Abstract

In this experiment, 1.41M sodium chloride aqueous solution was prepared and quantitatively transferred into a 250ml volumetric flask to further diluted to 0.141M. The transfer was inspected to be complete by attempting to precipitate with silver nitrate solution. Sand was successfully separated from 600ml water by vacuum filtration.

The Pasteur pipet was calibrated to have an average 0.043ml/drop with a standard deviation of 0.0026ml/drop, and an average 0.03800g/drop with mass per drop error (standard deviation) of 0.002332g/drop, giving a calculated water density of 0.88g/ml. The deviation from usual water density was interpreted. The absorbance of FDC blue solution measured 4 times by Spec 20 had an average of 0.925, a standard deviation of 0.000 after the removal of an outlier. Its percentage error related to the true value was 5.17 and 51.7 in parts per thousand. Possible sources of error were discussed.

Introduction

This experiment is a practice collection of fundamental lab techniques that will be heavily relied on and frequently applied in formal chemical laboratory operations. Mastery of these techniques will produce scientific and conclusive experimental data with both accuracy and precision to ensure the success of future experiments. This series of techniques includes weighing by difference, quantitative transfer of solids and liquids, volumetric dilution of a solution, vacuum filtration to separate a solid and liquid mixture, quantitative calibration of a Pasteur pipet, and taking absorbance of a given solution by UV-vis spectrometry.

Specified masses of solid chemicals can be taken by a technique called “weigh by difference”, which can be done in 2 different ways. If the reagent is needed in a relatively ample amount (for instance, approximately greater than one decigram), it can be added to a tared weighing bowl or other pre-weighed vessel from the storage bottle using a spatula (for solid), or a Pasteur pipet (for liquids).

For most situations when the container can be simply tared before putting in reagents, the settled reading on the analytical balance after closing the slide door directly indicates the mass of solids in the container. If the taring function of the analytical balance is not properly working, or the container needs to collect reaction product before weighing to determine the mass of product, the container needs to be pre-weighed instead.

The exact mass of interested substance is the final reading on the balance (total mass of container & solid) subtract the pre-weighed mass of the empty container. If a solid reagent is required on a milligram scale, it can be precisely weighed out by first scooping slightly more amount than required into a tared weighing bowl. Then tare the solid and weighing bowl again, followed by carefully taking sprinkles of solid from the balance into another container until the reading reaches the negative of a specified amount.

The digital display on the analytical balance is negative in this case because both the solid and weighing bowl is tared and become a part of the closed system inside the balance. When actively transfer granules of solid into another container, the total mass of the system decreases, which is exhibited by the negative reading on the balance. The mass of solid gathered is the magnitude of the reading on balance.

Solute and solution can be transferred quantitatively from one vessel to another according to the need of the lab.  In making a solution, the solute needs to be thoroughly stirred or swirled with solvent so that particles of solute are evenly distributed in the solution and the concentration of the solution is homogeneous everywhere. In the context of this experiment, sodium chloride is used as solute and water is solvent, the dissolving process can be summarized by equation (1):

When transferring a concentrated original solution completely into another container for subsequent dilution, every part of the dispersed solute should be ensured to completely move into the new container so that the total moles of solute is unchanged and the desired concentration after dilution will not differ too much from the theoretical calculation. This is achieved by thoroughly rinsing the original container with small portions of solvent to pick up residual original concentrated solution retained on the inner wall, then carry the rinse into the new container.

The more repeated this procedure, the better transfer will result. The chemical principle behind is equilibrium shift during the re-dissolving process, which can be explained by Le Châtelier’s principle. When rinsed with fresh solvent, the solute particles in the original concentrated solution encounter a concentration difference from the solvent. In order to reduce the low solute concentration environment in the solvent, the solute particles in the original solution will redistribute until a new even concentration is reached.

The effect on the left side (reactant side) is that the original solution is diluted every time it is rinsed by the fresh solvent. Therefore, multiple rinsing will drive the equilibrium to the very right (product side) end so that the droplets of solution retained in the original container has a negligible concentration of the solute of transfer (Equation 2)

Since this experiment uses a chloride salt to make the solution, the completeness of transfer can be visually examined upon the addition of a silver salt solution.  If a trace amount of chloride ion still remains in the original container, a distinguishable white cloudy silver chloride precipitation will instantly form. Since the solubility product (Ksp) of NaCl is approximately 36, that for AgNO3 is 51.6 and that for AgCl is 1.8 ×10-10, such a sharp difference enables AgNO3(aq) a good indicator of chloride ions. The ionic equation for the formation of AgCl precipitation is (3):

Volumetric pipets are laboratory instruments designed to accurately transfer liquids. They are usually calibrated with one etched mark near the top of their opening and can only transfer liquids of certain volumes, but they provide extra precision than graduated cylinders. They are often used to accompany a squeezable rubber bulb to provide suction to draw liquids into its chamber. When a flattened bulb is air-tightly attached to the top of a volumetric pipet and then gently release, the air trapped in between the bulb and the top of the liquid consists a closed system.

Restoration of the bulb into its original shape provides a reduced pressure that differs from the external atmospheric pressure. The resultant pressure pushes the liquid level in the original container and forces it into the chamber of the pipet, creating a phenomenon of “suction”. After 2~3 times of drawing, the liquid level in the pipet should be a few centimeters above the etched mark, but no liquid should go into the bulb to prevent chemical contamination for the next person’s use. Then the upper opening of the pipet is covered with an index finger to carefully loosen the seal and adjust the meniscus to the etched mark, which is the desired volume of the transfer.

The upper end of the pipet should be tightly sealed again to stop further descending of liquid and the entire pipet with volumetric liquid should move into another new container to release the transfer. Most pipets manufacturers take the error of remaining liquids in the tip after dispensing into its calculation of design, thus there is no need to “blow the last drop”. However, there might still be droplets clinging to the exterior side of the tip during dispensation that are still parts of transfer. The correct operation to do is to rotate the tip of the pipet along the inner wall of the new container to get rid of these droplets.

Experimental

2.0575g NaCl was weighed in a tared weighing bowl on an analytical balance, then it was dissolved by approximately 25.0ml DI water in a 250ml beaker. First, an empty plastic light weighing bowl as placed on the pan of the analytical balance and tared to make the reading zero before placing any content. Then solid was scooped out from its container using a microscale spatula each time until the digital display reaches around 2 grams.

The final mass was recorded to be 2.0575g once the reading stabilized. 25.0ml deionized water was measured in a 100ml scale graduated cylinder using a squeeze bottle and a Pasteur pipet to adjust small drops of liquids when the meniscus approached the designed mark. Both the NaCl solids and the measured DI water was transferred into a clean dry 250ml beaker. The residual NaCl crystals clung to the weighing bowl was rinsed by small portions of 25.0ml DI water from the graduated cylinder and transferred all into the beaker. The mixture was stirred till completely dissolved using a glass rod. A 1.41M Sodium chloride aqueous solution was prepared.

The solution was transferred into a 250ml volumetric flask by carefully decanting the liquid from the 250ml beaker along the same glass rod used for previous stirring. Then both the empty beaker and the glass rod was rinsed with small portions of DI water (around 10-15ml), then transferred into the volumetric flask in the same fashion for 5 times. 5 drops of AgNO3 aqueous solution was added into the beaker and on the rod, no precipitation was detected.

DI water was kept adding into the flask with a squeeze bottle and paused when the flask was one thirds and two-thirds filled, then swirled to obtain a homogenous solution to avoid visible volume change due to dilution. Then DI water was continuously added until the meniscus was less than 1cm below the etched mark. A Pasteur pipet was used to drop DI water cautiously until the bottom of the meniscus was reached parallel to the etched mark. Next, the mouth of the volumetric flask was covered tightly by a piece of parafilm. The content was thoroughly mixed by 5 times of inversion.

With a clean dry 25ml volumetric pipet and a rubber bulb, 75ml of said diluted solution was transferred into another clean dry 250ml beaker by repeating 3 times of transfer. First, the bulb was squeezed to expel air, then it was attached to the end opening of a pipet dipping into the NaCl solution in the volumetric flask. Gently released the bulb and let the solution to be drawn halfway into the middle chamber of the pipet, the left hand index finger quickly covered the upper opening to seal the pressure while the other fingers grabbed the pipet to avoid the tip falling into the bottom of volumetric flask, where insoluble impurities may accumulate.

Using another hand, the bulb was squeezed again to evacuate and attached to the top opening as soon as the left index finger was removed. When the solution level was drawn till around 2 inches above the mark, the bulb was removed and the opening was promptly resealed by the same index finger. The press was loosed lightly to vent the pipet and let the meniscus descend slowly to the mark when the opening was firmly repressed.

The pipet was lifted from the 250ml volumetric flask, placed into the 250ml beaker, and released to dispense the solution. Once the pipet was empty, the tip was rotated 2 turns around the inner wall of the beaker to deliver any clinging drops on the tip. After the experiment was done, all liquid waste went to their designated container in the hood.

600ml mixture of sand and water was filtered through the sinister crucible sat on the vacuum filtration apparatus. Before any operation, all filtering flasks, glass crucible and connecting pipes are examined free of cracks. Then, the vacuum filtration was assembled according to the following diagram (fig.1).

After turning on the vacuum, a few squirts of DI water were filtered through the crucible to test the goodness of the seal. No hissing of air was heard, indicating a tight seal. The filtered water quickly dropped to the bottom, boiled and vaporized in a few seconds at room temperature, and form small droplets of water fume, indicating an over-powerful vacuum. The suction pressure was adjusted until no fumes were readily formed. Then approximately 600ml sand and water mixture were filtered in separate portions through the crucible and let dry by suction for about 5 minutes. All sands were collected in the crucible while only water contained in the primary filtering flask. No liquid went into the secondary trap flask.

A plastic Pasteur pipet was calibrated by volume and then by mass. The volume was measured in a 10ml graduated cylinder every 10 drops and the resulted volume increase was recorded in a table.

To obtain the mass of DI water per drop, drops of DI water were added into a tared weighing bowl on an analytical balance, and accumulative masses were recorded every 10 drops. The absorbance of FDC blue was measured via UV-vis spectroscopy using Spec 20. First, the instrument was turned on for at least 15 minutes to warm up.

During this time, clean, free-of-scratch cuvettes were picked and loaded 2/3 DI water blank solution and FDC solution to be tested. Then, the absorption wavelength was adjusted to 628nm with the wavelength filter set to a long-wavelength range, followed by zero transmittance calibration without the holder.  Next, a cuvette filled with blank was placed into a cuvette holder, which was inserted into the spectrometer to adjust transmittance to 100%. Finally, 3 trials of FDC solution were taken absorbance and recorded in a table.

Results

After adding 5 drops of AgNO₃ aqueous solution into the beaker and on the rod that used to prepare and transfer NaCl solution, no precipitation was detected, indicating a successful and complete transfer. The mass of weighed NaCl solid is 2.0575g, which is equivalent to 0.0352 mol (Eq. 4). The molarity of the first made NaCl solution is calculated to be 1.41M (Eq. 5).

The molarity of the solution after dilution in a 250ml volumetric flask can be computed from the known condition that the total moles of solute does not change during dilution (Eq.6), which is 0.141M. The original solution was ten times diluted.

For the pasteur pipet calibration by volume experiment, the volume was taken at the beginning of 20 drops because the first 10 drops of liquids did not give an obvious meniscus. All the data is recorded in the following table (Table 1):

Table. 1 Experimental data of Volume Calibration of Pasteur Pipet.

Total Number of Drops Added into the Cylinder	Approximate Volume Reading (ml)	Volume Difference of Water at every 10 Drops (ml/10drops)
20	0.66	N/A
30	1.13	0.47
40	1.55	0.42
50	2.00	0.45
60	2.40	0.40
70	2.80	0.40
80	3.33	0.53
90	3.77	0.44
100	4.20	0.43
110	4.60	0.40

The data 0.53ml/10 data is suspicious as an outlier. Performing Q-test on this value yields a Q_epx of 0.46, which is greater than 90% confidence level of Q_erit with 9 data (0.44) and should be discarded. The new average calculated from these 8 groups of is 0.43ml/10 drops and the sample standard deviation is 0.026ml/10 drops. Dividing onto each drop, the average is 0.043ml/drop and standard deviation is 0.0026ml/drop.

The table below(Table 2) gives the experimental data of the calibration by mass.

Table. 2 Experimental Data of Mass Calibration of Pasteur Pipet.

Total Number of Drops Added into the weighing bowl on balance	Mass Reading on the Balance (g)	Mass Difference of Water at every 10 Drops (g/10drops)
10	0.3678	N/A
20	0.7801	0.4123
30	1.1452	0.3651
40	1.5407	0.3955
50	1.9433	0.4026
60	2.3067	0.3634
70	2.6568	0.3501
80	3.0280	0.3712

Both the Q_epx values of maximum and minimum stay in the 90% confidence level of Q_erit with 8 data, no data need to be discarded. The average mass per 10 drop is 0.3800g/10 drops, with a sample standard deviation of 0.02332g/10 drops. Dividing onto each drop gives 0.03800g/drop and mass per drop error (standard deviation) of 0.002332g/drop.

Table. 3 Absorbance Data for FDC blue solution at 628nm

Trail Number	Absorbance	Outliers?	Sample Mean	Sample Standard Deviation	True Value	Relative   Error of the Mean (%)	Relative Error of the Mean  (ppt)
1	0.915	Yes	0.925	0.000	0.870	5.17	51.7
2	0.925	No
3	0.925	No
4	0.925	No

The 4 trials of absorbance data of FDC blue solution taken by Spec 20 and their mean and standard deviation are summarized in Table 3.

Discussion

The quantitative transfer of sodium chloride solution into the volumetric flask was very successful. The silver nitrate test was negative, namely, no visible precipitate formed. Such observation indicates the absence of chloride ions in the original beaker and stirring rod used to prepare the sodium chloride solution.

One of the main contributing details of operation is that both the beaker and the rod that used to make the solution was rinsed 5 times with small portions of DI water and all rinse was carefully transferred into the volumetric flask, which ensures the equilibrium drives to the very right end of forming more and more hyper-diluted NaCl solution from the residual amount of previous NaCl solution that retained in the beaker. After final transfer, the concentration of chloride ions from the water film covering the inner wall of the beaker is lower than the minimum concentration of chloride ions that will lead to AgCl precipitation.

Even after increasing the concentration of a silver ion by adding a dropper of AgNO3(aq) into the beaker, still, no precipitation formed. Thus it is concluded that the concentration of residual chloride ion is far below the precipitation threshold (Ksp  1.8 ×10-10). But the addition of AgNO_3(aq) into a small sample of prepared solution created white precipitate immediately, indicating the silver nitrate indicator is effective and the presence of chloride salt. This result further confirmed the solution was made with the correct chemical.

The volume per drop of water of a Pasteur pipet was calibrated by measuring the volume of each 10 drops in a 10ml graduated cylinder and subtract each group’s volume from its previous term to get the intergroup difference of volume per 10 drops. Then the average of 9 groups of data was taken and then divided by 10 to figure out the volume of every drop.

Since the precision scale (0.2ml/grid) of 10ml graduated cylinder is too large to measure the volume of a single drop (less than 0.1ml), only when the volume of water accumulated to a readable amount by continuously adding counted drops of water can the average of total drops’ volume be taken to obtain the volume per drop. This experiment was designed to measure the volume of a known counted drops instead of the reverse for 2 main reasons.

First, it is a more sensible approach since the number of drops is the independent variable as it is actively being added into the cylinder during the experiment, and the result is the volume increase in the cylinder, which is recognized as the dependent variable. In addition, measuring the approximate volume after adding a designated number of water drops is more precise and doable than trying to fit a specific number of drops up to a volume mark, which oftentimes is hard to achieve and will introduce greater error.

Because of the inevitable presence of all sorts of errors in every experiment, the procedure was repeated at least 5 times until enough representative data was collected. The same reasoning can be applied to justify the experiment design of mass per drop calibration.

As the average volume per drop of water is 0.043ml/drop and the average mass per drop of water is 0.03800g/drop, the average density of water is 0.88g/ml, which is less than the theoretical value of 1g/ml at room temperature. This deviation can be explained by the following 4 main sources: First, since both calibrations were carried out by hand with thumb and index finger holding the bulb of the Pasteur pipet, it is likely that the water was warmed by the heat of hand slightly above the room temperature and had a lower density than normal. Second, the evaporation of water can lead to a decrease in volume and mass, especially the mass calibration because it was done in an open weighing bowl with a wide opening and thus gave the water a greater surface area of exposure that speeded up evaporation.

This was confirmed by the observation that the last digit of analytical balance drops 0.0001g every 5 seconds. Third, it was observed that the size of each drop slightly differs depends on how the Pasteur pipet was used. The horizontally tilted pipet gave smaller drops each time than holding vertically. For the volume calibration experiment, most times the pipet was held upright for ease of adding drops into the narrow opening of the small graduated cylinder.

However, for the mass calibration, which was done with one hand putting into the chamber of the analytical balance, the pipet had to be tilted and gave significantly smaller drops of water each time, contributing to smaller mass per drop that caused lower computed density than normal. Fourth, the uncontrollable random error in the experiment. Each drop differed from one the other depended on the force of squeeze. There were times when extra drops of water were accidentally squeezed into the container and taken into account of mass or volume, which generated outliers.

The absorbance of FDC blue solution was taken to have three times repeated data of 0.925 and one time of 0.915. The absorbance of 0.915 was identified as an invalid outlier because the cuvette that use to prepare the solution was just rinsed with DI water without thorough drying, which diluted the solution and causes the value to be slightly off. Since this operation error was later realized at the second trial, the cuvettes were more carefully selected to be free of scratch and dried completely using laboratory Kim wipes. Then they were cleaned on the side of the light path to eliminate the contamination of fingerprints before being place into the holder.

The later following three trials agreed so well that they have no difference in absorbance, which gave a more reliable average of 0.925. However, the true value was told to be 0.870, leading to a percentage error of 5.17 and 51.7 parts per thousand. The measurement was precise but inaccurate, inferring the systematic error of UV-vis spectrometry is the main cause. It is possible that the spectrometer was not properly calibrated.

Though very unlikely, there might be a problem with the internal mechanics of Spec 20 that caused it to malfunction. It can also be explained that the FDC blue solution changed its concentration over the week due to frequent use. The bottle of the solution was stored in a constantly operating hood and it was sometimes spotted uncapped. The evaporation of the solvent caused the solution to concentrate and thus produced a higher value of absorbance.

In general, this experiment was a success. The quantitative transfer of NaCl solution from the 150ml beaker to 250ml volumetric flask was complete as no precipitation formed upon adding a few milliliters of sliver nitrate solution into the beaker and glass rod that used to prepare the solution. A 250ml diluted NaCl solution was precisely made in a volumetric flask by carefully approaching to the etched mark.

The molarity of the first made NaCl solution was calculated to be 1.41M and that of diluted was 0.141M. The sand was successfully separated from 600ml water by vacuum filtration. The Pasteur pipet was calibrated to have an average volume per drop of water 0.043ml/drop with a standard deviation of 0.0026ml/drop, average mass per drop of water of 0.03800g/drop with mass per drop error (standard deviation) of 0.002332g/drop.

The calculated density was 0.88g/ml, possible sources of error were accounted for in the previous discussion. The absorbance of the FDC blue solution measured by Spec 20 had an average of 0.925, the standard deviation of 0.000 after the removal of an outlier. Its percentage error related to the true value was 5.17 and 51.7 in parts per thousand. Possible sources of error were discussed.