Standardization of Sulfuric Acid against Sodium Carbonate E. W. BALIS, L. B. BRONK, H. A. LIEBHAFSKY, and H. G. PFEIFFER General Electric Research Laboratory, Schenectady,
N. Y.
is 3s ( 2 ) . The worst case in Table I is carbonate 3, where 3s is 0.04 or 1 part in 1200 for a single titration. The reliability of the mean of the five titrations for each carbonate is improved by a
Sulfuric acid, LY, has proved to be a satisfactory reference standard. Sodium carbonate is a reliable and convenient acidimetric standard. The difference between six cornniercial sodiuni carbonates is less than l part in 1000.
factor of 2.24, since sm =
2dG,Thus, the reliability
of the mean
becomes 1 part in 2700 in the worst case (carbonate 3) and is 1 part in 8000 in the best cases (carbonates 1 and 4).
I
S OIIDER to achieve a highly precise standardization of a
1 S sulfuric acid solution that has been in use in this laboratory as a reference standard since 1944, the acid was titrated in 1951 against each of 6.; of the heqt sodium carbonates available commercially.
RELIABILITY O F SODIUJI CARBOR-ATE
The standard deviation-.
E:XPERI31 ENTAL
Six different sodium carbonates of the highest quality obtainable were bought for this work. A portion of each, taken from a newly opened bottle, n-as heated in a weighing bottle t o constant, weight at 250" C., and stored in a desiccator. On the day of titration for a given carbonate, 5 grams were taken from the weighing bottle and used in the preparation by weight of about 100 grams of aqueous solution. Five separate 10-gram portions of this solution lvere then compared with the reference stand:irtl, 1N sulfuric acid. One drop of 0.1% methyl orange was added to the weighed portion of carbonate solution to serve as preliminary indicator. The reference standard was then added from a weight buret to pH 3, this end point being determined b y visual comparison to 3 standard buffer. After carbon dioxide had been expelled by boiling, and the flask washed down with freshly boiled distilled water, 2 drops of 0.1% bromothymol blue were added and the titration was finished volumetricaally with dilute sodium hydroxide. Each carbonate solution was titrated completely in one day to reduce any risk of errors due to c,fieniical changes. The mixed indicator gives a satisfactory warning that the end point is being approached, and a sharp end point at pH 7 . No end point correction Jyas applied. Calculation emploving the dissxhtion of bisulfate ion s h o w that the presence of this ion causes the results t o be low by less than 1 part in 100,000. The precision of the titrations was improved measurably hy adding the sulfuric acid to p1-T 3 in all titrations. Complete expulsion of carbon dioxide by Imiling is certain at this pII. Several preliminary sets of data in which the p H a t this stage was allowed t o vary were comparatively erratic. This euperience indicates the control to n.ithiri ahout r 0 . 2 unit of p H 3 is desirable. RESULTS
The evperimental results, all corrected to vacuum, are given in Table I. The grand mean for the table is 8 = 48.679 mg. of sodium carbonate per grain of reference standard, the unit used throughout Table I. I n Table I, XI to Xg are the results of individual titrations, 2 is the mean for each carhonate, and the standard deviation is
for the six 2 values iii Table I may be taken to measure the over-all reliability of sodium carbonate as an acidimetric standard. The value of s' is 0.3 part per 1000 and 3s' is 0.9 part per 1000. The conclusion is that the strength of sulfuric acid standardized as described by five replicate titrations against a high quality sodium carbonate may be guaranteed ( 2 ) to 1 part per 1000. Inasmuch as the sulfuric acid was standardized against six sodium carbonates of high quality, its strength can be guaranteed accordingly to 0 . 9 / ~ ' 0 ,or 0.1part per 1000.
Table I . Carbonate x 1
X2 X3 x 4
XS -
X S
Sunimarj- of Titration Results
1
2
48.667 48,658 48.659 48.663 48.668 48.663 0.0045
48.662 48.660 48.654 48.662 48.664 48 65SQ
3
4
48.703 48.693 48,679 48,693 48.708 48.690 48.688 48.682 48.708 48.692 48.697 48.690 0.005- 0 . 0 1 3 1 0.0045
5
6
48.689 48.694 48.684 48.678 48.674 48,684 0.0081
48.670 48. 679 48.685 48.681 48.686 48 680 0 0064
Carmod\- ( 1 ) used a titration against standard hydrochloric acid approximately 0.5AVto measure the purity of his carbonate; but distilled only two batches of acid. The reliability of constantboiling hydrochloric acid as a standard has been discussed previously ( 2 , 3). The sulfuric acid was standardized against 14 batches of constant-boiling acid over a 3-year period. The average normality obtained for the reference acid agreed with that from all the data in Table I to within less than 1 part in 25,000. This agreement is one order of magnitude better than the foregoing guarantees. The data of Table I suggest that measurable differences (less than one part in 1000) in equivalent weight exist among the six sodium carbonates used. Variance analysis of the results from an experimental design providing for complete randomization should reveal any such differences. LITERATURE CITED (1) Carmody, W. R., IND. ENG.CHEM.,ASAL.ED.,17, 577 (1945).
PRECISIOY OF TITR4TIONS
The precision attained in the titration of any one carbonate is indicated by the corresponding standard deviation, s. A conservative estimate of the reliability of any single titration
(2) Liebhafsky, H. A., Pfeiffer, H. G., and Balis, E. W., ~ A L CHEM.,23, 1531 (1951); 25, 677 (1953). (3) Williams, S., and Weiss, W., J . Assoc. Of.Agr. Chemists, 36, 354 (1953).
RECEIVED for rewew February 19. 1954.
1173
Accepted January 1 4 , 1955.
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