Patent Application
20250216367
Hydrogen sulfide (H2S) is a common contaminant of water sources that requires monitoring. Hydrogen sulfide is formed when soluble sulfides are hydrolyzed in water. In water, hydrogen sulfide dissociates, forming monohydrogensulfide (1-) (HS−) and sulfide (S2−) ions.
Hydrogen sulfide may be produced by bacteria, or released from naturally occurring geologic deposits. For example, the presence of hydrogen sulfide in a water sample can be indicative of microbial activity or volcanic gas production. Thus, its presence can reflect the existence of other underlying problems.
Additionally, hydrogen sulfide is highly corrosive and can lead to premature damage and failure of equipment. Corrosion caused by hydrogen sulfide increases production well operating costs significantly when equipment is damaged and needs replacing.
Hydrogen sulfide is also toxic, even in very low amounts. Once ingested, it will form a complex with the iron (III) ion of the mitochondrial metalloenzyme cytochrome oxidase, blocking oxidative metabolism. It also alters or inhibits many other enzymes, rapidly leading to acute complications.
Exposure to minimal amounts of hydrogen sulfide can range from more minor symptoms of irritation (e.g., irritation of the eyes and respiratory tract, nausea, vomiting, epigastric pain) at amounts ranging from 20 to 250 ppm, to more severe symptoms impacting the nervous system (unconsciousness and respiratory paralysis) at amounts higher than 250 ppm. The National Institute for Occupational Safety and Health (NIOSH) considers hydrogen sulfide concentrations of 100 ppm to be immediately dangerous to life or health. Concentrations greater than 500 ppm can cause a person to collapse within five minutes, and concentrations exceeding 700 ppm can cause immediate collapse, and death, within just one or two breaths. Accordingly, the Occupational Safety and Health Administration (OSHA) regulates that the permissible exposure limit for hydrogen sulfide is 20 ppm.
Although hydrogen sulfide has a recognizable odor likened to rotten eggs, this odor is only perceptible at very low concentrations (less than 10 ppm in air). Beyond this, the odor can be deceptively sweet, and higher concentrations will deaden the sense of smell. This means that someone can be exposed to potentially toxic amounts of hydrogen sulfide without there being a detectable smell.
Current methods for detecting hydrogen sulfide in aqueous samples are generally limited to offsite testing applications. For example, hydrogen sulfide is traditionally detected using an acid displacement procedure in which the hydrogen sulfide is displaced by acidification and detected by gas chromatography. So-called “dip strip” techniques, although more portable, have low accuracy.
Thus, there is a need to quickly and easily test for hydrogen sulfide in aqueous samples from sources such as wastewater, drinking water, and well water. In order to do so, the test should be accurate at detecting low concentrations of hydrogen sulfide, and be capable of being performed in situ (at the site of the water source).
The disclosed embodiments describe a hydrogen sulfide test kid, a method of using the hydrogen sulfide test kit, and a method for detecting hydrogen sulfide in an aqueous sample.
According to one aspect of the embodiments, a hydrogen sulfide test kit includes a first reactant powder comprising citric acid; a second reactant powder comprising sodium bicarbonate; and a test paper configured to change color upon contact with hydrogen sulfide. The first and second reactant powders are physically separated from each other.
Another aspect of the embodiments includes a method of using the hydrogen sulfide test kit. The method includes adding the first and second reactant powders to an aqueous sample; allowing the citric acid and the sodium bicarbonate to react in the aqueous sample to form air bubbles, wherein the air bubbles release any hydrogen sulfide in the aqueous sample into gas form; contacting the released hydrogen sulfide in gas form, if present, with the test paper; and detecting the presence of hydrogen sulfide in the aqueous sample based on the color of the test paper.
Another aspect of the embodiments includes a method for detecting hydrogen sulfide in an aqueous sample. The method includes adding citric acid and sodium bicarbonate to the aqueous sample; allowing the citric acid and the sodium bicarbonate to react in the aqueous sample to form air bubbles, wherein the air bubbles release any hydrogen sulfide in the water sample into gas form; contacting the released hydrogen sulfide in gas form, if present, with a test paper that is configured to change color upon contact with the hydrogen sulfide; and detecting the presence of hydrogen sulfide in the aqueous sample based on the color of the test paper. In this method, the citric acid and the sodium bicarbonate have not been in contact with each other for more than 24 hours before adding the citric acid and the sodium bicarbonate to the aqueous sample.
A more particular description of the invention will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, embodiments of the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
Existing methods for testing for hydrogen sulfide in aqueous samples generally require offsite analysis or can only determine the amount of hydrogen sulfide with low accuracy. Thus, there is a need for a kit that would allow for onsite testing for hydrogen sulfide in an aqueous sample with high accuracy and resolution.
The inventors found that hydrogen sulfide could be detected and quantified with high accuracy in an aqueous sample by releasing the hydrogen sulfide into gas form and detecting the hydrogen sulfide in its gas form.
For example, effervescent tablets such as Alka-Seltzer Gold™ could be used to form air bubbles in the aqueous sample. The tablets include sodium bicarbonate (NaHCO3) and citric acid (HOC (CH2CO2H)2)—a base and an acid, respectively. When the tablet is dissolved in water, bicarbonate (HCO3−) and hydrogen ions (H+) are formed. Once in solution, the bicarbonate and hydrogen ions react according to the reaction shown below.
HCO3− (aq)+H+(aq)→H2O (l)+CO2 (g)
The carbon dioxide bubbles to the surface, allowing any hydrogen sulfide in the aqueous sample to be released in gas form. Specifically, it is believed that the hydrogen sulfide is released by the shift in pH caused by the reaction, and the effervescence brings the hydrogen sulfide out of solution.
Once in gas form, the hydrogen sulfide could then be permitted to contact a test paper that is configured to change color upon contact with hydrogen sulfide. It was found that even very small amounts of hydrogen sulfide could be detected with high resolution (e.g., 0.1 or 0.2 ppm differences in concentration) when detected in gas form. By contrast, similar methods for detecting hydrogen sulfide in liquid form using color-changing paper (so-called “dip strip” methods) do not allow for distinction between concentrations with differences of less than about 2 ppm.
Despite this improvement, the method's reliance on effervescent tablets was found to be disadvantageous. By combining the reactive ingredients in tablet form, the method did not allow for any adjustment in the amount of reactive ingredients. Thus, the method could not be easily adapted for different applications.
Additionally, the tablets contained other ingredients in order to prevent premature reaction of the active ingredients. For example, Alka-Seltzer Gold™ tablets contain citric acid and sodium bicarbonate as active ingredients, potassium bicarbonate as a stabilizer, and magnesium stearate as a desiccant. Without the stabilizer and desiccant, the citric acid and sodium bicarbonate would begin to react with each other. Thus, the reactive ingredients cannot be packaged together without requiring additional filler ingredients. These filler ingredients add to cost and bulk and have the potential to interfere with the reaction process and reduce testing accuracy.
The inventors conducted numerous studies and found that hydrogen sulfide could be detected and quantified with high accuracy in an aqueous sample by releasing the hydrogen sulfide into gas form using citric acid and sodium bicarbonate in powder form, and then detecting the hydrogen sulfide in its gas form. The citric acid and sodium bicarbonate can be stored separately in powder form to prevent them from reacting with each other, and to allow for precise adjustment of reaction amounts depending on the circumstances of the test sample.
In particular, the disclosed embodiments include a hydrogen sulfide test kit that includes citric acid and sodium bicarbonate. For example, the citric acid and sodium bicarbonate can be isolated from each other in separate containers until ready for use.
In one embodiment, a first container includes a first reactant powder comprising citric acid, and a second container includes a second reactant powder comprising sodium bicarbonate. In order keep the citric acid and sodium bicarbonate isolated from each other so that they do not react with each other, the first reactant powder is substantially free of sodium bicarbonate, and the second reactant powder substantially free of citric acid. Herein, “substantially free of” means that the component is contained in amounts that do not significantly affect the reaction chemistry. For example, because the first reactant powder is substantially free of sodium bicarbonate, any sodium bicarbonate present in the first reactant powder would react with the citric acid to a negligible extent. In one embodiment, the first reactant powder does not include any sodium bicarbonate, and the second reactant powder does not include any citric acid.
The first and second reactant powders can optionally include other non-reactive ingredients. For example, the first and second reactant powders can include a stabilizer and/or a desiccant. Alternatively, the first and second reactant powders can be free of any stabilizer or desiccant. In one embodiment, the first reactant powder consists of citric acid, and the second reactant powder consists of sodium bicarbonate.
The citric acid and the sodium bicarbonate are substantially pure (meaning that they contain less than 5 wt % of impurities). For example, the citric acid and the sodium bicarbonate contain less than 3 wt %, preferably less than 1 wt %, and more preferably less than 0.5 wt % of impurities.
The sodium bicarbonate can have a particle size in the range of 50 to 600 μm, 75 to 500 μm, 100 to 400 μm, or 200 to 300 μm, for example.
The kit also includes a test paper configured to change color upon contact with hydrogen sulfide. For example, the test paper can include copper sulfate, which will react with hydrogen sulfide to form copper sulfide according to the following reaction.
CuSO4+H2S→CuS+H2SO4
The copper sulfate may be coated on a surface of the test paper in order to ensure contact with the hydrogen sulfide.
The quantity of reaction product will depend on the amount of hydrogen sulfide that comes into contact with the test paper, which in turn depends on the amount of hydrogen sulfide present in the aqueous sample. A larger amount of reaction product can be visualized as a change in color. Thus, the test paper allows for both the detection and quantification of hydrogen sulfide in the aqueous sample.
In order to allow for quantification of hydrogen sulfide, the test kit also includes a test chart. An example of a test chart of the disclosed embodiments is shown in
The disclosed embodiments also relate to a method for detecting hydrogen sulfide in an aqueous sample, as summarized in
Preferably the citric acid and sodium bicarbonate are not combined with each other until immediately before conducting the test. For example, the citric acid and the sodium bicarbonate are in contact with each other for 15 minutes or less before adding the citric acid and the sodium bicarbonate to the aqueous sample for testing. Preferably the citric acid and the sodium bicarbonate are in contact with each other for 10 minutes or less, 5 minutes or less, or 1 minute or less before adding the citric acid and the sodium bicarbonate to the aqueous sample. In an embodiment, the citric acid and sodium bicarbonate are separately added to the aqueous sample, and thus are not combined in advance of being added to the aqueous sample.
The aqueous sample for testing may be a water sample such as wastewater, drinking water, or well water, for example. The aqueous sample comprises at least 95 wt %, 98 wt %, 99 wt %, or 99.8 wt % water.
The aqueous sample may contain hydrogen sulfide. The hydrogen sulfide content in the aqueous sample may be 10.0 mg H2S/L sample (i.e., 10.0 ppm H2S) or less, 5.0 ppm or less, 2.0 ppm or less, 1.0 ppm or less, 0.7 ppm or less, 0.5 ppm or less, 0.3 ppm or less, or 0.1 ppm or less. The hydrogen sulfide content in the aqueous sample may be 5.0 ppm or more, such as 8 ppm, 10 ppm, 15 ppm, or 20 ppm, for example.
In order to improve the accuracy of the testing, the aqueous sample should ideally be maintained at a temperature below 50° C. during testing. For example, the temperature can be in the range of from 5° C. to 40° C., 10° C. to 37° C., 12° C. to 30° C., or 10° C. to 23° C.
The aqueous sample can have a pH in the range of from 3.7 to 9.7. For example, the aqueous sample may have a pH in the range of from 4.0 to 9.5, 5.0 to 8.0, or 6.5 to 7.5.
The aqueous sample can have a hardness of 0 to 180 ppm calcium carbonate (CaCO3). For example, the aqueous sample may have a hardness in the range of from 10 to 150 ppm, 25 to 130 ppm, or 50 to 100 ppm.
As shown in
Next, a test paper is positioned in proximity to the aqueous sample so that any hydrogen sulfide released from the aqueous sample will contact with the test paper. For example, the test paper is positioned inside the test container, such as inside the lid. The test paper can be positioned inside the lid so that it is in fitted arrangement with the lid and remains fixed inside the lid when the lid closes the opening of the test container, as shown in
Once the test paper is positioned inside the container (for example, inside the lid), the first and second reactant powders are measured and added to the aqueous sample inside the container. The first and second reactant powders can be added together or separately, as discussed above. A total amount of the sodium bicarbonate and citric acid added to the aqueous sample can be in the range of from 1 to 8 mg per 100 mL of the sample. For example, the total added amount can be in the range of 2 to 6 mg, or 3 to 4 mg.
Depending on the source of the aqueous sample, the amounts of the first and second reactant powders that are added to the aqueous sample can be adjusted as desired. For example, the first and second reactant powders can be added so that a ratio of sodium bicarbonate to citric acid added to the aqueous sample is 30-70:70-30. For example, the ratio can be 35:65, 40:60, 45:55, 50:50, 55:45, 60:40, or 65:35.
As mentioned above, the aqueous sample should be analyzed as soon as practically possible after it has been collected in order to ensure that the measurements accurately reflect hydrogen sulfide concentrations in the source of the aqueous sample. Accordingly, the first and second reactant powders should be added to the aqueous sample within 2 hr of collecting the aqueous sample, and preferably within 1 hr, 30 min, 15 min, 10 min, or 5 min or less.
After adding the first and second reactant powders, the container is closed (for example, by closing the lid) and the first and second reactant powders are allowed to react, creating bubbles. The reaction is allowed to progress for 30 s to 10 min, 45 s to 5 min, 1 min to 4 min, or 1.5 min to 2 min, for example. During this time, the container can be agitated as needed to ensure that the reaction runs to completion. For example, the container can be gently moved so that the contents swirl inside the container, mixing the first and second reactant powders with the aqueous sample. It is expected that all of the hydrogen sulfide in a 100 mL sample will be released from solution within approximately 2 minutes.
After waiting for a period of time, the test paper is removed from the container and visually observed. The observations can be done with the naked eye or with magnification. If visual observation reveals that the test paper has undergone a color change, then it is determined that hydrogen sulfide is present in the aqueous sample. The test paper can be compared to the test chart to confirm the presence of hydrogen sulfide and optionally quantify the amount of hydrogen sulfide present in the aqueous sample. Visual observation is preferably performed within 15 minutes of removing the test paper from the container. For example, the test paper is visually observed 10 minutes or less, 5 minutes or less, 2 minutes or less, or 1 minute or less after removing it from the reaction conditions.
Modifications if this test method can be made. For example, the test paper can be added to the container before adding the aqueous sample, after adding the aqueous sample but before adding the first and second reactant powders, or even after adding the first and second reactant powders if done quickly. The test can be performed over a range of environmental conditions, such as a relative humidity in the range of from 20 to 90%, 30 to 80%, or 40 to 60%.
It will be appreciated that the above-disclosed features and functions, or alternatives thereof, may be desirably combined into different kits and methods. Also, various alternatives, modifications, variations or improvements may be subsequently made by those skilled in the art, and are also intended to be encompassed by the disclosed embodiments. As such, various changes may be made without departing from the spirit and scope of this disclosure.
