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HK-11603UNCLASSIFIED'

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THE DETERMINATION OF FREE NITRIC ACID

in s o l u t io n s c o n ta in in g

URANYL NITRATE, ALUMINUM NITRATE AND SODIUM PICHROMATE

R o b e r t Lee Moore end

H . R , S chm id t

TOttiiAioAnoN I anciLU^™• * * FEB 19 1957Fw Th» Attml« In iff; 0«mmlt«lm

C h e m ic a l R e se a rc h S e c t io n T e c h n ic a l D iv is io n s

October 21, 1949

HANFORD Y.ORK5 RICHLAND, WASHINGTON

f o r t h e A tom ic O ie rg y C om m ission by

G e n e r a l E l e c t r i c Company u n d e r

C o n t r a c t if W -3 1 -1 0 9 -en g -5 2

Photostat Price $ *^q

Mfcrofllm Price S *f-0

Avollable from the Office of Technical Servicet Deportment of Commerce Woihington 25, D. C.

UNCLASSIFIED

HW-H603-4-

THE DETERMINATION OF FREE NITRIC ACID

IN SOLUTIONS CONTAINING

URANYL NITRATE. ALUMINUM NITRATE AND SODIUM DICHROMATE

i . INTRODUCTION

Late in 1947 a need was f e l t f o r an accurate method fo r the determ ination of n i t r ic ac id in the presence o f uranyl n i t r a te (UNH), aluminum n i t r a te (ANN) and sodium dichrociate. A s u ita b le method was developed based on e lec trom etric t i t r a t i o n and elim ination o f aluminum and uranium in te rfe rence by treatm ent with potassium flu o rid e . The use of KF for th is purpose had previously been reported^1 ' but only in connection with the use o f a phenolphthalein in d ic a to r which was not considered to g ive su ff ic ie n tly accura te resu lts* Since t h i s work was o r ig in a lly intended f o r in te rn a l process research application only, no formal report was issued . However, in te re s t by o th er Hanford Works tech­n ica l groups led to pub lica tion of development work'■2) based on the o rig in a l worn and has since led to req u ests th a t complete d e ta i ls of the o r ig in a l work be made a v a ilab le . This has been done in th e p resen t rep o rt.

I I . SUMMARY

A method is described fo r th e determ ination of free n i t r i c acid in so lu tio n s containing uranium n i t r a te , aluminum n itr a te and sodium dichromate. The uranium and aluminum are p re c ip ita te d with potassium flu o rid e and the acid t i t r a t e d with standard a lk a l i . A pH meter equipped w ith a glass e lec trode and a calomel reference e lec tro d e is employed to fo llow the t i t r a t i o n . The hydroxide equivalent of the dichromate, i f p re sen t, i s subtracted from th e to ta l t i t r e , however i t is n o t necessary to remove th e dichromate, as in th e oxalate method. Analysis o f a s e r ie s of synthetic so lu tions gave an average e rro r o f 0.14 gA HN0*j or 0 .86$. The method is app licab le to hexone so lu tio n s and to hexone-saturated aqueous so lu tions and may be scaled down to a micro scale w ithout serious loss o f p rec ision or accuracy.

Ammonium n i t r a te and hydrazine are found to in te r fe re due to evolution o f ammonia.

I I I . EXPERIMENTAL

Prelim inary te s t tube experiments showed th a t add ition o f KF to aqueous UNH so lu tio n s caused the form ation o f a fa s t s e t t l in g , lemon-yellow colored c ry s­t a l l in e p re c ip ita te which appeared to carry down almost a l l of the uranium, leaving a w ater-clear supem ate . This m aterial i s probably the known compound, K^UO^Fc* I t is in te re s tin g to note th a t 'a d d itio n of NaF did not re s u lt in a p re c ip i ta te . Addition o f excess KF to an ANN so lu tion gave a fin e white p re ­c ip i ta te or suspension, p ossib ly the potassium analogue of c ry o lite (Na^AlF^)or c h io l i te (5NaF*3AlF3).

5- HW-U603

i l l . SXFERIHEIITAL (C o n t'd )

A s e r ie s o f t i t r a t i o n s were c a rr ie d out u sing a g la ss e lec tro d e fo r pH d e te r­m ination . A Leeds and Northrup U niversal pH m eter was used fo r a l l th e meas­urem ents and gave read in g s reproducib le to 0 .02 pH u n it . P o rtio n s o f standard HNO3 so lu tio n co n ta in in g UNH, ANN, o r Na^irjOy and an excess o f KF (Merck Reagent Grade KF*2H20) were t i t r a t e d w ith standard N/2 KOH o r NaOH. Figure I shows the t i t r a t i o n o f 10 ml o f 0.25 M HNOo in the presence o f a la rg e excess o f KF. I t w ill be no ted th a t although most o f the p o in ts are in th e basic reg ion the shape o f th e curve i s th a t ty p ic a l o f a weak acid and th e in f le c ­t io n po in t corresponds to th e s to ic h io m e tric concen tra tio n o f a c id . In a c tu a l i ty th e curve corresponds to th e t i t r a t i o n o f the equ iv a len t amount o f hydro fluo ric ac id re le a se d from th e potassium f lu o rid e by th e s tro n g e r a c id .

Figure I I shows the t i t r a t i o n o f HNOj in th e presence o f ANN (and excess KF) and Figure I I I in th e presence o f UNH (th e d iffe re n c e in pH a t th e end po in t was due to th e d i lu t io n s employed). The curve with aluminum p re se n t i s sim­i l a r to th a t fo r HNO3 ^ alone. With UNH p re sen t, the low er p o rtio n of th e curve i s u n a lte re d , however somewhat beyond the end p o in t th e curve tu rn s sharp ly and becomes p r a c t ic a l ly h o r iz o n ta l. This i s due to conversion of th e f lu o rid e to u rany l hydroxide a t th e h ig h e r a lk a l in i t i e s , th e p r e c ip i ta te changing co lo r and becoming f lo ccu len t a t t h i s p o in t. I t i s n ecessary to employ a s u f f ic ie n t excess of potassium f lu o r id e to insure th a t t h i s con­v ers io n w ill begin a t a pH h igher than th a t corresponding to th e end p o in t.The 34 gms KF*2H20 used in th e se experim ents corresponds to a fo u r fo ld excess over the uranium p resen t in 10 ml o f 1 .7 M UNH, assuming th e p rec ip ­i t a t e to be K3U02F5«

Figure IV i l l u s t r a t e s the t i t r a t i o n o f sodium dichrom ate. The in f le c tio n p o in t a t 7.25 ml corresponds to th e re a c tio n

Cr20y ♦ 20H“ -----» 2CrO® ♦ H20

fo r which th e th e o re t ic a l end p o in t would be 7.22 m l. F igure V i l l u s t r a t e s the t i t r a t i o n o f a so lu tio n con ta in ing UNH, ANN and Na2Cr20n, as w ell as HNO3 and excess KF. I t i s id e n tic a l in fo ra to th a t o f Figure I I I . The volume 01 base equ ivalen t to th e f re e n i t r i c ac id would be obtained by su b tra c tin g from th e to ta l t i t r e th e c a lc u la te d volume o f a l k a l i equ ivalen t to th e dichrom ate.

The g la ss e lec tro d e was found to behave norm ally in the presence o f potassium f lu o rid e and su ffe re d no d e tec tab le a t ta c k . The same se t o f e le c tro d e s was s t i l l perform ing w e ll a f t e r more than one hundred t i t r a t i o n s . I t was found, however, th a t f lu o r id e a tta c k on th e Pyrex beakers in which th e t i t r a t i o n s were ca rr ied out was s u f f ic ie n t to cause low r e s u l t s , even though no etching was v is ib le . Most o f th e t i t r a t i o n s were accord ing ly c a r r ie d ou t in p a r ra f f in - coated beakers o r in L ucite c o n ta in e rs .

The ea rly t i t r a t i o n s were c a rr ied out in 100 ml beakers a t a t o t a l so lu tio n volume, befo re t i t r a t i o n , o f 40-50 n l . Using 10 ml samples o f a stock solu­t io n 0.25 M in HNO3 and 0.7 M in UNH, th e amounts o f n i t r i c a c id found were

HW-146Q3- 6-

IIp. (Cont Td)

uniformly low by about 5*. This was attributed to co-precipitation o f acid, probably as HF or as KHF2 .

I t was found that precip itation from heated solution or digestion o f the p recip ita te did not appreciably decrease the lo s s o f acid. Sim ilarly, i t did not appear to matter whether the potassium fluoride was added as a solu­tion to the UHH - HNO3 so lu tion , whether a reverse strike was employed, or whether the KF^H^O cry sta ls were simply added slowly to the solution with constant motor stirr in g . The la tter method of addition was accordingly adopted as being the simpler and more convenient.

Increase of the volume before precipitation to 15&-20C ml (in 250 ml beakers) was found to give e sse n tia lly complete recovery of the n itr ic acid. At th is d ilu tio n there was apparently no co-precip itation of acid. At th is d ilu tion the end point occurred at a pH of 8.7 to 9 .2 , With 40-50 ml volumes the end point pH averaged about 10.6 (the same as the pH of 3A gms KF*2H20 dissolved in th e same volume of w ater). The shape of the curve was l i t t l e affected by d ilu tio n , although the break was somewhat more pronounced in the smaller volume.

Potassium hydroxide was employed in most of the t itra tio n s on the theory that the common ion effect would be helpful, however i t was found that sodium hydroxide gave identical r e su lts .

The method arrived at for the determination o f HNO in a macro sample o f IAF type solution was as fo llow s:

IV. HETHOD

Pipet 10 ml of sample into a paraffin-coated 250 ml beaker. Dilute to approx­imately 150 ml with d is t i l le d water And add 34 gms of KF*2H2C slowly with e f f ic ie n t motor stirr in g . After a l l the potassium fluoride has d issolved , lower the electrodes in to the solution and t i t r a t e with standard N/2 K0H (or HaOH). The t i t rant may be added rapidly u n t il the pH has increased by one pH unit and should then be added in 0.05 or 0 .1 ml increments u n til beyond the end point. Remove the electrodes from the solution immediately and rinse. Run in duplicate.

Compute the successive pH differences and ca lcu late the end point o f the t it r a t io n .-* I f dichromate i s present, ca lcu late the volume of standard alkali equivalent to the dichromate in the sample (as determined by an independent dichromate analysis). Subtract th is number from the to ta l volume of a lk a li used.

# HW S ta tis tic s Group formula:End Point = V ♦ f A V)(A2 - A l )

2 - (A 3)where ^2 the largest pH increase, and ^3 those inmediately preceding and following th is value, AV the increment o f titra iit added, and V the volun£ of t itra n t p reced in g ^ . / / 1

-7- HW-14603

IV. METHOD (Cont *d)

The potassium fluoride should be checked for free acidity or basicity by running a blank t i t ra t io n on the sane amount of KF in the presence of standard acid and the t i t r e compared with that of the acid alone.

V. RESULTS

A series of synthetic solutions were analyzed by the above method to obtain a measure of the accuracy to be expected. The results are presented in Table I and the data fur a typical t i tra tio n are shown in Table I I . The Mallinckrodt UMH used in preparing the ^oLuticrs contained 0 , 025% HNOj or 0.2 gA for a 1.7 M UNH solution. The numbers presented in the column headed Net HNO3 have accordingly been corrocted for the n itr ic acid in the UNH. The Addednumbers are the quantities of HND added in making up the solutions. The error in the analysis for UNO3 is shown both .in gA of HHOn and pareen*age-wise.The average of the magnitudes of the errors is seen to be O.li* gA or 0.8856.

Hexonc-saturated aqueous solutions wero found to behave normally. Hexene phase samples were t itra te d by pipetting an aliquot d irec tly into ca^ 140 ml of water, adding potassium fluoride and carrying out the t i t ra t io n with e ffi­cient s tirr in g . The titra tio n curves and the recovery of n itr ic acid did not d iffer appreciably from those obtained with hexone free solutions.

Ammonium n itra te and hydrazine were found to interfere with the n itric acid dsteradnrtion. This was apparently due to the buffer effect of ammonium ion and to the evolution of ammonia a t the higher pH*s. At an anmonium n itra te concentration of 8 M, no d istinct break was obtained in the titra tio n .

Although the method as presented was carried out on a macro scale with ''cold" solutions, there is no evident reason why i t could not be reduced to a micro scale for application to "hot4' solutions. The reaction could be carriod out in a modified "tea kettle" with pulser stirring and snail size Beckman elec­trodes used to follow the t it ra t io n . A 100X sample would resu lt in \ solu­tion volume of 2 ml. A micro burette could be u tilized for addition of the standard a lk a li. By proportionally reducing the increments of base added, the pH A*s should be identical to those obtained on a macro scale.

VI. REFERENCES

1. Clinton Analytical Methods, Clinton Central File Number Z.7-9-347.2. A. M. Ross, HW-9898.3. R. L. Moore, H. R. Schmidt, HW-3940, p. 1.4. R. L. Moore, Secret Notebook HSW-2106-T, p. 27-130.

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• • • s e t; • •• •

. 8 - HH-14603

TABLE I

POTASSIUM FLUORIDE titrations

Number HHO3

M

UNH

N

A1(H03)3

M

RsgCr^Oy

M

O.4508KNaOHml.

HKO3Added

g/1

lint* Pound HNOi

«AError

e/ i

Error*

1 0.* 1.7 - 10.55 30.96 ,30.96 0 02 0.25 1.7 • 5.40 15.95 15-76 -0.19 •1.19

• t • •* « 9

•••«3 0.25 1.7 - 5.51 15.95 16.08 ♦0.13 +0.814 0.25 1.7 1.0 - 5.46 15.95 15.93 -0.02 -0.13

• •5 0.25 0.85 - 5.45 15.95 16.00 40.05 ♦0.31

•• ♦ 6 0.25 0.3** - 5.44 15.95 16.03 40.08 ♦0.50

• •• • * 7 0.125 1.7 - 3.80 8.03 8.07 40.04 40.50• • • • 8 0.25 1.7 1.0 - 5.68 16.13 16.58 ♦0.45 42.70

9 0.25 1.7 1.0 - 5-56 16.13 16.22 40.09 +0.56• 10 0.25 1.7 0.5 - 5.55 16.13 16.19 +0.06 +0.38

• • • 11 0.25 0.8 1.0 - 5.03 16.13 16.53 ♦0.40 ♦2.48• • • •• • 12 0.25 1.7 0.1693 12.62 16.13 15.96 -0.17 -1.05

13 0.25 1.7 1.0 0.1693 12.63 15.13 15.99 -0.14 . -0.87Average ■ 0.14 0.88

# Corrected fo r HNO In UNH

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Ml. Q..W88 KOB

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• {JM» 7.98 IV3.*3 6.13 16 t .p5.30 8.38 285.33 8.9$ 305.60 8.66 22 :3 .63 9 .o e 103.70 9.183.60 9 2 9

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6 .00 9 3 97.00 9.308 .00 9,3*9 .00 9.36

10.00 9.53

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