OXIDE OF MERCURY WITH OXIDE OF ANTIMONY. VIL-On a New Method for the Quantitative Estimation of Nitric Acid. BY DR. E. PUGH. IN a paper read before the Chemical Section of the British Association for the Advancement of Science at Leeds for 1858 the author gave the chemical principles involved in a new method for the determination of nitric acid together with some results illustrating the accuracy of the same The object of the present paper is to give some details of manipulation a knowledge of which is essential to the successful use of the method as also to indicate some collateral points involved in cases of nitric acid determinations that are likely to occur. The difficulties of estimating small quantities of nitric acid by any of the known methods were sufficient to make it desirable that some better methods should be known.The great reducing power of the subchloride of tin suggested the idea of using it to deoxidise nitric acid; and the very exact method of August Streng (Yogg. Ann. xcii 57) of estimating the amount of tin oxidised and hence the amount of nitric acid reduced seemed to offer the necessary conditions of success. 8 1. Streng’s method consists in ascertaining how much of a solution of bichromate of potash of known strength is necessary to convert a given amount of protochloride of tin in chlorhydric D2 PUGH ON A NEW METHOD FOR THE acid solution into perchloride ; the point of complete chloridation being known by the deep blue colour produced by the liberation of iodine from iodide of potassium in presence of starch by the first drop of bichromate solution above that necessary to raise the protochloride to perchloride.62. This method is recommended by the ease with which the reagents are obtained in R state for use and by the absence of any tendency on the part of the bichromate solution to change on keeping and most particularly by the characteristic action that marks the point of complete oxidation. 63. An attempt to reduce nitric acid in open vessels gave no constant results; nor mas the experiment more successful when conducted in vessels from which the air had been exhausted by repeated pumpings and subsequent influx of carbonic acid. This corresponded with the results of Dr. Mohr who in 1855 says in his Lehrbuch der Titrirmethode p.218 that he found an amount of oxidation corresponding to more than three atoms of oxygen for each atom of nitric acid present from which he erroneously concluded that protoxide of nitrogen or even nitrogen gas mas evolved in the process. 0 4. In a great number of experiments in which the time of boiling under carbonic acid gas varied from 30 minutes to 6 hours and in which almost all possible proportions of nitric acid chlo- rhydric acid and protochloride of tin were used no conditions could be found that mould give constant results. The amount of oxidation obtained varied from 3 to 6 atoms of oxygen for every atom of oitric acid present; and ten hours boiling was not suffi-cient to raise the oxidation above 7 atoms.And what was re- markable it was possible to get an amount of oxidation from 3 or 4 millegrammes of nitric acid corresponding to 4 or 5 atoms of oxygen while with -060grins. of nitric acid the proportional amount of oxidation was very little higher. This would seem to indicate either that on long boiling the nitric acid acquires a passive state with regard to the proto- chloride of tin or that there is a more or less stable interme- diary compound formed during the reaction beyond which the oxidation cannot be carried without a higher temperature. 8 5. An examination of the carbonic acid over the fluid after the reaction showed the absence of nitrogen and protoxide of nitrogen. The fluid on the contrary was found to contain am- monia which corresponded in quantity with the amount of tin QUANTITATIVE ESTIMATION OF NITRIC ACID.subchloride oxidised and hence suggested an explanation of the reaction by the formula NO + 8 (SnC1 + HC1) = NH + 8SnC1 + 5HO 0 6. On raising the temperature in a closed vessel to 140"for half an hour the oxidation effected was equal to that of 8 atoms of oxygen for each atom of nitric acid present ; arid at a tempera-ture of 170° the reaction with *060grms. of nitric acid was COM-pleted in 10 minutes; it being indeed only necessary to raise the temperature to this point ill an oil-bath and then remove the lamp and allow the reaction to take place during the few minutes the temperature of the bath was rapidly falling. These reactions suggested the following process 7.To make an aqueous solution of bichromate of potash of such strength that a. times the unit of weight of salt wiil be contained in b. times the unit of volume of the solution. Then the unit of volume of the solution will contain -a units of weight b of the salt. And take of a solution of protochloride of tin with great excess of clilorhydric acid a quantity sufficient to reduce at least one- fourth more nitric acid than is supposed to be present in the substance to be examined. Then ascertain the number n of units of volume of the bichro- mate solution required to chloridate this quantity. Digest a like quantity in a sealed tube with the nitric acid to be determined in an oil-bath at 170° for 10 minutes and then ascertain the uurnher n' of units of volume of the bichromate solution required to complete the chloridation.Then -a (n-n') = the number of units of weight of the b bicliromate solution required to osidatc an amount of proto-chloride of tin equal to that oxidated by the nitric acid acted upon. And from the formulae hTO + 8 (SnC1 + WCl) =NH + 8SnC1 + 5H0 and K0,2Cr03 + 3SnC1 + &HC1= Cr,O + ShCL + YHO + KCl we get tlie equivalent valuc of nitric acid aid bichrornate of pt ash cxpressed by NO5 = +KO 2Cr.0 PUGH ON &4 NEW METHOD FOlt ‘L‘IiI< That is one unit of weight of bichromate of potash corrcsporids to a number of units of weight of nitric acid equal to -. NO5 ) = 0.13775 +(KO ZCrO,) KO 2Cr0 consequently if x equal the quantity of nitric acid present we have -13775 x a (n-n’) = x.b 0 8. This result is obtained without paying any regard to the atomic weight of tin; indeed the absolute quantity of tin ixsed need not be known. It is also much better to pay no regard to the atomic weight of chromic acid in fixing the strength -a of b a the solution since whatever value 8-may have the co-efficient of (n-n’) can always be reduced to a single number of 4 or at most 5 digits; and the labour of repeating a single multipli- cation for each determination is of less importance than that of getting the solutions of an exact strength to correspond with the atomic weights as is usually recommended. Besides it is of more importance in all volumetric analyses that the strength of the solutions be properly adapted to the size of the burette and the pipettes used.so that the maximum error of reading off the amount of solution required shall be inappreci- able than that they should be chosen with a view to facilitate the calculation of the result. And the latitude of this choice is too limited when confined to a multiple of so large a number as 10 as must be the case when simplicity of calculation is sought by such means. Q 9 VITe may now pass to the subject of the preparation of the reagents used. The bichromate of the shops may be used after recrystallisation and drying. The subchloride of tin may be prepared by hanging a piece of block tin or t,in foil with a platinum wire in a flask containing concentrated chlorhytkic acid.The wire should pass it few times round the tin in order by multiplicity of contact to promote the eiectro-chemical action without which the tin dissolves very slowly. A drop of bicliloride of platinum has also been recornmended to hasten the dissolution but the precipitated platinum in the solu- tion stop up the pipettes and is therefore objectionable. When so much of the tin has dissolved that it requires ahout 3 units of volume of the liichromate solution to oxidatc 1 volumc QUANTI'L'ATIVE ESTIMATION 0%'NITRIC A4<'113. of it the remaining tin may be removed by the platinum wire and the solution retained in a well-stoppered bottle for use. If the chlorhydric acid used contained either nitric acid or chlorine as the purest article of the shops always does it will have been destroyed by the tin.A weak solution of iodide of potassium free from iodate of potash must be used. As only 3 or 4 drops of a weak solutioii are used the presence of traces of iodic acid is immaterial. The starch paste or mucilage should be so thin that it Ciktl readily be drawn into a small pipette. A few miliigrammes of starch added to half a pint of' boiling water will answer. 0 10. Supposing the unit of volume to be the cubic centimetre tlie unit of weight the gramme and the diameter of the burette to be such that the maximum error of reading is less than & of this unit as was the case in my own experiments then for ordinary determinations 40 gram of the bichrornate solution may have so much water added to it that the whole shall amount to 1000 cubic centimetres.And *OPO + -13775 = 90551 grms. = NO correspondirig to 1.cc. of bichromate solution. the burette. And ~00551(n-78') = whole nitric acid in a cletermiiiation. Man&alation. 3 11. If no other substance capable of oxidating tile tin solution be present in the aqueous extract of the substance to be examined for nitric acid that extract is evaporated in a small capsule with excess of base (potash soda or lime) to as small a volume as possible. A 6 or 8 c. c. pipette is then filled with the tin solution xnd emptied into a small tube with narrow neck and funnel shaped top. (See Fig. 1.) The concentrated nitrate is brought into the saiiic tube hy means of a 1 c.c. pipette. The capsule is washed and thc wasli-ings brought into the tube in like manuer. If desirable the concentrated nitrate can be 1 ery accurately divided into two equal parts with this small pipette arid brought iirto two scparatc tabcs for duplicate analysis. PUGH ON A NEW METHOD FOR THE When ready for closing the tube should contain- 6 to 8 c. c. subchlorideof tin. 10 to 15 c. c. of nitrate and washings. 2 to 3 c. c. air below the point of closing.* This air if allowed to remain gives up its oxygen to the tin. It must be removed; this is effected by dropping a few small pieces of marble as large as a pin-head into the solution and ~10s- ing the tube after the evolution of carbonic acid gas has ceased.The tube thus prepared is placed in an oil-bath for which a small porcelain dish holding half a pint of oil will answer and the temperature is raised by a common spirit-lamp. A ther-mometer protected in a glass tube has its biilb placed in the oil to mark the temperature which to avoid danger of explosioli is read off through a small telescope. It should here be remarked however that in nearly 100 trials only three explosions took place and these were owing to the tubes being too full of fluid so that on expanding by heat the fluid filled the tube and burst it by the expansive force of water rather than that of steam. In 15 minutes the temperature may be brought to 170° and after resting 5 minutes at this point the reaction is completed.0 12. When sufficiently cooled down a drop of fluid which mostly adheres to the apex of the tube must be chased away by a gentle heat from a lamp and then the apex is broken off and the contents brought into a small beaker ;a little starch mucilage and a few drops of iodide of potassium are added and then the requisite quantity of bichromate is brought in from the burette. We thus get the value (n-n’) which multiplied by -13627 b gives the amount of nitric acid present. 0 13. The contents of the beaker may be treated with potash to alkaline reaction and then distilled till three-fourths of the fluid have gone over. The distillate may be caught in a titrated acid solution and the quantity of ammonia formed estimated from which the nitric acid present may be calculated ; or the contents * These tubes are easily made from glass tubing of the size of the largcst or-dinary combustion tnbing (+ in.diameter). The end B is closed and then ahout 5 to 6 inches up the tube it is drawn out as at 8,care being taken tEat the glass dops not liecome too thin at this point ; then after the introduction of the marble to drive out the air this neck is heated and drawn out almost to capillary fineness; 2nd the instant the marble is dissolved the tube is closed. QUANTITATIVE ESTIMATION OF NITRIC ACID. of the tube may be distilled with potash at once and the nitric acid thus determined. This method has the advantage that small quantities of other substances capable of oxidising the subchloride cannot if present affect the result.On the other hand the acids used must be free from ammonia as also must the solvent of the tin ($9) be free from nitric acid. And further when very small quantities of nitric acid are operated upon (as when the whole does not exceed -002grms.) it is not possible to determine ammonia with that extreme exactness which the above method affords in the determination of nitric acid. 0 14. Organic matters capable of oxidising the tin-solution must be removed by boiling the nitrate with permanganate of potash and then removing the excess of permanganate by carbonate of lead." Sulphuric acid was found to yield sulphurous acid but sulphate of baryta did not do so. This acid must therefore either be removed or saturated with chloride of barium.Q 15. Or no regard need be paid to these oxidising substances ; the whole may be heated as above (0 ll),and the products treated as in Q 13. Q 16. Mohr found that the oxidating value of the bichromate solution varied for different quantities of water containing equal quantities of tin; but he found that if all the air had previously been expelled from the water no suchresult was obtained. These observations of Mohr are easily confirmed; but a little care will enable the operator to use so nearly the same quantity of solution each time that the differences due to the cause just noticed will be inappreciable in ordinary determinations ;but for very small quantities of nitric acid the air must be removed. 617. Before finding that a high temperature was essential to give constant results and finding it impossible to complete the decomposition of the nitrate by the subchloride of tin and knowing that such decomposition is effected by subchloride of iron and that the perchloride of iron is reduced hy the protochloride of * Recent researches of CloEz and Guignet (Conzpt.rend. xlvii 710 1858) show that a number of nitrogeneous substances (ammonia aniline quinine cin- chonine the cyanides and sulpho-cyanides urea and gelatine as also several bodies in which hydrogen 18 replaced by hyponitric acid) give nitric ac d i\ith perman-gauate of potash. These substances may be got rid of by boiling w'th potash,-by distilling with sulphuric acid,-by destroying them with b.chromate of potash and sulphuric acid and then destroying the excess of chromiz acid with suhchlor de of tin and distilling with excess of sulphuric acid or by boiling with peroxide of manganese.My own experiments showed thit with meal from cereal and legu- rninoiis grains either fresh or partly decomposed In a so 1 no uitrate was formed by pcrmangauatc of potash. PUGH ON A NEW METIIOD kC. tin an idea was suggested that the iron might be made a means of conveying the oxygen from the nitric acid to the tin thus- NO + 6FeC1 + 3HC1 = NO + 3Fe,Cl + 3HO Fe,Cl + 2SnC1 = 2FeC1 + Sn2C1 and 2CrO + 3Sn2C1 + 3HC1 = Cr20 + 6SnC1 + 3H0 2Cr0 + 3SnC1 + 3HC1 = Cr,O + 3SnC1 + 3H0 from which a relation between the oxidating power of nitric acid and chromic acid may be obtained.But this method did not give satisfactory results when the iron and nitric acid acted on each other alone arid the tin-solution was afterwards made to act on the products thus formed and the tertiary products thus obtained were treated with the bichromate solution; nor did it when the tin and iron were acted on bothat the same time. Although the subchloride of iron alone had no power to prevent the liberation of iodine from iodide of potassium yet when mixed with the subchloride of tin this latter substance required more chromate to oxidate it than alien alone thus indicating a property in the iron subchloride to exhibit reducing forces in connection with the tin which alone were not manifested. A nitrate when ignited in a glass tube with protochloridc of tin in a current of hydrogen was found to yield ammonia; but not in sufficient quantity to indicate the possibility of obtaining a method founded upon the reaction.Nitrate of potash added to a concentrated hot aqueous solution of potash and subchloride of tin in crystals added to the mass and the whole ignited gave copious ammoniacal fumes; and several qnactitative trials by this method seemed to indicate that if suitable vessels could be obtained for igniting the mixture a good method for commercial purposes where great accuracy was not required might be obtained. But for accuracy of result the above method with sealed tubes will give results equal to those of any other dcpartment of volu-metric analysis. For those who do not like to usc sealed tubes sniall glass stoppered bottles may be used the stoppers beitig bound in by a small copper wire passing over thein and around thc neck of the bottle. In conclusion I must express my obligation to J. B. Lawes Esq. for hi! kindness in allowirig me the use of the Rothamstecl laboratory arid its ;tpparatus nucl reagents for these expcrinients.