Consumption of Base by Glassware Allen A. Smith Widener University, Chester, PA 19013

The effect of strong bases on glass is well known. The reciprocal effect of glass on standardized solutions of base is rarely considered. The few writers who have considered the problem have either assumed that base consumption is pro- portional to glass consumption (1,2) or confined themselves to a qualitative comment (3,4).

Several analytical techniques depend on the measurement of hase consumed by a hot solution of the analyte.

Saponification equivalents are determined from the base consumed by the ester during prolonged refluxing (5-7).

Mohr (8) introduced an indirect method for the quantita- tive analysis of ammonium ion in which the sample is react- ed with a known quantity of strong base, the resultant am- monia boiled off, and the remaining base determined by titration. Bradstreet (9) commends the method's simplicity and suggests its adaptation to the determination of organic nitrogen but notes that it gives estimates 1-2% lower than direct titration of the captured ammonia. Other modern authors (10, 11), including those of two popular laboratory manuals (12,13), give the method without comment.

Bezier (14) recommends adding an excess of hase, boiling to break up complexes in the precipitate, and titrating the excess base hack to pH 7 in the alkalimetric analysis of zirconium ion.

Determining the concentrations of carbonate and hydrox- ide in lye by titration before and after heating with barium chloride is sometimes offered as a laboratory exercise (12).

To determine how much of the base in these reactions might he consumed by the glassware rather than the analyte, four 25-ml samples each of several concentrations of base standardized to a bromthymol blue endpoint against appro- priate dilutions of hydrochloric acid, were simmered gently for one hour in horosilicate glass Erlenmeyer flasks, and titrated with the same hydrochloric acid and indicator. To ensure that the loss of base was due to reaction with the glass rather than to spatter, the one-hour simmering was repeated and the silicate and borate concentrations in the base solu- tions were measured. Silicate was measured by reading the color developed with acidic ammonium molyhdate a t 400 nm (15). Borate was measured by reading the color developed with carmine in sulfuric acid a t 585 nm (16). The flasks were also weighed before and after simmering. The results are shown in Table 1; values in the first three columns are for Kimax flasks; values in the last column are for a Pyrex flask.

The effect of boiling on the titrahle base is much less than would be expected from the quantity of silicate and borate produced. Silicates and horates, as salts of weak acids, are basic and consume some acid before reaching the turning point of bromthymol blue. Using an indicator with a more acid turning point should decrease the loss of titrable base. Soluble silicates polymerize to colloidal silica gels when neu- tralized, and one must wait several minutes for desorption of ions from the silica gel when using an acid range indicator. Alkali metal and alkaline earth oxides released from the glass may add to titrahle base (3, 17) when an acid range indicator is used.

0.5Malcoholic KOH consumes less glass than 0.1 Maque- ous NaOH does, probably because ethanol boils a t a lower temperature. The tendency of alcoholic solutions of silicate to hydrolyze rapidly to polysilicate and hydroxide (18) mini- mizes the effect of the dissolved glass on the titrable base.

The Qlo for the reaction of base with glassware is appar-

Table 1. Effects of Glass on Standardized Solutions of Basea

Kimax Kimax Kimax Pyrex

0.4956 Methanolie KOH moisrity 0.4954 change in -0.04%

molarity peq lost 5 silica (as 2.59 mg

SO2) borate (as 0.53 mg

8203) wt. lost 3.92 mg

by flask

0.1 116 Maqueous NaOH maiarity 0.1078 change in -3.4%

molarity peq iost 95 silica (as 6.45 mg

SiOd borate (as 1.69 mg

8203) wt. lost 9.07 mg

by flask

0.01081 Maqueous NaOH malarity 0.00956 change in -11.6%

molarity uea iost 31 . . silicate (as 1.72 mg

SiOd borate (as 0.50 mg

820,) wt lost 2.65 mg

by flask

0.001148 Maqueaus NaOH molarity 0.000955 change in -16.6%

molarity peq lost 5 silica (as 0.34 mg

SOl) borate (as 0.13 mg

8203) wi. bst 0.73 mg

by flask

(2.04 mg) 1.84 mg

(10.13 mg) 9.60 mg

0.000953 0.000970 0.000944 -17.0% -15.5% -17.8%

5 4 5 0.29 mg (0.46 mg) 0.34 mg

0.11 mg (0.13mg) 0.11 mg

0.60 mg (0.98 mg) 0.75 mg

dl~tilled water silica (as 0.04 mg

SO2) borate (as 0

E'20,) wi. lost 0.13mp

by flask

s v a ~ ~ ~ s m @rmrnJes are h a o nsrsnt I arrs man tne other valber m mat colmn ~.tfsrences oetweon f m k s expowa to ms same treatmem orobaoy amm trom dlHer- sncn nine h ,101 BS 01 me I a m

ently large, since one week of storage in glass a t room tem- perature changes the average molarity of aqueous hase by only 0.2% (Table 2).

Literature Clted Ill Wichers, E.. Finn, A. N.. and Clabaugh, W. S., Ind. Eng. Chsm. A n d . Ed., 13, 419

11941).

Volume 63 Number 1 January 1986 85

Table 2. Effects of Glass on a Base Solution after One Week (5) Chemnis. N. o. .~ntrikin, ~ . ~ . , a n d ~ o d n ~ d n t t , ~ . M., ' " ~ ~ ~ i ~ i ~ ~ ~ ~ i i t ~ t i ~ ~ organic Analysis." 3rd ed., Wiley-Interscience, New York, 1965, pp. 111&976.

Kimax Kimax Kimax FF3x (61 Vogel, A. I., '"Practical Organic Chemistry." 3rd ed., Longmans, London, 1957, pp. 392-393.

(7) Shriner, R. L.,Fuson. R. C.. Curtin, D. Y.,snd Morrill,T. C., '"Systematicidentifica- 0.1029 Maqueous NaOH: 1 w e e k at 25% tion of Organic Compounds." 6thed.. Wiley, New York. 1980, pp. 297.298.

moiarity 0.1027 0.1027 0.1028 0.1026 (8) Szabadvh. F., "History of AnaiytiealChemiatJY." Pwamon, Oxford, 1966.p. 247. c h a n g e i n -0.20% -0.19% -0.12% -0.29% (9) Bradstreet, R. B., "The Kjddahl Method for Organic Nitrogen." Academic Presa,

New Ymk, 1965,pp. 6,158. molarity (10) Williams. A. F., in "Comprehensive Aoal*ical Chemistry." (Editors: Wilaon. C. L.,

peq lost 5 5 3 7 and Wilson, D. W.), Elsevier, Amaterdsm. 1962, uol. IC , p. 203. silica (as 0.44 m g 0.38 m g 0.40 m g 0.41 m g (11) C1ear.A. J., snd Roth, M., in "T7eatise on Analytical ChamistJY." 1Fditora:Kolthoff.

I. M.,snd E1vinp.P. J.), Wiley-lnter8cience. New York. 1961, part. II,val. 5,p. 282. SiOd (12) Vogei,A.I.;'QuantitatlvelnorgsnieAnal~ia,"2nded..Lanpmans.London.L951,pp.

wt. lost 0.85 mg 0.73 m g 0.82 mg 0.82 m g 246218. by flask (13) RoheM, J. L.. and Eft, J . B., "Frentz/Maim'8 Chemical Principles in the Labmato-

ry? 2nd ed.. Freeman. San Francism. 1977, pp. 267-268. (14) Berier,D.,Chim. And.,36,175 (1954). (15) Knudaan, H. W., Juday, C., and Meloehe, V. W., Ind. Eng. Chem. A d . Ed., 12,270

(2) Koltholf. I. M.. Sandell. E. B., Meehsn. E. J.. and Bruckenstein, S.. "Qusntifetiw (1940). Chemical Analysis," 4th ed., MscMiilan, London, 1969, pp. 452-454. (16) Furman, N. H., (Editor), "Standard Methods of Chemical Analysis," 6th ed., Vsn

(3) Wyatt,G. H.,in~'ComprehensiveAnaiyficd Chemistry." (Editors: W i h n , C.L..and Natrsnd. Princeton. 1962 Vol I. p 217. Wi1~on.D. M.),Eiseuier,Amsterdam, 1962,voi.IA,pp, 13-22. (17) Dean, R. B., "MadernCoiloid%"Van Nastrsnd, New York, 1948, p. 250.

(4) Ayers. C., in "Comprehensive Analyficsi Chemi8try." (Editors: Wilson, C. L., and (18) Iier, R. K., "Colloid Chemistry of Silica snd Silicates." Cornell Uoiv. Prsu, Ithaea, Wilson, D. M.), Elsevier, Amsterdam. 1962, voi. I B, p. 219. 1955, p 110.

88 Journal of Chemical Education