitrite

2NO;

of’ inorganic mtrutes and nitrites

The applicability of the reaction to the analysis of inorganic nitrates and tutrites was also tested. The results obtained. based on the consumption of three and one equivalents of potassium iodide per nitrate and nitrite group. respectively, according to equattons (4) and (5). show an average recovery of 99’” for the nitrates and 97.5’, for the nitrrtes (Table 4). The quantitative liberatton of one mole of nitric oxide gas per mtrate and nitrite group’.” IS in a good agreement with these data.

(4)

+ 2HZSOd + 2Hg = Hg,SO, + 2HrO i- 2NO + SO:-

(5)

Advantages The proposed procedure has advantages on the score of sensitivity, selectivity and simplicity over many of the methods used for analysis of the nitrates.ls2 It is more than 60 times as sensitive as the micro-gasometric methods, since it is applicable to the analysis of lOOr(g of the nitrates, an amount which are totally inadequate for many gasometric and titrimetric procedures: 1Oig of nitrate-nitrosten = 1.07 ml of 0.002 M KI ~0.0160 ml of NO at NTP. It is not affected by environmental conditions such as pressure. temperature, solubthty, and vapour-pressure factors, which have a direct effect on the measurement by the gasometric procedures. Also, the reagents are stable and require no special precautions such as those needed with the reductants used in the titrimetric methods. Finally, it avoids the difficulties and errors arising from the time-consuming reduction’s and hydrolysis’ procedures. REFERENCES

1. W. Becker and W. Shaefer in J. Mitchell, Jr., I. M. KolthotT, E. S. Proskauer and A. Weissberger, eds., Organic Analysis, Vol. II, p. 97. Interscience, New York, 1954. 2. N. Cheronis and T. S. Ma, Organic Functional Group Analysis by Micro and Semimicro Methods, p. 181. Wiley, New York, 1964. 3. N. Allport and 1. Brocksopp, Coiorimetric Analysis, pp. 222-230. Chapman and Hall, London, 1963. 4. R. Hutton, S. Salam and W. Stephen. J. Chem. Sot. A, 1966, 1573. 5. W. Awad and S. S. M. Hassan, Talanta. 1969, 16, 1393. 6. G. Lunae, J. Sot. Chem. Ind., 1901, M. 100. 7. E. Beckett, J. Chem. Sot., 1920, 117, 220, 8. T. Murakami. Bunseki &aaku, 1958. 7. 681. 761. 9. S. S. M. Has&, Analyst, ‘i971; %, 59. 10. W. Awad, S. S. M. Hassan and M. T. Zaki, Taianta, 1971. 18. 219. 11. S. S. M. Hassan, Mikrochim. Acta, 1970, 1109. 12, Orion Research Inc., Bibliography, April 15, 1970. 13. J. Davies, G. Moody and J. R. D. Thomas. Analyst, 1972. 97. 87. 14. 1: Dobbelstein and H. Diehl, Talantu, 1969, 16, 1341. 15. L. McKenzie and P. Young, Rnafvst, 1975, 100, 620. 16. 1. DiGregorio and M. MO&S, A&f. Chem., 1970, 42. 17. M. Busch. 2. Angew. Chem. 1906, 30, 1329. 18. H. Muraour, Bull. Sot. Chim. France, 1929, 45-46. 1189.

19. J. Meltor, .4 Comprehensive Treatise on Inorganic and ~heo~eri~a~ Chemistry. Vol. IV. p. 967. Longmans, London. 1957 20. Orion Research Inc., Newsletter, 1970, 2, 41. 21. R. Overman. Anal. Chem.. 1971 43, 610. 22. J. Smeenk, ibid.. 1974, 46, 302.

Summary-A simple. selective and accurate method has been described for the rapid micro and submicro determination of organic nitrates and mtramines. It is based on reaction with mercury~ulphu~c acid mixture for 5 mm at room temperature followed by potentiometric titration of the mercurous ions released. a solid-state iodide-sensitive eIectrode being used. Three equivalents of potassium iodide as titrant are consumed per mole of nitrate or nitramine group. The results obtained, with sample sizes ranging from 1.0 to 50 pmole, are precise to h 0.2g; and the average recovery is 99’4. None of the other nitrogenous functional groups responds to this reaction.