James M. Thoburn
Central Scientific Company Chicago 13, Illinois
Weight Titrations Revived
The method of weight titrations, although seldom used, is an old procedure.l The method has never been popular. There are some obvious reasons for this situation. First, weight burets2 are cumbersome and unhandy to use. Second, they require the use of an analytical balance, which is usually tedious. Third, they give an order of precision which is seldom called for in most analytical work. In general, they seem to have little to offer. However, with the use of syringes and automatic balances, it is possible to make weight titrations more practical than ordinary volumetric determinations. This particular combination has many distinct advantages and in many cases should replace the time-honored methods of volumetric analysis. Much of the value of weight titrations is lost, however, if the other parts of the analysis are carried out with the usual volumetric flasks and pipets. (This would introduce two sets of units, which could only be interconverted by a determination of the density. Moreover, it would be inconsistent to carry out the various steps of an analysis with the different degrees of precision.) It is therefore desirable to devise an appropriate counterpart for volumetric flasks and pipets if the general method is to be made practical. Syringes os Weight Pipets
We should note a t the outset that pipets need not be reproducible. There is no part of analytical chemistry which demands reproducible delivery, as such. In the use of weight pipets, therefore, no attempt has been made to provide reproducible delivery. The syringe is simply weighed before and after delivery of liquid, and the loss in weight is the amount delivered. These weight pipets are much easier to use than the volumetric pipets. They are not subject to drainage errors as is the volumetric pipet. One may handle highly poisonous, corrosive, or radioactive material with ease and without danger. One does not need rubber bulbs, which tend to make the use of volumetric pipets awkward. Syringes os Weight Burets
The same piece of equipment can be used as a weight. buret. The syringe is weighed before and after the titration, and the loss in weight is the amount of titrant delivered. In this case, however, the control of liquid delivery is a little more difficult. The "head" of liquid within the syringe provides a pressure which tends to deliver the liquid into the titration beaker without any force from the operator. Toward the end of the titrations some pull on the plunger is needed to prevent it from delivering too rapidly.
A standard 20 ml-B-D Multifit (registered trademark of Becton, Dickinson, and Company) Syriuge was modified by sealing a short length of Pyrex capillary to it (Fig. 1). The delivery tip has been drawn out, ground, and beveled.
Figure 2.
Figure 1.
Weight pipet.
Commercial syringes are generally not made of Pyrex. However, they have a coefficient of expansion close enough to that of Pyrex so that a good seal can be made between the two. An amateur glass blower can do a satisfactory job with a little care. The syringe glass seems to have a lower softening point and a slightly greater coefficient of expansion. It will crack more easilv than Pvrex. "
'
WASHBURN, E. W., J. Am. Chem. Soe., 30, 39 (1908). 'FRIEDUN, H. B., AND LAMER, V. K., Znd. Eng. Chem., Anal. Ed., 2, 54 (1930).
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Journal of Chemicol Education
Weight buret.
We have improved the control immeasurably by providing a lightweight screw mechanism (Fig. 2 ) . At the beginning of the titration when rapid delivery is needed, a quick release mechanism permits the syringe to he operated in the usual manner. When this mechanism is not actuated, the movement of the plunger is controlled by the knurled end of the screw, and the delivery is slowed to any desired rate. When the syringe is filled, the entire unit weighs less than 200 g and will not overload an analytical balance. The amount of titrant delivered is determined as it is with the usual weight buret. Both the pipet and buret are much too long to fit on a standard analytical balance conveniently. All of our work was carried out using an Ainsworth Type S automatic balance. During the weighing operation one door was left open; the syringe extended out this side.
In the delivery of 20 g, we are not interested in weights below 0.002 g (one part in 10,OM)), if that; any errors due to air currents or fingerprints can be ignored. Weight Flosks
I n order to make our method of analysis consistent, we must have a substitute for volumetric flasks. As in the case of the pipet, a volumetric flask need not be reproducible. To make up one liter of a standard solution by weight, one simply weighs out the appropriate sample from a weighing bottle into a 1500-ml beaker. The weighing bottle is, of course, weighed on an analytical balance to obtain the loss in weight. The beaker plus sample is placed on a large capacity balance and weighed; approximately one liter of solvent is added and the beaker reweighed. The increase in weight gives the exact amount of solvent added, and one can immediately calculate the exact weight concentration in a straightforward manner. Unfortunately, there are few balances capable of reighing these relatively large amounts to the desired accuracy and with a minimum of effort. Mettler Instrument Corporation probably has the best balance for such an operation; their type K5 weighs up to 2000 g with a precision of plus or minus 0.2 g and does it automatically. Of course, any large capacity balance can be used; it need not be automatic since standard solutions are made up only infrequently. Moreover, the usual practice is to make up large quantities of secondary standards which are made up only approximately and subsequently standardized against a primary standard. One must also be able to take an aliquot on a weight basis. For this operation we need to know the weight of the sample and the weight of the aliquot; multiplying our result by the ratio of sample weight to aliquot weight gives us the total active ingredient in the sample. The aliquot weight is simply the weight delivered from the weight pipet. The sample weight is obtained by subtracting the container weight from total weight of container plus sample. For this work we used 250ml beakers of known weight, or which are weighed immediately before adding the sample. After adding the sample, the total weight is obtained, and the difference is the sample weight. We have thus outlined a method by which any volumetric procedure can be transformed to a corresponding gravimetric procedure since every volumetric operation has a gravimetric counterpart. 'Experimental Results
A sample standardization of NaOH with primary standard potassium acid phthalate was used to test the operation of the weight buret and to obtain figures for the standard deviation. Phenolphthalein was used as an indicator. Two series were run: one with a 20-ml syringe and one with a 30-ml syringe (see table). One would expect that the largest error in the titration would be in the repeatable location of the end point; the weighing errors must certainly be small. Assuming, then, that the end point location introduces the biggest source of variance about the average, the variance will
be smallest percentage-wise for the larger samples. This is borne out by the data: the standard deviation, in parts per thousand, is smaller for the larger samples. I n order to obtain the most sensitive indication of the approaching end point, the tip of the syringe buret was immersed below the surface of the titrating medium. After the titration was complete, the tip was wiped to dryness and then weighed. The titration itself was carried out in a 250-ml beaker, using magnetic stirring. RQsumQ of Experimental Results
20-ml 30-ml weight buret weight buret Number of determinations Average weight of NaOH added per titration, in grams Concentration of titrating solution, in moles of NsOH ner kilogram of solution, average of 7 diterminations Standard deviation, in moles of NaOH oer kiloeram of solution ~ t m d a r d .d e v i a t b in parts per thousand
7 17.725
7 26.979
0.10715
0.10721
0.000065
0.000047
0.607
0.444
One might well ask why weight titrations should constitute an improvement over the usual volumetric procedure. A discussion of the advantages should include : There are no drainage pmb1ems.a There is no need to consider speed of delivery or drainage time. In weight titretions the drainage problem is simply nonexistent; liquids of any reasonable viscosity can be handled with equal accuracy. A corollary to this is: Cleaning is easier and less critical. A volumetric buret or pipet must not only be cheinically clean, but physically clean. Whereas a. slight amount of grease on a syringe causes no noticeable difference, s. greasy buret is worthless. Even when burets are kept filled with liquid they tend to require frequent cleaning. The analyst who runs only an occasional titration spends more time cleaning the buret than he does in actually carrying out the titration. Sometimes, if some silicone grease has been introduced via stopcock grease, i t may he virtually impossible ever to get the buret clean. This brings us to the third consideration: There i s no stopcock to grease or to leak or to fall apart. The syringe may be used an any chemical that will not attack glass, and there is no need for having gresse of any kind. There is no meniscus to read. We can throw away our buret reading cards and magnifying glass. Even such titrants as perrnanganste cannot cause difficulty. The syringe, as we have devised it, gives much better control of deliwry of titrant than either the standard buret or the present weight huret. A stopcock is an abominable device for controlling the flaw of liquids; a buret with a needle valve is something of an improvement. With a. syringe i t is quite easy to "sneak up" on sn end point. A weight titmtion can be made more preeise. This, in itself, is seldom needed since one or t x o parts per thoussnd is usually sufficient far most analyticsl work. On those occasions, however, when the answer is the difference between two titrations, the increased precision is sometimes the difference between a usable method and one which is not.
By eliminating glassware ceslibration we do awa; with changes in calibration caused by temperature fluctuations. Syringes w e easier to rejill. A simple reverse motion on either pipet or huret relills them. No buret funnels are needed, no standing on stool to reach the top of the buret and pouring a Kimhle Glass Co., Toledo 1, Ohio, "The Care and Handling of Glass Volumetric Apparatus," 1957. 'MANVILLE,R. L.,A N D HENDERSON, S. R., Anal. Cham., 25, pp. 840f (1953).
Volume 36, Number 12, December 1959
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from a one-liter bottle, no liquid slopping an the buret and equipment. Because of their compactness, syringes can easily be adapted to dry box operations. It i s easier to protect litrants from atmospheric contamination. Carbon dioxide is easily excluded in titrations with alkali: the glass tip of the syringe can be lowered into the stock bottle through a slit in a sum-rubber The standmduahon is such that there ore no changes in concentratzon of solultons due to temperature changes. This effect, while not pronounced for aqueous solutions a t ambient room temperature, can be very important in nonaqueous titrations, where the liquid coefficient of expansion is much larger. This has been noticed particularly in the ease of acetanitrile. There is an overall sauzng i n lime. The actual titration time is roughly equivalent, but the subsidiary work, such its cleaning, calibrating, greasing stopoook, etc., is almost nonexistent. As a rule, smaller volumes are used; a 15-20 ml weight titration replaces a 40-50 ml volumetric titration. This means that costs of reagent should be decreased by 50%, that stock solutions will last two or three times as long, and that less time will be spent making up these solutions. Fnver sizes oj pipets and buvets t i l l be needed since each syringe can cover a large range without loss in accuracy.
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The method of weight titration does have one disadvantage: an analytical balance is needed, and weighings are more numerous. The average chemist would sooner put up with all of the above enumerated disadvantages than have to use an analytical balance. Fortunately, we now have semiautomatic balances such as the Ainsworth Type S, the Mettler Type B5, and the Sartorius Selecta. While it cannot be recommended that a laboratory purchase one of these $900 balances just to carry out weight titrations, the laboratory which is presently equipped with one can use this method to advantage.
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Deflnition of a Molel Solution
I n the case of a weight titration it is convenient to have a new unit of concentration. Thus in our laboratory, we have defined a one molel (sic) solution (abbreviated A?) as one gram molecular weight dissolved in one kilogram of solution, and a one normel solution (IT) as containing one gram equivalent weight in one kilogram of solution. The molel unit is, of course, implicit in any weight titration. One can carry out a determination without ever using this terminology. (The same can be said for normal and molar units.) However, if any considerable amount of work is done with weight titrations, a new unit is certainly desirable. Two things are immediately apparent. First, molel and molar solutions approach the same numerical value as the solution approaches the density of pure water (where one liter equals one kilogram). In the 0.1 concentration range the difference between the two units is seldom more than a few parts per thousand. Secondly, the molel concentration can be obtained by dividing the weight per cent by the molecular weight. Since many handbook tables list the density of solutions as a function of weight per cent, it is possible to convart from molel to molar units, or vice versa. Acknowledgmenl
The author wishes to acknowledge the invaluable assistance of J. LeBlanc who designed the screw drive mechanism on the weight buret.
