containing less than O . O l ~ o water. Figure 2 shows the rate of hydrochlorination of 3,3-bis(chloromethyl)oxetane obtained with it. The advantage of this reagent over aqueous hydrochloric acid plus pyridine or dry pyridinium chloride in pyridine-chloroform (1 to 1) appears to be a combination of low water content and relatively high boiling point. Figure 3 shows the effect of raising the temperature through the use of sealed glass tubes heated in a n aluminum block. Water may affect the reaction by competing with pyridinium chloride as a ring-opening reagent to form a 1,3diol, which would then react very sloaly to form the desired 1,3-chlorohydrin. With compounds having easily hydrolyzed substituents, the anhydrous reagent has obvious advantages. DISCUSSION
Table I1 shows the results obtained using Procedure A on bis(chloromethy1)oxetane of high purity. Table I11 shows results obtained on nine other oxetanes of high purity. The results are reasonable, since the compounds were known to contain small amounts of impurities. Table IT’ shous the effect of temperature on the rate of reaction of 3,3-bis(cliloromethyl)oxetane with dry pyridinium chloride in pyridine reagent. The elimination of water in the reagent lessens the reaction time and minimizes hydrolysis problems with esters. JTater acts as a n inhibitor to the hydrohalogenation reaction if present in more than small amounts ( > O . l % ) . Low results are obtained if the condenser and joint are rinsed with water nhile the solution is still warm. This may be due to hydrolysis of the 1,3c*lilorohydrin to the corresponding diol.
Table IV.
Effect of Temperature on Rate of Hydrochlorination of
3,3-Bis(chloromethyl)oxetane
Temp., ’ C.
Time, Hour
Oxetane Found, %
100 115 150 li0
1 1 1 0.5
170
0.25
14.6,15.2 34.1,33.8 35.6,35.8 36.0,35,9 35.9,36.1 34.8,33,6
Phenols and alcohols interfere slightly and should be removed if present in large amounts. Most oxiranes interfere quantitatively. Esters do not interfere. hromatic and noncyclic ethers probably do not interfere, since oxetane compounds containing ether linkages in substituent groups give essentially theoretical results (Table 111). Olefins such as oleic acid and 3-chloro-2chloromethyl-1-propene do not add hydrogen chloride during the reaction. The latter compound does precipitate as a quaternary ammonium salt even when heated with pyridine alone; however, the salt dissolves later when diluted and does not disturb end point detection. I n Procedure B, considerable degradation of pyridine takes place unless the air is swept out with nitrogen before sealing.
Condition Sealed tube Reflux Sealed tube Sealed tube Sealed tube
culty with some resins is the rather rapid saponification during back-titration with base. This difficulty u n s eliminated through the use of sodiuni methoxide in benzene-methanol (8 to 1) as titrant ( 2 ) . The pyridine x i s diluted with benzene instead of n ater, and thymol blue was used as indicator. This system gave satisfactory end points and can also be used to measure the amount of acid catalyst in a re,‘Qlli so that a correction can be made. ACKNOWLEDGMENT
The author would like to express his appreciation to W.D. Coder, Jr., A . S.Matlack, and S.F. Dieclimann for supplying thp ovetanes for thi- study.
OTHER APPLICATIONS
The method has been applied to many oxetane polyester and polyamide resins by following cross linking through the decrease in oxetane ring content. In general, the procedure worked satisfactorily until very highly cross-linked resins were encountered. X a n y of these were insoluble in pyridine as well as in other solvents tested. On such materials. the method fails. Another diffi-
LITERATURE CITED
(1) Farthing, A. C., Reynolds, R. J. K., J . Polymer Sci. 12, 503 (1954). ( 2 ) Fritz. J. S.. Lisicki, S . ll.,.ls.i~. CHEST.23,‘ 589 (1951). (3) Jungnickel, J. L., Peters, E. I)., E’ol\
,
gar, A., Weiss, F. T., “Determination of the epoxy Group” in “Organic Analysis,” 5‘01. 1, pp. 12T54, Interscience, New Tork, 1053. (4) Searles, S., others, J . Am. C h e m Soc.
73, 124, 4515 (1951); 75, 1175 (1953); 76, 56, 2313, 2780 (1054).
RECEIVEDfor review Yovemher 2 7 , 1956. Accepted February 8, 1957. Delaware Chemical Symposium, Xewark, Del., Fehruary 16, 1957.
Volumetric Determination of Potassium E. D. SCHALL Department of Biochemistry, Purdue University, lafayette, Ind. ,The volumetric method described for the determination of potassium compares favorably with the flame photometric method in speed and with gravimetric procedures in accuracy. Potassium in the sample solution is precipitated with an excess of a standard solution of sodium tetraphenylborate; the excess is titrated with a standard solution of a quaternary ammonium salt in the presence of bromophenol blue as an indicator.
1044
ANALYTICAL CHEMISTRY
T
of the tetraphenylborate complex and of the unusual properties of its potassium salt by Wittig and coworkers (7-9) has led to the development of several novel methods of determining potassium during the past 5 years. Many of these procedures were reviewed by Gloss (5) in 1953 and were included in the comprehensive bibliography prepared recently by Barnard (5). The precision and accuracy of methHE DISCOVERY
ods for determining potassium utilizing this unusual reagent have been n-ell established, I n general, these methods also have an advantage in speed over the conventional gravimetric procedures but do not compare with the flame photometer in this respect. This is due primarily to the physical characteristics of the precipitate, which is h e and often flocculent in nature, tends to stick to the sides of the vessel, and is difficult to transfer and mash.
These difficulties have been circumvented by eliminating the necessity of transferring or recovering the precipitate. 111 the method described below, the potdlisium in a sample is precipitated R-ith a standard sodium tetraphenylborate Folution. A slight excess of the reagent is added to ensure complete precipitntion and then the amount of the excess is determined by titration with a st:indnrd quaternary ammonium salt solution in the presence of bromophenol blue. The procedure compares favorably TI ith the flame photometer method in speed and with gravimetric procedure< in accuracy. Thtb :ipplication of the method to the determination of potassium in fertilizers is presented in detail below. The titer suggested for the standard solutions is convenient for this application, but is not critical and could be adjusted as de-ired for other types of samples. METHOD
Reagents. Sodium hydroxide. Dissohe 20 grams of reagent grade sodium hydroxide in 100 ml. of water. Formaldehyde, 37%, reagent grade. Sodium tetraphenylborate (STPB). Dissolre 23 grams of sodium tetraphenj borate (J.T. Baker Chemical Co., Phillipsburg, PI'. J.) in approximately SO0 WJ. of mater. Add 20 to 25 grams of a h ~mnumhydroxide, stir 10 minutes, and filter. Collect the initial cloudy filtrate (100 to 200 ml.) separately and refilter. Add 2 ml. of 20% sodium hydroxide to the clear filtrate, then dilute to 1 liter with water and mix. To standardize this solution, dissolve 2.5 grains of potassium chloride (63.177, 1120) in water in a 250-ml. volumetric flask, add 50 ml. of 4% ammonium osalate solution, dilute to volume with water, and mix. Transfer a 15-ml. aliquot (94.755 mg. K20) to a 100-ml. volumetric flask, add 4 ml. of 20y0 sodiuin hydroxide, 10 ml. of formaldehyde. and 36 ml. of sodium tetraphenylbomtc reagent. Dilute to volume with n-ater mix, allow t o stand 5 to 10 minute.. and pass through a dry filter. Transitbr a 50-ml. aliquot of filtrate to a 125-inl. Erlenmeyer flask, add 8 to 10 drops of indicator, and titrate the excess reagent with cetyltrimethylamnioiiium bromide (CTAB) solution. Calculate the titer of the solution as fOllO\\
F = -
.:
36 nil.
-
63.17 (2 X ml. CTAB)
-
% KzO per ml. of reagent Thi. factor, F , then applies to all fertilizer samples if a 2.5-gram sample is diliited to 250 ml. and a 15-ml. aliquot is taken for the determination. For e:iqe of calculation when handling large numbers of samples, adjust the reagent so that F = 2.00. If results 3re to be expressed on the elemental rather than the oxide basis, substitute 52.44 for 63.17 in calculating F .
Cetyltrimethylammonium bromide. Dissolve 2.5 grams of the reagent in water and dilute to 100 ml. To standardize transfer 2 ml. of sodium tetraphenylborate solution to a 125-ml. Erlenmeyer flask, add 20 ml. of water, 1 ml. of 20% sodium hydroxide, 2. 5 ml. of formaldehyde, and 8 to 10 drops of indicator. Titrate to the blue end point with cetyltrimethylammonium bromide solution. Adjust the concentration so that 1 ml. equals 1 nil. of standard sodium tetraphenylborate. Bromophenol blue indicator. Dissolve 0.040 gram of tetrabromophenolsulfonphthalein in 3 ml. of 0.1N sodium hydroxide and dilute to 100 ml. with water. Potassium chloride. Recrystallize reagent grade potassium chloride twice from water and dry a t 105' C. to constant weight Ammonium oxalate, 4%. Determination. Place a 2.5-gram sample of fertilizer in a 250-ml. volumetric flask and add 50 ml. of 4% ammonium oxalate solution and 125 ml. of water. Boil for 30 minutes, then make slightly alkaline with ammonium hydroxide. Cool, dilute to volume with water, mix, and pass through a dry filter or permit the solution t o stand until clear ( 1 ) . If the sample contains 50% or less potash, transfer a 15-ml. aliquot to a 50-ml. volumetric flask and add 2 ml. of 20% sodium hydroxide and 5 ml. of formaldehyde. Add 1 ml. of standard sodium tetraphenylborate solution for each 2% potash expected in the sample plus an additional 2 ml. in excess to ensure complete precipitation. Dilute to volume with water, mix, allow to stand 5 to 10 minutes, and pass through a dry filter. (Whatman KO.12 or the equivalent is satisfactory.) Collect 25 ml. of the filtrate and transfer t o a 125ml. Erlenmeyer flask, add 8 to 10 drops of indicator, and titrate the excess reagent with standard cetyltrimethylammonium bromide solution. If the sample contains more than 50% potash, transfer a 15-ml. aliquot to a 100-ml. volumetric flask. Add 4 ml. of 207, sodium hydroxide and 10 ml. of formaldehyde; proceed as above evcept use a 50-nil. aliquot of the filtrate for the titration with cetyltrimethylammonium bromide. 70
potash in sample = [ml. STPB added - (2 X ml. CTAB)] X F
where F
=
70potash equivalent to 1 nil. STPB RESULTS
Recoveries of potash in amounts up to 100 mg. indicate that the average error of the method is about 0.370 (Table I). Comparison of results obtained by the A.O.A.C. official gravimetric method ( 1 ) with those obtained following the above procedure on fertilizer samples ranging from 4 to 30% potash showed good agreement (Table 11). Com-
parisons were made on a total of S i samples and the standard deviation of the differences between the two methods was 0.14%. The volumetric results averaged 0.09% higher than those obtained by the official method.
Table 1. Recovery of Potash from Potassium Chloride Solution K20, hlg. Error 0 Mg. /G Taken Found 10 10.08 0.08 0.80 0.09 0.45 20 20.09 0.08 0.20 40 40.08 0.14 0.23 60 60.14 0.20 0.25 80 79.80 100 99.65 0.35 0.35
Table II. Comparison of Results Obtained by Gravimetric and Volumetric Methods Sample Potash Found, yo
Gravimetric
Volumetric
1 2
4.35 8.50 10.55
4.24 8.66 10.64
8 9 10 11
19.00 22.75 26.10 29.88
19.00 22.86 26.32 29. i 6
SO.
DISCUSSION
Ammonium ions also form a materinsoluble precipitate with the tetraphenylborate complex. Because the majority of fertilizers contain ammonium salts and other ammonium compounds are added during the sample preparation, it is necessary t o complex these ions before precipitating the potassium. This is accomplished by adding formaldehyde to a slightly alkaline solution of the sample 3'. suggested by Berkhout (4). The amount of these reagents suggested in the present method is sufficient to handle two t o three times the maximum amount of ammonia likely to be encountered in a G m l . aliquot of the sample solution. Formaldehyde does not complex quaternary ammonium compounds and thus does not interfere with the titration. The only other ions known a t the present time to interfere with the method are cesium, rubidium, silver, mercury(II), and the nitrogen bases, but these are not expected to occur in fertilizers. The formaldehyde and 20% sodium hydroxide should not be mixed together prior to use. Mixtures of these two components, upon standing about 1 day, will no longer complex animonium ions. Although the reaction was not VOL. 29,
NO. 7, JULY 1957
1045
investigated, it apparently involves a condensation of the formaldehyde in the presence of alkali. Any quaternary animoniuni compound capable of forming colored salts with bromophenol blue may be substituted for cetyltrimethylammonium bromide. These include compounds in the general class (R1R2R3RJ) -, where Ri, Rz,and RB are methyl or longer chained alkyls and R4 is a benzyl, butyl, or longer chained aryl-alkyl group ( 2 ) . Zephiran chloride (Rinthrop-Stearns, Inc., Kew York 18, N. Y.) has been used routinely in this laboratory without difficulty. It is available a t local pharmacieq a3 a 12.870aqueous solution. The solubility of the quaternary ammonium tetraphenylborate compounds is less than that of the potassium salt; therefore, filtering is necessary before back titrating the excess reagent I n this step it is not necessary to filter the entire amount or t o transfer the precipitate quantitatively. Collection of sufficient filtrate for the titration (one half of the initial volume) suffices. Where possible, the filtering should be done in a hood because of the volatility of the formaldehyde. The color change of the indicator a t the end point is from the alkaline purple to a light blue. With certain types of fertilizers a dark-colored solution is obtained when the sample solution is prepared, and some of this color, although diluted severalfold, carries over to the final filtrate. This does not interfere with the titration, as the end point remains sharp, but the final color usually is a blue-green rather than blue. The indicator solution should be prepared fresh weekly. If permitted to
stand longer than this, it sonietinies gives an indistinct end point. The titration requires approximately 1 ml. under normal conditions; a 10-nil. semimicro buret is recommended for the cetyltrimethylammonium bromide solution. Titrations of less than 0.05 ml. indicate a lack of excess sodium tetraphenylborate reagent, and thus that the potash content of the sample was at least 4% higher than anticipated. I n this case the determination should be repeated, adding sufficient reagent to ensure an excess. Titrations in excess of 6 to 8 ml. of cetyltrimethylammonium bromide should also be avoided because the end point is less distinct in the presence of a large amount of precipitate. Neutral or acidic solutions of sodium tetraphenylborate tend to be unstable, as evidenced by the appearance of cloudiness and the odor of benzene or phenol in the solution on standing. Slightly alkaline conditions result in much greater stability (6). Solutions of this reagent in 0.01S sodium hydroxide remain clear when stored a t room temperature for periods of a t least 1 week and daily checks on the standardization show that the titer does not change during this period. -4s a precautionary measure, hom-ever, the standardization should be repeated frequently. This can be done quickly and conveniently if a standard solution of potassium chloride is kept in the laboratory for this purpose. The reaction between sodium tetraphenylborate and quaternary ammonium salts proposed above to determine the excess reagent appears to be applicable to the volumetric determination of those quaternary ammonium com-
pounds capable of forming colored salts with bromophenol blue ( 2 ) . Thi; application has not been investigated in detail but preliminary experiments in which sodium tetraphenylborate solutions, containing alkali and formaldehyde as indicated earlier, were titrated with several compounds of this class revealed no difficulties. Primary animonium salts would not interfere under these conditions, but potassium, rubidium, cesium, and mercury compounds would because they form insoluble salts with the reagent. ACKNOWLEDGMENT
The author is indebted to F. L. Aldrich for the analyses of the fertilizer samples and for many suggestions during the course of this wvork. LITERATURE CITED
(1) Assoc. Offic. -4gr. Chemisthi, '*Official Methods of Analysis,' ' 8th ed., 1955.
(2) Auerbach, AI. E., IXD. ENG.CHEM., ANAL.ED. 15,492 (1943). (3) Barnard, A. J., Jr., Chemist B m l y s t 44,104 (1955). (4) Berkhout, H. W., Chem. T e e k b l a d 48, 909 (1952). (5) Gloss. G. H.. Chemist diialust 42, 50 i1953). '
La": P..'An%. 563. 100. 126
RECEIVED for review September 21, 1956. .4ccepted February 21, 1957. Journal Paper S o . 1029, Purdue University .4gricultural Experiment Station
Use of Subsampling in Control Laboratory Problems R.
H. MATTHIAS
Electrochernicals Department, E. 1. du Pont de Nernours &
b The determination of variation in the multistep analytical procedures used by many control laboratories does not solve the basic problem of where in an analysis the major portion of the variation is encountered. A statistical design of experiment known as nested sampling or repeated subsampling is useful for isolating these components of variance. A scheme of this design is presented, as well as examples of its economic usefulness in pin-pointing the
1046
ANALYTICAL CHEMISTRY
Co.,lnc.,
Niagara Falls, N. Y.
source of variation as compared with the usual method of repeating the complete analysis.
T
use of sound statistical methods can be an invaluable aid in the chemical industry, where precision of analytical work in the control laboratory contributes to effective process control in the plant. A series of determinations run in duplicate indicates to the analyst whether a determination HE
is good to 1 1 % or is%, but it does not point to steps that need improvement. By properly designed experiments, components of variation can be separated to point out where the major variations occur. A statistical design known as subsampling is ideally suited for this. The multistep determination of viscosity is one of the problems of controlling the quality of a polymeric material. Assume that the specification range for
