Glue - Analytical Operations
( Originally Published 1906 )
WHILE not at all averse to consulting a qualified chemist when need of his services arises, progressive superintendents frequently desire to carry out for them-selves the simpler analytical operations, if only that they may thereby better comprehend the import f the chemist's work. In the interest of such, it has been deemed advisable to add to this work a limited exposition f analytical technique. No attempt is made to outline quantitative processes save such few as pertain to the standardization of solutions; nor is it to be expected that the perusal of this chapter will enable the inexperienced to qualify as expert analysts. For the details of specific determinations, the reader is referred to any one f the many excellent works extant upon the subject of quantitative analysis.' Since, however, the success f analytical operations is largely subject to the rules f common sense, if the reader will combine the instructions for a specific determination with the suggestions as to technique herein embodied, fairly accurate results should be obtained. All analytical work presupposes the greatest care in execution, lest errors of manipulation creep in to invalidate the result.
Apparatus Required. — The following apparatus is essential to the proper execution f the majority of investigations pertaining to glue and gelatine.
1. A good balance for weighing, provided with an accurate set f weights. The balance should be sensitive to at least one milligram (1–28000) ounce. Balances of precision, sensitive to 1–10 milligram, are best. These, as a rule, are costly, and instruments less sensitive will supply results correct within technical limits.
2. A drying oven. This may be of block tin, or better, copper-jacketed.
3. A copper water-bath fitted with concentric rings.
4. Assorted Erlenmeyer and Florence flasks.
5. Two burettes, 50 c. c., graduated in tenths, with glass stop-cocks.
6. Volumetric pipettes, 10, 25, and 50 c. c. capacity.
7. A 100 c. c. graduated cylinder.
8. A 10 c. c. pipette, graduated in 1-10 c. c.
9. Glass funnels of various diameters.
10. Watch- and clock-glasses, assorted sizes.
11. Filter paper, Swedish, 1 F, assorted diameters.
12. Sohxlet extraction apparatus, medium size, complete with flask and condenser, also capsules to fit.
13. Test-tubes, 6 inches long, 1–2 inch diameter.
14. Beakers, Griffith's shape, several nests.
15. Assorted glass and porcelain evaporating dishes.
16. Two separatory funnels, 300 c. c. and 500 c. c.
17. A short piece of platinum wire and a small square of platinum foil.
18. Several Bunsen burners, tripods and ring-stands.
19. Squares of iron wire gauze for tripods.
20. Glass stirring-rods and assorted glass tubing.
21. Volumetric flasks for the preparation of standard solutions. Sizes, 100 c. c., 250 c. c., 500 c. c., and 1000 c. c. (1 liter).
22. Half a dozen porcelain crucibles, Royal Berlin, No. 00, with covers.
23. A platinum crucible, 15 grams.
24. A platinum evaporating dish, 15–20 grams.
25. A desiccator, chloride of calcium variety.
26. A gas blast-lamp, with adequate bellows.
Reagents. The reageants required are those commonly employed in qualitative work, a list of which will be found in the appendix.
1. Desication.- The case in which the balance is placed must be kept absolutely dry, lest hygroscopic substances, in the course of weighing, absorb moisture therefrom. Not only this, many substances, previous to weighing, must be heated to expel moisture and then cooled in a perfectly dry atmosphere. For maintaining the balance case in a dry condition, a small glass vessel should be filled with pure, granular chloride of calcium, and this vessel kept constantly in the case. The chloride f calcium must be renewed from time to time. For cooling objects and substances to be weighed, the desiccator is employed. The lower chamber of this is filled with granular chloride of calcium. A partition separating the upper chamber from the lower, which serves also as a platform on which to rest objects, is made by affixing corks of uniform size to the four corners of a piece of wire gauze that has been fitted to the dimensions of the chamber. Holes may be cut in this to accommodate crucibles. The lid of the desiccator should be slightly greased with vaseline, to insure perfect contact with the ground edge of the upper chamber.
All porcelain apparatus should be heated over the Bunsen flame for at least fifteen minutes before introduction into the desiccator. In many instances it is advisable to subject the object to the blast flame as well. Porcelain requires at least twenty minutes' cooling in the desiccator. Platinum will require ten minutes' heating and fifteen minutes' desiccation.
2. Weighing. — That the life and usefulness of the instrument may be prolonged, certain simple precautions are to be observed in connection with the use of a balance. Before any attempt is made at weighing, it should be seen that the instrument is perfectly level. It should rest on a table of such height that the ivory scale is at the level of the eyes when the operator is seated before the table. The balance should never be exposed to direct sunlight nor to the fumes f the laboratory, nor should it be kept in too close proximity of any source of heat lest this cause undue expansion f any of the parts. It should be kept scrupuously clean, the scale-pans being dusted with a fine camel's-hair brush. Under no circumstances should oil be applied to any part of the balance; proper cleaning, from time to time, of the various parts will insure the perfect working of the instrument. In releasing the pan-rests or beam, care must be taken not to jolt the apparatus unnecessarily. Nothing will ruin a sensitive instrument so quickly as the too sudden arrest of its oscillation.
Methods of Weighing. The importance of accuracy in the use of the balance cannot be over estimated. The slightest error in weighing out the original sample or the final precipitate from which some constituent element f the sample is to be deduced, will invalidate the analysis no matter how carefully intermediate, operations may be conducted. Objects to be weighed should be at the same temperature as that obtaining in the balance-case. Hence the desiccator should be kept next to and under the same conditions as the balance. Hot, or even warm, objects should never be placed upon the scale-pan, for, not only will the heat emanating from them cause the expansion of certain parts of the balance, rendering the instrument worthless for the time being, but the objects themselves will increase in weight as they cool.
Even though in apparant equilibrium, the zero or position f rest of a balance is never exactly at the zero mark f the scale. To assume that such is the case, is to sacrifice a degree f accuracy in weighing. The most accurate method f weighing presupposes the determination of the true zero or position f rest, and that weight which will restore this position of rest is the true and final weight of an object.
The method of weighing to be adopted will depend upon the sensitiveness of the balance at hand. If this does not exceed one milligram, it will suffice for all ordinary purposes to weigh by the simple expedient of placing the properly desiccated and cooled object on one pan, and adding weights to the other until equilibrium is re-stored. If such balance has been correctly set up, and is kept under conditions where it is not affected by vibration or other undue influence, the position of rest will remain practically at the zero line of the scale and any slight deviation will be offset by errors in the weights.
Such a balance will not permit of weighing closer than one milligram or to third decimal place.
On the other hand, if the balance be one of great precision, weighing to tenths of milligrams, the position of rest is far more variable, and cannot be assumed to be at the zero line f the scale unless the needle has been deliberately adjusted to the latter mark. Even then, the position of rest will remain constant but for a short time under the most favorable conditions.
Determination of the Position of Rest. — Each of the ten scale divisions on either side of the dividing line is mentally further divided into tenths. Swings of the needle to the right of the dividing line are arbitrarily designated positive or +, and those to the left, negative or — . The balance is set in motion and the plus and minus swings noted.
The first pair of swings indicates that the needle oscillated further to the right than to the left, by two scale divisions. The oscillation would be f the same proportional value, though of smaller amplitude, had the needle started at the dividing line, traveled two scale divisions to the right, and then returned to the dividing line and stopped. Were the balance in equilibrium, this would correspond to a swing of one scale division on either side of the dividing line. It will at once be seen that one scale division equals half the difference between the first two swings. Similarly the half-differences between the remaining swings or oscillations are 0.50, 1, and 0.50. These are designated as positive or +, since the swing to the right of the dividing line was in each instance of greater amplitude than that to the left. Continuing our hypothesis, the tabulated statement of results at this stage.
By dividing the sum of the half-difference by 4, the number of such half-differences, we learn that the position of rest of the balance for this determination is 0.75 f a scale division to the right f the dividing line f the scale. Five such determinations as the above should be made and the arithmetical mean of the result taken as the position of rest.
Extension f the above procedure enables us to determine the delta of our balance. This means the milligram equivalent of each scale division. To do this, we determine the position of rest as above. The rider is now placed at the 1 milligram mark on the beam, and the position of rest determined under these conditions. This time it will be negative or —, as the weight causes the needle to swing further to the left than to the right.
Let us assume that the position of rest, as first deter-mined, is + 0.75; and that, with the weight, it is -1.25. If we subtract these two algebraically we find the difference to be two scale divisions. That is, the difference of position of rest, due to the milligram weight, equals two scale divisions. Hence the milligram equivalent of one scale division is found by dividing 1 milligram by 2. The figure so obtained, 0.5 mg., is the so-called delta of the balance. This changes about once per month, under normal conditions, and more frequently if the balance be exposed to undue influences. The function for the figure for delta in weighing operations will be seen from the following.
Weighing by Half-Differences. In its entirety, the process of weighing by swings necessitates the determination of the initial position f rest, which, let us say, is + 0.75. The object is then placed upon the pan and weights added to three decimal places. Assume that the weight thus far is 9.542. We have now to determine what weight will restore the balance to the original position of rest. To do this, we again determine the position of rest, and this, for example, is found to be +1.25. The difference between this and the original is 0.50 scale division. Since each scale division is equivalent to 0.5 milligram (delta), we require the additional weight of 0.25 milligram to restore the balance to the original position of rest. We accordingly add this weight or rather 0.3 milligram (since our balance does not weigh to hundredths) and once more determine the position of rest. If this proves to be + 0.75, the final weight f our object is 9.5423 gram.
It may be objected that the process of weighing by swings consumes too much time, and the latter is assuredly a factor in chemical analysis. True, the novice will find the method tedious, until he has had consider-able practice. At the same time, the importance f accustoming himself to accurate methods f weighing cannot be too strongly urged, provided he has equipped himself with a sensitive instrument.
Less troublesome and tedious is the method of weighing by half-differences, a method that presupposes that the position of rest of the balance is always at the zero or dividing line f the scale. That such is not the case with an instrument of great sensitiveness has already been pointed out, and the reliability of the method will depend, first, upon the maintenance f the balance under strictly uniform conditions, and, secondly, upon the adjustment f the needle to the zero f the scale each morning before starting work. This done, it will be necessary to let the balance rest for an hour before attempting to use it, in order that conditions of temperature, etc., become normal. Before attempting this adjustment, it should be first seen whether any deviation from the adjustment of the previous day has taken place. If not, there is no need f readjustment. These precautions observed, the method f half-differences be-comes satisfactorily reliable and possesses the advantage over the complete oscillatory method, in that it consumes far less time, a weighing being accomplished in five minutes with reasonable practice. In operation, the method is as follows:
An object is being weighed, and the operator has already placed upon the pan 9.542 gram. The balance is now set in oscillation and the swings to right and left of the dividing line noted. These, let us say, are 8 scale divisions to the right and 6 to the left. The difference, + 2, shows that the weight is still deficient. The weight required is that which will cause the needle to swing one scale division on either side of the dividing line. One scale division is seen to be half the difference between the swings + 8 and — 6. We have already determined that the milligram equivalent f our hypothetical balance is 0.5 milligram. Hence, if this weight be added by means f the rider, we shall find that the balance is restored to equilibrium, oscillating equidistantly on either side of the dividing line. The final weight of the object is thus 9.5425.
The novice is at times troubled by the failure f the needle, when the beam is first released, to oscillate on both sides of the dividing line. This difficulty may be overcome by causing a gentle draft to impinge on the right pan by gently waving the hand toward this pan. It is well to adopt this practice at all times, disregarding the first pair of oscillations and relying on the third for the reading. Thus, if after stimulating the motion of the balance as suggested, the swings are we take the half-difference between the third pair, viz., one scale division for our calculation, and multiply this by the delta to determine the final weight required to restore the balance to equilibrium.
Preparation of Samples for Weighing. — Substances to be analyzed are, as a rule, dried prior to weighing out the initial sample or samples. Save such as would lose water of crystallization by crushing, they are first reduced in a mortar to an impalpable powder and then placed in the oven until all moisture has been expelled. The temperature of drying will depend upon the nature of the substance. Much importance attaches to the method of transferring the dried substance to the weighed container in "weighing out." The practice of first weighing crucible or other container and, as this rests upon the scale-pan, introducing the substance by means of a spatula, is a simple and convenient one, but requires great care lest any substance be spilled onto the pan with the result that the sample in the crucible, etc., is less than the weight recorded. It were better for the novice, once he has weighed the container, to remove this from the balance and, placing it on a clean sheet of paper, introduce the substance and proceed to the second weighing. The removal and replacement of the container must be effected by means of a pair of tongs. Under no circumstances should contact of container with the hand be permitted.
Where the sample is to be placed in containers either too large or too heavy for the balance, the following method f weighing out will be found convenient. As the substance is drying in the oven, a test-tube, fitted with a smooth, tight cork should be dried at the same time and the substance subsequently introduced into the tube. This is then corked and placed in the desiccator to cool. Before weighing, the tube is permitted to rest in the balance case for fifteen minutes. The container, into which the sample is to be weighed, is placed near at hand upon a sheet of black glazed paper.
The tube plus cork and contents is carefully weighed, then uncorked and a little of the contents shaken into the container. A piece of bibulous paper is employed to handle the tube and cork to avoid contact with the hand. The tube is recorked, permitted to rest a few minutes in the balance case and again weighed. The difference in weight represents the sample taken. If, inadvertently, any particles f the sample have fallen upon the paper, these may readily be added to the main portion within the container. Two samples require but three weighings by this method.
It is advisable always to express the weights taken in milligrams and to base all subsequent calculations on milligrams.
In order that the results may be controlled, it is customary to run duplicate samples and these should approximate each other in weight. Where the analyst has at his command a sensitive balance, large samples should never be taken for analysis. A large sample means subsequent bulky precipitates awkward to handle and entailing mechanical loss in washing. On the other hand, it must be remembered that, with a small sample, each loss through faulty manipulation counts more than with a large sample. The analyst must regulate the sample taken through his knowledge of the properties f the substance sought. Where he suspects a constituent which he knows will yield a bulky precipitate, the minimum working sample should be weighed out; and vice versa, since there is added safety in large samples.
Weighing out Liquids. — For this, a weighing bottle, provided with ground stopper, should always be employed. It will save time if a lead counterpoise is made for this bottle and subsequently used to tare the bottle in weighing.
In weighing potash bulbs of any description, they must be permitted to rest for a time in the balance case that the contents may acquire the temperature obtaining therein. This applies equally before and after absorption f carbon dioxide.
Calcium chloride tubes that have been employed for the absorption of moisture should be accorded the same treatment.
Filtration. — Separation of filtrates and precipitates entails certain precautions, neglect of which will seriously hamper the operation. In selecting funnels, only those with long, narrow stems and flawless sides should be accepted. Such will permit f rapid and uniform filtration. Some precipitates, because of their fineness, require double filters. In preparing the filter, the paper is folded in half, and this again in half, taking care that the edges are even. It is now a quadrant. Opening the filter again to semi-circular form, a small piece is torn from one end of the diameter. This subsequently serves, after moistening with alcohol, to detach any particles of dry precipitate adhering to the funnel itself, in which case it is incinerated with the main filter.
After detaching the portion of paper, the filter is once more folded to a quadrant and inserted into the perfectly clean and dry funnel, conforming its shape to that of the latter by pressing, care being taken not to rupture the point. One side of the filter represents a single thickness of paper and the other three thicknesses. If a double filter is required, a second paper is prepared in the same manner as the first and introduced into the funnel superimposed upon the first paper in such way that both sides f the complete filter represent four thicknesses of paper. Whether single or double, the filter, after conforming its shape to that of the funnel, is moistened with boiling water and firmly pressed into the funnel to expel all air bubbles between it and the surface of the funnel. Let boiling water pass several times through the filter, and note if the stem of the funnel holds the column of water, free from air bubbles. A well-made filter is in reality a miniature air pump. Filtration will never proceed rapidly nor efficiently unless the stem of the funnel holds the column of water.
In filtering precipitates, a glass rod is held against the lip f the beaker, the end of the rod directed to the center f the filter. Under no circumstances should liquids for filtration be poured onto the filter direct. The filter should never be filled to its capacity, as in this way much precipitate collects at the edges and difficulty in washing ensues. The bulk of the precipitate is washed from the beaker down the rod, into the funnel.
Washing Precipitates. — Before drying, precipitates must be washed free of the precipitating medium, else this dries with them, augmenting the weight and producing a fallacious result. Save in certain specific analyses, the washing is effected with distilled water, which is delivered from a wash-bottle equipped with a fine-pointed nozzle attached to the main delivery tube by a short piece of rubber tubing, enabling the operator to direct the stream at will. Care must be taken to wash the edges f the filter, it being customary to wash the precipitate down from the sides of the filter towards its apex. It must be remembered that many precipitates are not wholly insoluble in water and hence only a minimum of wash-water may be employed. Nevertheless, washing must be complete.
In many instances, filtration is effected by holding back as much of the precipitate as possible, by means of the rod, and washing the precipitate in the beaker, bringing the washings and a little of the precipitate each time on the filter and washing the latter in between. The bulk of the precipitate is finally brought upon the filter by means of a stream from the wash-bottle, aided, perhaps, by a trimmed feather or rubber "policeman," and the filter given a final washing. This method is known as "washing by decantation."
Certain precipitates attach themselves to the sides of the beaker, and are difficult of removal. For detaching these, a trimmed Guinea fowl feather is most useful. The feathers are stripped in toto from one side of the shaft, and the balance cut down short with a scissors. Where a feather of this description is employed, it must be rinsed thoroughly lest precipitate adhere to it and loss thus occur. Another aid in rinsing beakers of precipitates is the "policeman." This consists of a short piece of rubber tubing, of the same diameter as the stirring rod to which it is attached by slipping over the end of the rod. One end f the rubber tube is firmly sealed. To be at all serviceable the " policeman " must be perfectly smooth.
Drying Precipitates. — The washed precipitate is now dried. The funnel, still retaining the wet filter, is covered so as to exclude dust and placed in the drying-oven, the temperature of which should not exceed that of boiling water.
Incineration of Filters. — Before the dried precipitate can be weighed, it must be detached from the filter and the latter burned to an ash. The weight of this ash (which is usually recorded on the package of filter paper) is then deducted from the total. The entire operation is one of great nicety. The crucible, in which the precipitate is to be placed for incineration and weighing, is itself desiccated, cooled, weighed, and then placed upon a sheet of black glazed paper. The funnel containing the filter with dried precipitate is uncovered, the filter removed and inverted upon a clean watch-glass. By gentle tapping, the bulk f the precipitate is freed from the paper. If any has adhered to the funnel, the strip of paper originally torn from the filter is moistened with alcohol, and the funnel wiped with this, which is subsequently incinerated with the main filter. The precipitate detached from the filter as far, as possible, the latter is folded, the apex placed in the flame until it is ignited and the whole rapidly introduced into the crucible which is supported on a triangle. The ignited filter is permitted to burn of its own accord, and as soon as it smolders a small flame is placed beneath the crucible. If this causes the half-burnt paper to burst into flame again, remove the Bunsen flame until the former has again died out. As soon as the filter is thoroughly charred, and no longer burns of its own accord, replace the Bunsen flame beneath the crucible and gradually raise the flame until all carbon is burned off, leaving a good white ash. The crucible is now cooled, and the bulk of the precipitate is added, care being taken to brush the surface of the watch-glass, as well as that of the glazed paper, with a feather, any particles of precipitate adhering to either being transferred in this way to the crucible. The precipitate is now ready for simple ignition or prolonged blasting, as the conditions demand.
Volumetric Processes. — Volumetric processes involve the use f graduated flasks, cylinders, pipettes and burettes. The success of such processes depends upon consistency in reading the instruments, as well as upon their preservation in the utmost state of cleanliness.
(a) The Pipette. — A description of the use f this instrument for a specific purpose will be found under the discussion f Viscosity, in Chapter 2. The pipette is most convenient for delivering definite quantities of liquid, but its proper use presupposes certain precautions. As remarked, the instrument must be scrupuously clean, lest globules of the discharged liquid adhere to the glass. The last drop may be discharged by placing the index finger of the right hand over the aperture of the upper tube, holding the body of the instrument in the palm of the left hand, when the natural heat of the hand will cause the air to expand, driving out all liquid. Unless this practice is observed, the full capacity of the pipette is not delivered and an ultimate error f analysis. results.
(b) The Burette. — Many substances are analytically determined while in solution or "volumetrically" rather than gravimetrically i.e., by precipitation and weighing. For example, if the strength of an acid solution be known, — that is, if we know the exact amount of acid contained in each cubic centimeter, and if, further, 10 c. c. of this solution are required to neutralize x c. c. f an alkali solution f unknown strength, from the data at hand we may readily determine the strength f the alkali solution. Such analytical process is termed "Titration"; and in the above instance, we speak of titrating the alkali against acid.
The instrument employed for titration is the burette, which consists of a stout, long glass tube, provided at one end with a glass stop-cock or a rubber tube fitted with a pinch-cock. The tube is graduated up to 50 c. c., each subdivided into tenths. The correct reading f a burette presupposes considerable practice. The meniseus, or heavy stratum of liquid at the top, common to all columns of liquids, is the guiding point, where no "float" is at hand to assist the reader. The meniscus will be seen to consist essentially of three distinct lines, upper, lower, and a very dark line between. Any f these may be employed as the guiding line in the reading; but it behooves the operator to read from the same point throughout any one titration or set of titrations.
As in gravimetric, so in volumetric analysis, duplicates are run for the purpose of checking the results. Duplicates must be titrated under identical conditions of temperature and concentration, if concordant results are to be obtained. Each should contain the same amount of indicator.
The flow of solution from a burette should be so regulated that 10 c. c. are discharged per minute, or the entire contents of the burette in five minutes. If discharged with greater rapidity, some drops of solution tend to adhere to the upper portions of the tube, and unless the operator waits for these to flow down to the level of the column before taking the reading, this will be incorrect.
(c) Graduated Cylinders. — The correct reading f a column of water contained in a graduated cylinder is dependent upon the identical principles obtaining with the burette. The cylinder is not reliable for measuring liquids in analyses presupposing quantitative accuracy.
(d) Volumetric Flasks.— These flasks, usually of 100, 250, 500 and 1000 c. c. capacity are accurately graduated and are most useful, not only for the preparation of standard solutions, but for measuring liquid samples, for which latter purpose they should always substitute the graduated cylinder. In adjusting the contents to the graduated mark on the neck of the flask, the operator has to observe the same precautions governing the reading of the burette. Liquids, while hot, must never be adjusted to the mark, as shrinkage occurs through cooling.
Cleaning Volumetric Apparatus. A solution of alcholic potash is an excellent medium for cleaning all glassware. This need not necessarily be concentrated and may be prepared by dissolving about 10 grams of dry, chemically pure caustic potash in 100 c. c. of 80 per cent alcohol with the aid of heat. The solution should. be kept in a dark bottle sealed with a tuft of cot-ton in preference to a cork, and may be used repeatedly for cleansing purposes until exhausted. The apparatus cleansed by its use should be subsequently rinsed with pure, distilled water and inverted to drain.
Equally efficacious, and less expensive, is a solution of potassium dichromate in concentrated sulphuric acid. Such solution may be prepared either by making a concentrated aqueous solution f the dichromate and cautiously adding an excess of concentrated sulphuric acid, or dissolving the salt in the acid direct. Its use is governed by the same precautions applying to alcoholic potash.
It is at times inexpedient to rinse burettes and pipettes with water, since, unless the apparatus be subsequently dried, the strength of a standard or other solution will be altered by admixture with the water remaining. Burettes, when not in use, may be kept filled with distilled water and well corked to exclude dust. When it is desired to use the apparatus, this water may be run off and the instrument rinsed with about 20 c. c. of the solution with which it is to be filled. This is then run off and the main solution added.
Pipettes and volumetric flasks, after cleaning, rinsing and draining, may be dried by rinsing with alcohol to remove the balance of water, then with ether to remove the alcohol. The ether is then removed by suction, either of the lips or pump.