Acidimetric Titration of Inorganic Salts in Glacial Acetic Acid CHARLES W. PIFER AND ERNEST G. WOLLISH Products Control Laboratory, Hoffmann-La Roche Inc., i\’utley, Little information concerning titration of illorganic salts in nonaqueous sohitioris has been available. Inorganic salts have been found to be acidic, neutral, or basic when dissolved in glacial acetic acid. i method for the determination of this behavior is desrrihetl. >\Ianp cations of basic inorganic salts can be titrated potentiometrically or .risually using perchloric acid i n p-dioxane as titrant. Salts containing chloride, bromide, or iodide anions are acidic in glacial acetic acid. These are converted to basic salts by the addition of mercuric acetate, and thus are titratable with perchloric acid. The proposed method permits the direct titration of a great number of inorganic salts (if soliible in glacial acetic acid).
T
I-IE behavior of a salt respondiuy aa a n acidic, neutral, or basic compound in any given solvent is governed by both the cation and the anion forming the compound. Kolthoff and \\-illinan (3, 4)investigated this property of cations and anions, recording the following. Inorganic Cations, Having the Same Anion. “The acidity of inorganic cations increases with decreasing size and increasing charge of the ion.. . Salts having a common anion show decreasing acidity in the order ;Ilg+f > c a f + > S r + + > B a t + ( > Ag+) > Li+ > X a + > Y H 4 + = K + > Rb+. “Since the sizes of the ammonium and potassium ions are about the same, potassium and ammonium acetates in acetic acid behave like bases of approximately the same strength.. . The dissociation of the monovalent inorganic acetates in acetic acid decreases with decreasing ionic size, the order being Cs+ > K + > S a + > Li+. “The dissociation of the alkaline earth acetates decreases also with decreasing ionic size, the order being B a + + > S r T + > Ca++ > Mg++.” Inorganic Anions Combined with the Same Cation. “Salts which do not change the reaction of water undergo solvolysis in acetic acid. The acidity of the potassium salts of the following inorganic acids decreases in the order Clod- > I- > Br- > CI- > NOI-, potassium nitrate being more alkaline than acetic acid.”
Y .J.
If the eye opens, the compound is considered basic in glacial acetic acid. The degree of basicity can again be measured by recording the voltage difference required to restandardize the eye. EXAMPLE.If a 0.1 Jf solution of sodium chloride in glacial acetic acid is placed in contact with the standardized eye, the edges of the eye will overlap, indicating that sodium chloride is acidic in that medium. If a solution of mercuric acetate in glacial acetic acid (which has been found to cause no deflection of the standardized eye and theretore can be considered neutral) is added to the first solution, the folloning reaction u-ill occur: 2SaCl acidic
IIXC + HgAcr -+ HgC12 + 2Na+ ntutral
neutral
basic
The eye nil1 open iinmetliately, indicating the presence of the basic compound sodium acetate. Discussion. Within the aqueous solvent system the strongest acidic cation, the hydrogen ion (H)+, is combined with the strongest basic anion, the hydroxyl ion (OH)-, to form the neutral compound water. Lihewise, in the acetic acid solvent system the strongest acidic cation, the hydrogen ion ( H ) +, is combined with the strongest basic anion, the acetate ion (CZHaOi)-, to form the neutral compound hydrogen acetate. Thus, all compounds in the acetic acid system, as in the water system, will exhibit acid, neutral, or baqic properties, which are influenced by both the cations and anions. Influence of Cations. Cations exhibit different properties in glacial acetic acid medium. Compound Glacial acetic acid Alercurlo acetate Potassium acetate
Relative Behavior H +CaHaOiNeutral Hg * +(C?HsOdz- ’ Neutral K +GHsOnBasic
Influence of Anions. Salts of the same cation-other than hydrogen, mercury, and copper, for example-with varied anions will exhibit different properties in glacial acetic acid medium. Compound Potassium perchlorate Potassium bromide Potassium bisulfate Potassium chloride Potassium iodide Potassiiim phosphate Potassium nitrate Potassium organic salt Potassium acetate
In order to determine whether an inorganic or organic salt exhibits acidic, neutral, or basic properties in glacial acetic acid, a qualitative procedure wvas developed. A Fisher Senior Titrimeter equipped with calomel-glass electrodes was used for this purpose.
Relative Behavior
Examples of acidic anions are perchloIates, chlorides, bromides, iodides, ferrocyanides, ferricyanides, bisulfates, and persulfates. Inorganic salts of carbovylic acids have previously been titrated in nonaqueous medium (5,6‘). Salts, containing the acidic halide anions (chlorides, bromides, or iodides) can be converted to basic salts by removal of the halogen (1, 2, 7’) and thus can be made available for titration with perchloric acid. Acidimetric Titration of Inorganic Salts. Apparatus and reagents have been outlined ( 7 ) .
PROCEDURE
Standardization of Electronic Eye. Glacial acetic acid is used t o standardize the instrument and the electrodes a t 0.5 volt. The eye is adjusted to a very slightly opened position by regulation of the eye control. Measurement of Relative Acidic, Neutral, or Basic Property of a Compound. The compound to be analyzed is dissolved in sufficient glacial acetic acid to result in a 0.1 M solution. After the instrument and the electrodes have been standardized, the glacial acetic acid, which has been used for the standardization, is replaced by a 0.1 M solution of the sample. If no change occurs within the eye in the standardized position, the compound is considered neutral in comparison to glacial acetic acid. If the edges of the eye overlap, the compound is considered acidic in relation to glacial acetic acid. The relative degree of acidity can be measured by recording the voltage difference required to restandardize the eye.
PROCEDURE. The salt to be titrated is finely ground in a mortar and passed through a 100-mesh sieve. A sample of approximately 3.5 me. is weighed into a 250-ml. beaker, 80 ml. of glacial acetic acid are added and, if necessary, the mixture is heated to effect complete solution. If the sample is difficult to solubilize, as in the case of potassium sulfate, it is passed through a 200-mesh sieve, up to 5 ml. of water may be added to dissolve the sample prior to addition of the glacial acetic acid, and the titration is 519
ANALYTICAL CHEMISTRY
520 Table I.
Salts Assayed by Titration with 0.1 N Perchloric Acid
Table 111. .4ccuracy and Precision Perchloric Acid Titration, Yo
(Commercial reagent grade) Purity Found,
Salt (Cation Na) NaCzHs01 NaNs NaHCOa NaHSOr NazBzOi NaBrO3 NaBr NazCOI NaClOa NaCl NaCX NaF Na0H NsH1POZ.HaO NaIOs NaI NazMo04.2H10 NaNOz NaNOs Na202 NaH2POi NazHPOi NasPOi NalSiOi NazSOd NaaS.9HnO NatSOa NaSCX NazW04.2K10
Aluminum iimmonium Antimony Barium Bismuth
Anion of Snlt Acetate Azide Bicarbonate Bisulfite Borate Bromate Bromide Carbonate Chlorate Chloride Cvanide Fiuoride Hydroxide Hypophosphite Iodate Iodide Molybdate Nitrite Nitrate Peroxide Phosphate, primary Phosphate, secondary Phosphate, tertiary Silicate Sulfate Sulfide Sulfite Thiocyanate Tungstate
% 100.0 100.0 100.1 99.4 99.8 100.0 100.0 99.9 99.9 99.9 95.7 96.6 98.2 99.8 100.0 100.0 99.9 100.0 100.0 97.6 99.8 99.9 99.9 99.8 100.0 99.9 98.8 99.4 99.9
Table 11. Other Cations Cadmium Calcium Cobaltous Iron Lead hlagneaium
Labeled Purity,
7c
..
.. .. , .
..
..
..
..
.. 95.9 96.2 98.2
Sodium chloride A
Chloride Determination,
B
99.9 99.9 100.0
99.8 100.0 99.9 d c i d Titration
B
99.9 99.9 100.0
99.9 99.7 99.9
C Sodium carbonate 4 C
Precipitation as Bas04
Sodium sulfate A B C
100,o 99.9 100.0
99.6 99.4 99.7
..
.. ,
I
100.0 97.6
.. ..
..
Salts composed of the cations listed in Table I1 with any of the anions in Table I may also be successfully titrated in the manner described, provided they can be solubilized and react basic in glacial acetic acid.
, .
ACCURACY AND PRECISION 98.8
.. ..
Manganous Potassium Silver Strontium Zinc
performed potentiometrically. It was thus possible to dissolve all salts listed in Table I. If the anion is a chloride, bromide, or iodide, 10 ml. of mercuric acetate reagent are added and the solution is titrated with 0.1 N perchloric acid in p-dioxane, either potentiometrically or using crystal violet as indicator. Several compounds, such as those that react xith the indicator, must be titrated potentiometrically.
Accuracy and precision of titrations in nonaqueous solutions, using perchloric acid, have been evaluated ( 6 , 7 )and a reproducibility of ~k0.275or better has been reported for a majority of compounds. This is also borne out by results obtained in triplicate determinations of three different sodium salts, both by titration with perchloric acid and by conventional methods (Table 111). LITERATURE CITED
(1) Higuchi, Takeru, and Concha, Jesusa, J . Am. Phurm. Assoc., 40, 172 (1951). (2) Higuchi, Takeru, and Concha, Jesusa, Science, 113, 210 (1951). (3) Kolthoff, I. M., and Willman, A., J. Am. Chem. SOC.,56, 1007 (1934). (4) Ibid., p. 1014. (5) Markunas, P. C., and Riddick, J. A., ANAL. CHEW,23, 337 (1951). (6) Palit, S. R., IXD. ENG.CHEM.,ANAL.ED.,18, 246 (1946). (7) Pifer, C. W., and Wollish, E. G., ANAL.CHEM.,24, 300 (19323. RLCEIYED for reriew August 28, 1951.
Accepted December 6, 1951.
Determination of Small Amounts of Molecular Oxygen in Gases and liquids FRANCIS R. BROOKS, MARTIN DIMBAT, RICHARD S. TRESEDER, AND LOUIS LYKKEN
Shell Development Co., Emeryville, Calif.
I
N T H E petroleum and related industries the need frequently arises for a reliable method for determining dissolved oxygen in both aqueous and nonaqueous liquids. A knowledge of the dissolved oxygen content of liquids such as gasoline, kerosene, lubricating oil, solvents, extracting solutions, and treating solutions is often of value in the solution of both plant and laboratory problems involving corrosion, emulsions, and storage stability of refined products, A review of the literature reveals that little consideration has been given to this analytical problem. The method proposed by Schulze, Lyon, and Morris (6) for application to certain liquid hydrocarbons is a modification of the fundamental Winkler method (8) used in water analysis. It consists of bringing the hydrocarbon sample in contact with an aqueous reagent containing manganous hydroxide and, subsequently, measuring the degree of
oxidation of the reagent by iodometric titration. This method is subject t o serious error if the material analyzed contains reducing agents or oxidizing agents other than dissolved oxygen. The materials to be analyzed frequently contain appreciable quantities of substances, such as organic peroxides and mercaptans (thiols), which would interfere with the method. The same limitation applies t o other methods of this general type, which involve measuring the oxidizing power of the sample by mixing it with a standard solution of a reducing agent. MacHattie and Maconachie (5) devised a method for the determination of small amounts of oxygen in gases, whereby the gas sample was passed through a tube containing copper wire wetted with ammoniacal ammonium chloride solution and the resulting copper oxide was removed from the reaction tube by washing the copper further with ammoniacal ammonium chloride solution.
