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The preparation of standard solutions of sulphuric acid by means of specific gravity determinations
VOL.
ANALYTICA
11.(x954)'
THE PREPARATION ACID BY MEANS
CHIMICA
AGTA
501
OF STANDARD SOLUTIONS OF SULPHURIC OF SPECIFIC GRAVITY DETERMINATIONS bY
R. E. ESSERY + The
Institute
of
Brewing
Research Laboratories, (England)
The
University,
Birmingham
INTRODUCTION
As far back as xgo5, WORDEPU AND MOTIONI published a table of specific gravities of dilute solutions of sulphuric acid, for use in preparing standard solutions. At Chemists2 advocates, the present time, The Association of Official Agricultural as an official m&hod, the preparation of standard solutions of sulphuric acid, by means of a formula relating the specific gravity of an approximately 70% sulphuric acid solution to its exact percentage composition by weight. This formula is dcrivcd from MAIXSEIALI.~and is claimed to be accurate to I part in 1500. MARSIIALL'S formula is itself based upon the density figures of PICKERING~. The acid of 70% strength used by the A.O.A.C. suffers from the disadvantage so that special precautions must be observed in that it is very hygroscopic, weighing it, and it is, moreover, unpleasant material to handle for a specific gravity dctcrmination. HAKE~ showed that the maximum absorption of moisture by concentrated sulphuric acid reduces it to 35.37% by weight, so that acid of this strength or near it should show little or no hygroscopicity, and could therefore be weighed easily in open vessels. ~~JARSZIALL'S fommla is inapplicable to acids 0, however, pointed out that the concentrations of of this concentration. GAEHK dilute solutions are often proportional to the logarithms of their densities, and a relationship of this type was found to hold very closely for concentrations of sulphuric acid lying between 32% and 39% by weight. At concentrations higher than 39% the equation was not satisfactory. PICKERING'S figures have been criticized by DOMKE’ on the ground that PICI~~ING employed a relative and not an absolute unit for his work, and that his percentages by weight are high by about o.2G”h of their value. It has also been pointed out in THORPE'S Dictiomry of A#$~Zied Clzentist~~ that although the claimed accuracy of YICKERING'S results is 8 parts per x,ooo,ooo, the eight analyses of his strong acid yield a mean of gg.84g5°/o with a range from gg.7o4o/o to gg.g62%,
* Present
Surrey,
adclress:
England.
References
p.
506.
-
The
Brewing
Industry
Research
Foundation,
Nutfield,
502
R, E.
ESSERY
VOL.
I¶
(1954)
or about r part in 400. The standard deviation is & 0.09043 and the standard error & 0.03199, i.e. about 320 parts per I,OOO;OOO. It seems difficult to reconcile this uncertainty with the claimed precision. The results of DOMKE’ do not appear to have attracted similar criticism, and have been relied on for the work here reported. EXPER1MENTAL
establish the regression-equation, the whole-number percentages by weight of DOMKE were transformed to pcrccntages by weight in air at zoo C, and his corresponding figures for density i~c vfzczco converted to specific gravities in air at 2o” C/20” C. From these calculated results the following rcgrcssionequation was dcrivcd by the method of least scluares: P = 343.61 (log,, S.G.) Jr 0.1875 (I) whcrc P is the percentage of sulphuric acid by weight in air at zoo C, and S.G. is the specific gravity in air at zoo C, referred to water at 20’ C. The percentages calculatccl from the specific gravities by means of this equation are given in Table I, where it will bc seen that they corrcspond very closely with those calculated from DOMIU’S figures, the difference in no case exceeding I part in xo,ooo. To
in
VUCZIO
TABLE SPECIPIC
GRAVITY
AND
---.---_----~__.-
S.G.
20”
C/20” C
in air, talc. from nOMMIZ
d
_. --.
I.2457 1.2541. I .2G25 1.2710 I.2795 I .2881
in
ale. from DOMKI?:
__.._-
I.2374
Ou w/w
I .2968
PXRCENTAGE BY SOLUTIONS
WISIGIiT
OF
‘$2
__._._ --_. ____--_-. __ . .._ - __.. - .___.____ 3 I sQ7.5
+
32.973 33.972 34.972 g:;;:
32.971 33.975 34.971 35.974 3G*QO8 37.967 38.97 1
_t + +
.___- -. ._____^_ _.---. Stanclarcf
SULPlIURIC
ACID
___._ .__-. -- --,_ _ -. .-_ .___. -_ __________ Diff. between DOMKE 6: Eq.
air at 20~ C Calc. from Equation (I)
3Jr -974
37.970 38.969
_._______
I
-.
__-_ ._. . .
Mean Deviation
Parts per 10.000 ^
O.OOI 0.002
0.3 0.G
0.003 0.001 0.003 0.003 0.003 0.002
0.9 0.3 0.8
-
Normality
._--_
0.8
0.8 0.5
. _. _ .- -_____ -_._- _-______
.“.__ 8.072 8.380 8.692 9.008 Q-327 %%I
Q-978
10.310
0.00025 -t_ 0.00250
This agreement is very satisfactory in view of the fact that according to the Xntcrnational Table of Atomic Weights for x95x, there is an uncertainty of rt_: 0.003 in the atomic weight of sulphur, due to differences in isotopic ratio, which leads to an uncertainty of I part in 33,000 in the equivalent weight of sulphuric acid. Refm+cnccs
p.
506.
VOL.
STANDARD
11 (195-Q
Rational
atomic
SOLUTIONS
OF
SULPHURIC
ACID
503
and eqzrivalcnt weights
Tt has been pointed out by SCHOORL~ that International atomic weights arc based upon weighings i?t WCW, while ordinary analytical weighings arc not in general corrcctcd for buoyancy, and that unless the latter arc so corrcctcd, “rational” atomic weights based upon air-weighings should be employed in gravimctric and volumetric conversion-factors. In some cases an error of as much as 0.1% can lx introduced in this way. For tile present work, the density of barium sulphate was taken as 4.50, and that of IOO~/~ sulphuric acid as 1.8305 at zoo C. Using the xg5r International atomic wcigllts, the rational molecular weight of sulphuric acid is 98.032 -& 0.003, and its rational equivalent weight is dg.orG 4 0.0015. On this basis, the air-weight W of sulphuric acid of PO/, by weight in air required to give I litrc of IN H,SO, is given by (2) and the normality .N of a solutioii prepared by diluting 11” grammes of Pi: acid to I litrc is given by (3).
11’ = If
49or .G ---F- - grammes
the International Jj’
-_
figures
!?!!e 1’
-***
(2)
N
are used, the equations . . (2a)
*
N=
=
PlV’
- ___. ..- _
49or.6 bccomc:
(3)
P Iv’ . . . . . . (3a) 4904*=
the difference being 5 parts in IO,WO. Similarly, the rational molecular weight of barium sulphatc is 233.397 f. 0.003 which yields a factor for converting air-weight of BaSO, to air-weight of HeSO, of 0.42002, differing by 4 parts in ~o,ooo from the International factor of 0.42018. In the present communication, rational molecular and equivalent weights have becn used throughout.
The accuracy of equation (I) was checked at four points between 32”/ and 39% of sulphuric acid, by preparing suitable dilutions of pure concentrated sulphuric acid with distilled water. KUNZLER~~ has described five methods for the preparation of very pure sulphuric acid of known strength for use as a primary standard. They are not attractive for ordinary USC,as they involve the preparation of constant boiling sulphuric acid. This boils at 330” C at I atmosphere, and requires the use of boiling tubes or rods (ScHo~~dl) to minimize bumping. As the impurities (other than water) in AnalaR concentrated sulphuric acid total 0.5 parts in xo,ooo, which is much smaller than the coefficient of variation of calibration of a burette or pipette, KUNZLER'S methods would seem to add unnecessary complications for all but the most precise work, and AnalaR sulphuric acid (S.G. 1.84) was used as the starting material for this study. References
fi. 506.
so4
R. E. ESSERY
VOL.
11
(1954)
On the prcparcd sulphuric acid solutions, specific gravities were determined by means of a specific gravity bottle at 20’ C/20” C, temperature being controlled to rl: 0.1~ C. Sulphuric acid was determined gravimctrically as &SO,. The compositions calculated from equation (I), together with those found, arc given in Table I I, whcrc each gravimctric result is the mean of five determinations.
_ ._-. . _
--
S.C.
ANALYSIS
20~ C/2b”‘C
in air
- -.-_._.__..- _-~ ‘-A r.25’5 I .27Ho H I .2‘-428 . - : _-- . . _----_-.I .2’).{4 _-- ----_.-. ..-.
01’
STOCK
‘%, wLzcin ‘E .
SULPlIUIlIC
air %
33.GG5
WIW
ACID
Fount1 Standard Error
3H.G92 .__.-_2....___._--_.A
--_.
Lk 0.0393
33.GS’)
36.792 37.352
SOLUTIONS . ---
0.0442 0.0252 -----_ - _----- 0.0449 -* 0.0405
36.778 :$zW& ----
- _--- Mean - _._.--__
. -_--
Diff. from talc. _“/. ..- -._ - 0.000 - 0.014 - 0.006 - -__..__.___ 0.002 -
Solutions A, I3 and ‘D were analysecl soon after preparation, but the gravimctric results for Solution C were not obtainccl until six months after preparation, the solution mcanwhilc being used for the hygroscopicity and storage tests dcscribcd below. The figures arc satisfactory. In no case does the diffcrcncc bctwccn tile calculatcd figure and that found exceed the standard error of the gravimctric mcthocl, while the mean standard deviation of the latter is & 0.09049, to bc compared with ~‘fCl~l~lilNG’S & 0.09043. HygroscoPicity
Solution C of Table II (prepared by mixing 720 ml of conccntratcd sulphuric acid with 2 litrcs of water) was tested for absorption of moisture, by leaving wcighcd quantities, near those rcquircd for I litrc of clccinonnal and I litrc of normal sulphuric acid rcspectivcly, in various containers on a balance-pan for twenty minutes, and measuring the incrcasc in weight during that period. Results are given in Tnblc III, from which it is seen that the increase in weight was ncgligiblc, even in open bcakcrs, so that no special precautions are required during weighing out. TABLE I.11 lIYGI
ygy;
-
.-. -.---_-
-- -----
Increase in wt. in
__.._____20 minutes
Conditions _..._- parts .-._ -.. per. . x0,000 __ __ __ ____ ____ _-_---.-----.-.-.-.-..._ -. ..__ _. . . _.mg -. -g Open IOO ml conical flask 14.x 0.7 0.5 Open xoo ml beaker 1.6 1.1 14.L 0.8 100 ml beaker covered by watchglass 0.G 14.1 Open 250 ml conical flask 0.07 128.0 0.9 2.2 Open 250 ml beaker 0.17 128.0 250 ml beaker covered watchglass 0.07 I 28.0 ___________-------_.--- _.-by _--.--_-__..- ----__---... 0.9 . ._.--_-_ Rcfcrcwccs
p. 506.
VOL. II (1954)
STANDARD
SOLUTIONS
OF
SULPHURIC
ACID
SJS
Stovagc The remainder of Solution C (approximately 2 litres) was stored on tbc laboratory shelf, in a screw-capped glass Winchester, the cap being fitted with a rubber washer. At intervals averae;ing TO days, over a period of G months, portions of 102 ml (approximately that required for I litre of IN H,SO,) were poured into a cylinder and rejected, the mouth of the Winchester being immediately wiped with a dry filter-paper and the screw-cap replaced and tightened, after each withdrawal. The final 200 ml portion was cxamincd for specific gravity and percentage of sulplnrric acid. The specific gravity at zoo C/zoo C was 1.283r. an incrcasc of 0.0003 in si.u months. This is probably due to extraction of material from the glass of the Winchcstcr, and would be accounted for by about 0.04O/~ of sodium silicate. As seen in Table II, the perccntagc of sulphuric acid found was in satisfactory abmcmcnt with that calculated six months previously from the specific gravity then found. Reconrntendcd
f~oced~c~e for the prefiavation
of stzndavd
sotzrtion
of sul$hcric
acid
Pour 360 ml of concentrated sulphuric acid (S.G. 1.84) of analytical rcagcnt purity into I litrc of distilled water slowly with constant stirring. Cool, transfer to a suitably stoppered bottle. Determine specific gravity at zoo C/zoo C, and from equation (I) calculate the percentage by weight in air. l3y means of equation (2) calculate the weight of this stock standard acid rcquircd to proclucc the required volume of standard acid of the required normality. A single specific gravity determination is adequate for ordinary work, as the standard deviation of five measurements by specific gravity bottle on the same solution was found to bc & 0.000036, which corresponds to an error of the order of I part in IO,OOO. The temperature should be closely controlled as, according to DOMKE’Sresults, the mean temperature correction between 32yo and 3g’yo of sulphuric acid is 0.00074 per degree centigrade, so that an error of 0.1’ C would lead to an error of the order of o.oog”/o in the concentration or 2 parts per IO,OOO. Five-figure logarithms arc used in equation (I) and the pcrcentngcs rounded off to the third decimal place. CONCLUSIOSS I. The percentage composition by weight of sulphuric acid solutions bctwccn 32o/0and 3g0/0 can be accurately computed from specific gravity determinations. 2. Such solutions can bc weighed in open vcsscls without precautions against absorption of moisture. They can be lcept in suitably capped bottles under ordinary conditions of use for at least six months. 3. The absence of hygroscopicity renders them superior to the 70”/~ acid used
Refevenccs
9.
5oG.
506 by
I<.
tilt A.O.A.C.
direct
weighing.
for the prcpxration
Ix.
VOL.
13SSI51<\'
of standard
solutions
11
of sulphuric
(1954)
acid by
SUMMARY
The strength of solutions of pure sulphuric acid between 32”!, and 3g”,, can be accurately calculated from the specific gravity at 20~ C/zoo C l! y a simple cquatiou. Such solutions can be kept for at least six months, non-hygroscopic! and arc suitable for the preparation of standard ticall of su Yphuric acicl by direct weighing.
by weight means of are pracsolutions
La force clcs solutions d’ncicle sulfuriquc de 32 ri 3o”A, (on oids) peut Otre calculGe avcc pr6cision LLUmoycn d’unc simple Gquation G partir cPe leur poids specifique h 2o” c/zoo C. Ccs solutions peuvent 6trc conscrvccs au moins six xnois; pratiqucellcs conviennent pour la prhparation de mcnt, cllcs ne sont pas liygroscopiqucs; solutious &talon d’aciclc sulfuriquc par pcs6e directc. ZUSAMMENFASSUNG Die Stiirke dcr SchwefelsiLurc zwischcn 3~ uncl 39% (in Gewicht) bercchnet werdcn mit Hilfe ciner cinfachen GIeichung auf Gruncl cles Gcwichtes bci 20’ C/zoo C. Dicse Losungen kiinnen mindcstens scchs bewahrt wcrclen; sie sind praktisch nicht hygroskopisch. Sie cigncn Herstcllung von Schwefels;iure Stanclartllijsungen durch direktes
kann priizis speziflschen Monate aufsich fiir die Wiigen.
REFERENCES 1 3
I<. C. WORDEN AND J. MOTION, J. Sec. Ckem. Ind., 24 (1go5) 178. Ass. of Official Agric. Chemists, Met/rods o/ Analysis, Washington, D.C., 7th Ed. (I 950) 765. A. MARSHALL, J. Sot. Cl&em. Id., 18 (x&g) 4, rogr. S. U. PICKIZIIING, J. Chom. SOL Trans., 57 (I Sgo) 64. I-I. W. HAKE, PYOC. C&m. SOL, 12 (1896) 33. 1’. I?. GAEJIR, PJays. Rev., 32 (19x1) 476. FViss. AbJ~awZG. dev I<. Normal-eicitwzgs I~omm., rgoq. Heft V, 22 I. J. DOMKlC, See also G. LUNGX AND C. A. E;EANE, TecJr. Methods o/ Chem. Analysis. Gurney nncl JacksonF London, 2nd Ed. (1924) Vol. I, 424. Green & Co. London, 4th ‘I’IIOI~PLZ’SDzctionary of .4fipZied CJrem., Longmans Ed., (rg54), Vol. XI, p. zgr. N. SCHOORL, %. asal. Cl&em., 57 (1918) 209. See also I. M. KOLTHOF~: AND V. A. STENGPH, T/oZrrmelvic A,zaZysis, Interscicncc Pub. Inc., N.Y., 2nd Ed., 1947, Vol. II, 35. J. E. KuN%L~I<, Anal. CJtem., 25 (1953) 93. I). L. SCHOLES, J. Am. CJiem. SW., 48 (i) (1g26) 1Go5. For nnothcr pattern, see I’. 13. HAWIC, B. L. 0s~~ AND W. I-l. Sunrrmmsoxv, PracticaC Pl~ysiological Clrenrislry, The Blakiston Co., Philaclelphia, U.S.A. 12th Ed., (1947) 499, 500. 1Zeccived
May
I 7th,
1954
ANALYTICA
11.(x954)'
THE PREPARATION ACID BY MEANS
CHIMICA
AGTA
501
OF STANDARD SOLUTIONS OF SULPHURIC OF SPECIFIC GRAVITY DETERMINATIONS bY
R. E. ESSERY + The
Institute
of
Brewing
Research Laboratories, (England)
The
University,
Birmingham
INTRODUCTION
As far back as xgo5, WORDEPU AND MOTIONI published a table of specific gravities of dilute solutions of sulphuric acid, for use in preparing standard solutions. At Chemists2 advocates, the present time, The Association of Official Agricultural as an official m&hod, the preparation of standard solutions of sulphuric acid, by means of a formula relating the specific gravity of an approximately 70% sulphuric acid solution to its exact percentage composition by weight. This formula is dcrivcd from MAIXSEIALI.~and is claimed to be accurate to I part in 1500. MARSIIALL'S formula is itself based upon the density figures of PICKERING~. The acid of 70% strength used by the A.O.A.C. suffers from the disadvantage so that special precautions must be observed in that it is very hygroscopic, weighing it, and it is, moreover, unpleasant material to handle for a specific gravity dctcrmination. HAKE~ showed that the maximum absorption of moisture by concentrated sulphuric acid reduces it to 35.37% by weight, so that acid of this strength or near it should show little or no hygroscopicity, and could therefore be weighed easily in open vessels. ~~JARSZIALL'S fommla is inapplicable to acids 0, however, pointed out that the concentrations of of this concentration. GAEHK dilute solutions are often proportional to the logarithms of their densities, and a relationship of this type was found to hold very closely for concentrations of sulphuric acid lying between 32% and 39% by weight. At concentrations higher than 39% the equation was not satisfactory. PICKERING'S figures have been criticized by DOMKE’ on the ground that PICI~~ING employed a relative and not an absolute unit for his work, and that his percentages by weight are high by about o.2G”h of their value. It has also been pointed out in THORPE'S Dictiomry of A#$~Zied Clzentist~~ that although the claimed accuracy of YICKERING'S results is 8 parts per x,ooo,ooo, the eight analyses of his strong acid yield a mean of gg.84g5°/o with a range from gg.7o4o/o to gg.g62%,
* Present
Surrey,
adclress:
England.
References
p.
506.
-
The
Brewing
Industry
Research
Foundation,
Nutfield,
502
R, E.
ESSERY
VOL.
I¶
(1954)
or about r part in 400. The standard deviation is & 0.09043 and the standard error & 0.03199, i.e. about 320 parts per I,OOO;OOO. It seems difficult to reconcile this uncertainty with the claimed precision. The results of DOMKE’ do not appear to have attracted similar criticism, and have been relied on for the work here reported. EXPER1MENTAL
establish the regression-equation, the whole-number percentages by weight of DOMKE were transformed to pcrccntages by weight in air at zoo C, and his corresponding figures for density i~c vfzczco converted to specific gravities in air at 2o” C/20” C. From these calculated results the following rcgrcssionequation was dcrivcd by the method of least scluares: P = 343.61 (log,, S.G.) Jr 0.1875 (I) whcrc P is the percentage of sulphuric acid by weight in air at zoo C, and S.G. is the specific gravity in air at zoo C, referred to water at 20’ C. The percentages calculatccl from the specific gravities by means of this equation are given in Table I, where it will bc seen that they corrcspond very closely with those calculated from DOMIU’S figures, the difference in no case exceeding I part in xo,ooo. To
in
VUCZIO
TABLE SPECIPIC
GRAVITY
AND
---.---_----~__.-
S.G.
20”
C/20” C
in air, talc. from nOMMIZ
d
_. --.
I.2457 1.2541. I .2G25 1.2710 I.2795 I .2881
in
ale. from DOMKI?:
__.._-
I.2374
Ou w/w
I .2968
PXRCENTAGE BY SOLUTIONS
WISIGIiT
OF
‘$2
__._._ --_. ____--_-. __ . .._ - __.. - .___.____ 3 I sQ7.5
+
32.973 33.972 34.972 g:;;:
32.971 33.975 34.971 35.974 3G*QO8 37.967 38.97 1
_t + +
.___- -. ._____^_ _.---. Stanclarcf
SULPlIURIC
ACID
___._ .__-. -- --,_ _ -. .-_ .___. -_ __________ Diff. between DOMKE 6: Eq.
air at 20~ C Calc. from Equation (I)
3Jr -974
37.970 38.969
_._______
I
-.
__-_ ._. . .
Mean Deviation
Parts per 10.000 ^
O.OOI 0.002
0.3 0.G
0.003 0.001 0.003 0.003 0.003 0.002
0.9 0.3 0.8
-
Normality
._--_
0.8
0.8 0.5
. _. _ .- -_____ -_._- _-______
.“.__ 8.072 8.380 8.692 9.008 Q-327 %%I
Q-978
10.310
0.00025 -t_ 0.00250
This agreement is very satisfactory in view of the fact that according to the Xntcrnational Table of Atomic Weights for x95x, there is an uncertainty of rt_: 0.003 in the atomic weight of sulphur, due to differences in isotopic ratio, which leads to an uncertainty of I part in 33,000 in the equivalent weight of sulphuric acid. Refm+cnccs
p.
506.
VOL.
STANDARD
11 (195-Q
Rational
atomic
SOLUTIONS
OF
SULPHURIC
ACID
503
and eqzrivalcnt weights
Tt has been pointed out by SCHOORL~ that International atomic weights arc based upon weighings i?t WCW, while ordinary analytical weighings arc not in general corrcctcd for buoyancy, and that unless the latter arc so corrcctcd, “rational” atomic weights based upon air-weighings should be employed in gravimctric and volumetric conversion-factors. In some cases an error of as much as 0.1% can lx introduced in this way. For tile present work, the density of barium sulphate was taken as 4.50, and that of IOO~/~ sulphuric acid as 1.8305 at zoo C. Using the xg5r International atomic wcigllts, the rational molecular weight of sulphuric acid is 98.032 -& 0.003, and its rational equivalent weight is dg.orG 4 0.0015. On this basis, the air-weight W of sulphuric acid of PO/, by weight in air required to give I litrc of IN H,SO, is given by (2) and the normality .N of a solutioii prepared by diluting 11” grammes of Pi: acid to I litrc is given by (3).
11’ = If
49or .G ---F- - grammes
the International Jj’
-_
figures
!?!!e 1’
-***
(2)
N
are used, the equations . . (2a)
*
N=
=
PlV’
- ___. ..- _
49or.6 bccomc:
(3)
P Iv’ . . . . . . (3a) 4904*=
the difference being 5 parts in IO,WO. Similarly, the rational molecular weight of barium sulphatc is 233.397 f. 0.003 which yields a factor for converting air-weight of BaSO, to air-weight of HeSO, of 0.42002, differing by 4 parts in ~o,ooo from the International factor of 0.42018. In the present communication, rational molecular and equivalent weights have becn used throughout.
The accuracy of equation (I) was checked at four points between 32”/ and 39% of sulphuric acid, by preparing suitable dilutions of pure concentrated sulphuric acid with distilled water. KUNZLER~~ has described five methods for the preparation of very pure sulphuric acid of known strength for use as a primary standard. They are not attractive for ordinary USC,as they involve the preparation of constant boiling sulphuric acid. This boils at 330” C at I atmosphere, and requires the use of boiling tubes or rods (ScHo~~dl) to minimize bumping. As the impurities (other than water) in AnalaR concentrated sulphuric acid total 0.5 parts in xo,ooo, which is much smaller than the coefficient of variation of calibration of a burette or pipette, KUNZLER'S methods would seem to add unnecessary complications for all but the most precise work, and AnalaR sulphuric acid (S.G. 1.84) was used as the starting material for this study. References
fi. 506.
so4
R. E. ESSERY
VOL.
11
(1954)
On the prcparcd sulphuric acid solutions, specific gravities were determined by means of a specific gravity bottle at 20’ C/20” C, temperature being controlled to rl: 0.1~ C. Sulphuric acid was determined gravimctrically as &SO,. The compositions calculated from equation (I), together with those found, arc given in Table I I, whcrc each gravimctric result is the mean of five determinations.
_ ._-. . _
--
S.C.
ANALYSIS
20~ C/2b”‘C
in air
- -.-_._.__..- _-~ ‘-A r.25’5 I .27Ho H I .2‘-428 . - : _-- . . _----_-.I .2’).{4 _-- ----_.-. ..-.
01’
STOCK
‘%, wLzcin ‘E .
SULPlIUIlIC
air %
33.GG5
WIW
ACID
Fount1 Standard Error
3H.G92 .__.-_2....___._--_.A
--_.
Lk 0.0393
33.GS’)
36.792 37.352
SOLUTIONS . ---
0.0442 0.0252 -----_ - _----- 0.0449 -* 0.0405
36.778 :$zW& ----
- _--- Mean - _._.--__
. -_--
Diff. from talc. _“/. ..- -._ - 0.000 - 0.014 - 0.006 - -__..__.___ 0.002 -
Solutions A, I3 and ‘D were analysecl soon after preparation, but the gravimctric results for Solution C were not obtainccl until six months after preparation, the solution mcanwhilc being used for the hygroscopicity and storage tests dcscribcd below. The figures arc satisfactory. In no case does the diffcrcncc bctwccn tile calculatcd figure and that found exceed the standard error of the gravimctric mcthocl, while the mean standard deviation of the latter is & 0.09049, to bc compared with ~‘fCl~l~lilNG’S & 0.09043. HygroscoPicity
Solution C of Table II (prepared by mixing 720 ml of conccntratcd sulphuric acid with 2 litrcs of water) was tested for absorption of moisture, by leaving wcighcd quantities, near those rcquircd for I litrc of clccinonnal and I litrc of normal sulphuric acid rcspectivcly, in various containers on a balance-pan for twenty minutes, and measuring the incrcasc in weight during that period. Results are given in Tnblc III, from which it is seen that the increase in weight was ncgligiblc, even in open bcakcrs, so that no special precautions are required during weighing out. TABLE I.11 lIYGI
ygy;
-
.-. -.---_-
-- -----
Increase in wt. in
__.._____20 minutes
Conditions _..._- parts .-._ -.. per. . x0,000 __ __ __ ____ ____ _-_---.-----.-.-.-.-..._ -. ..__ _. . . _.mg -. -g Open IOO ml conical flask 14.x 0.7 0.5 Open xoo ml beaker 1.6 1.1 14.L 0.8 100 ml beaker covered by watchglass 0.G 14.1 Open 250 ml conical flask 0.07 128.0 0.9 2.2 Open 250 ml beaker 0.17 128.0 250 ml beaker covered watchglass 0.07 I 28.0 ___________-------_.--- _.-by _--.--_-__..- ----__---... 0.9 . ._.--_-_ Rcfcrcwccs
p. 506.
VOL. II (1954)
STANDARD
SOLUTIONS
OF
SULPHURIC
ACID
SJS
Stovagc The remainder of Solution C (approximately 2 litres) was stored on tbc laboratory shelf, in a screw-capped glass Winchester, the cap being fitted with a rubber washer. At intervals averae;ing TO days, over a period of G months, portions of 102 ml (approximately that required for I litre of IN H,SO,) were poured into a cylinder and rejected, the mouth of the Winchester being immediately wiped with a dry filter-paper and the screw-cap replaced and tightened, after each withdrawal. The final 200 ml portion was cxamincd for specific gravity and percentage of sulplnrric acid. The specific gravity at zoo C/zoo C was 1.283r. an incrcasc of 0.0003 in si.u months. This is probably due to extraction of material from the glass of the Winchcstcr, and would be accounted for by about 0.04O/~ of sodium silicate. As seen in Table II, the perccntagc of sulphuric acid found was in satisfactory abmcmcnt with that calculated six months previously from the specific gravity then found. Reconrntendcd
f~oced~c~e for the prefiavation
of stzndavd
sotzrtion
of sul$hcric
acid
Pour 360 ml of concentrated sulphuric acid (S.G. 1.84) of analytical rcagcnt purity into I litrc of distilled water slowly with constant stirring. Cool, transfer to a suitably stoppered bottle. Determine specific gravity at zoo C/zoo C, and from equation (I) calculate the percentage by weight in air. l3y means of equation (2) calculate the weight of this stock standard acid rcquircd to proclucc the required volume of standard acid of the required normality. A single specific gravity determination is adequate for ordinary work, as the standard deviation of five measurements by specific gravity bottle on the same solution was found to bc & 0.000036, which corresponds to an error of the order of I part in IO,OOO. The temperature should be closely controlled as, according to DOMKE’Sresults, the mean temperature correction between 32yo and 3g’yo of sulphuric acid is 0.00074 per degree centigrade, so that an error of 0.1’ C would lead to an error of the order of o.oog”/o in the concentration or 2 parts per IO,OOO. Five-figure logarithms arc used in equation (I) and the pcrcentngcs rounded off to the third decimal place. CONCLUSIOSS I. The percentage composition by weight of sulphuric acid solutions bctwccn 32o/0and 3g0/0 can be accurately computed from specific gravity determinations. 2. Such solutions can bc weighed in open vcsscls without precautions against absorption of moisture. They can be lcept in suitably capped bottles under ordinary conditions of use for at least six months. 3. The absence of hygroscopicity renders them superior to the 70”/~ acid used
Refevenccs
9.
5oG.
506 by
I<.
tilt A.O.A.C.
direct
weighing.
for the prcpxration
Ix.
VOL.
13SSI51<\'
of standard
solutions
11
of sulphuric
(1954)
acid by
SUMMARY
The strength of solutions of pure sulphuric acid between 32”!, and 3g”,, can be accurately calculated from the specific gravity at 20~ C/zoo C l! y a simple cquatiou. Such solutions can be kept for at least six months, non-hygroscopic! and arc suitable for the preparation of standard ticall of su Yphuric acicl by direct weighing.
by weight means of are pracsolutions
La force clcs solutions d’ncicle sulfuriquc de 32 ri 3o”A, (on oids) peut Otre calculGe avcc pr6cision LLUmoycn d’unc simple Gquation G partir cPe leur poids specifique h 2o” c/zoo C. Ccs solutions peuvent 6trc conscrvccs au moins six xnois; pratiqucellcs conviennent pour la prhparation de mcnt, cllcs ne sont pas liygroscopiqucs; solutious &talon d’aciclc sulfuriquc par pcs6e directc. ZUSAMMENFASSUNG Die Stiirke dcr SchwefelsiLurc zwischcn 3~ uncl 39% (in Gewicht) bercchnet werdcn mit Hilfe ciner cinfachen GIeichung auf Gruncl cles Gcwichtes bci 20’ C/zoo C. Dicse Losungen kiinnen mindcstens scchs bewahrt wcrclen; sie sind praktisch nicht hygroskopisch. Sie cigncn Herstcllung von Schwefels;iure Stanclartllijsungen durch direktes
kann priizis speziflschen Monate aufsich fiir die Wiigen.
REFERENCES 1 3
I<. C. WORDEN AND J. MOTION, J. Sec. Ckem. Ind., 24 (1go5) 178. Ass. of Official Agric. Chemists, Met/rods o/ Analysis, Washington, D.C., 7th Ed. (I 950) 765. A. MARSHALL, J. Sot. Cl&em. Id., 18 (x&g) 4, rogr. S. U. PICKIZIIING, J. Chom. SOL Trans., 57 (I Sgo) 64. I-I. W. HAKE, PYOC. C&m. SOL, 12 (1896) 33. 1’. I?. GAEJIR, PJays. Rev., 32 (19x1) 476. FViss. AbJ~awZG. dev I<. Normal-eicitwzgs I~omm., rgoq. Heft V, 22 I. J. DOMKlC, See also G. LUNGX AND C. A. E;EANE, TecJr. Methods o/ Chem. Analysis. Gurney nncl JacksonF London, 2nd Ed. (1924) Vol. I, 424. Green & Co. London, 4th ‘I’IIOI~PLZ’SDzctionary of .4fipZied CJrem., Longmans Ed., (rg54), Vol. XI, p. zgr. N. SCHOORL, %. asal. Cl&em., 57 (1918) 209. See also I. M. KOLTHOF~: AND V. A. STENGPH, T/oZrrmelvic A,zaZysis, Interscicncc Pub. Inc., N.Y., 2nd Ed., 1947, Vol. II, 35. J. E. KuN%L~I<, Anal. CJtem., 25 (1953) 93. I). L. SCHOLES, J. Am. CJiem. SW., 48 (i) (1g26) 1Go5. For nnothcr pattern, see I’. 13. HAWIC, B. L. 0s~~ AND W. I-l. Sunrrmmsoxv, PracticaC Pl~ysiological Clrenrislry, The Blakiston Co., Philaclelphia, U.S.A. 12th Ed., (1947) 499, 500. 1Zeccived
May
I 7th,
1954