In pure water, soluble copper hydroxo complexes form at low and high pH values. Precipitation is dependent on copper concentration, presence of other anions and cations, temperature, and time to thermodynamic equilibrium Jensen, Figure 1 demonstrates the distribution of individual hydroxo Cu II complexes in pure water as a function of pH and identifies where copper hydroxide precipitate will form.
While hydroxo complexes are always present in water, individual or combinations of anions can bind to cupric ion to form complexes based upon stability constants. Precipitates of these complexes form when the solubility product is exceeded. A common multianionic precipitate is malachite [Cu 2 OH 2 CO 3 ], which is a blue—green cupric-hydroxide-carbonate precipitate.
It is well established that copper speciation affects toxicity and bioavailability in aquatic organisms from algae to fish. Free copper II ion and monohydroxo Cu II are considered highly toxic, while other anionic complexes, especially carbonato complexes, are less toxic to aquatic organisms. Particulate copper is not toxic unless it is solubilized in water or the fluids within an organism. Copper is much less toxic to mammals, as reflected in the health-based standards previously discussed.
Although the role of speciation of copper is known to be important in aquatic toxicity, its role in human sensory response is not well established.
Only a few previous studies, summarized in Table 1 , addressed the taste threshold of copper with limited focus on the speciation of copper in water. Global locations of these studies are reported in the following discussion because mineral content of tap and natural waters are primarily influenced by local geography and may have been vastly different in these studies; however, authors did not report detailed water quality data.
One research study in the United States found taste thresholds of 6. Soluble copper was maintained by adjusting pH to 6. The triangle test method was used, and three sets of copper concentrations were administered per person per session with a test range of 1.
This technique did not specifically address copper taste adaptation of the 15—20 panelists, some of whom were smokers. Lower thresholds, 2. The importance of copper solubility to taste perception was identified, and solution pH was adjusted to control copper solubility.
A one-of-five test format was used to decrease the likelihood of guessing correctly. Only one concentration was given per session because panelists exhibited a decreased sensitivity to the copper stimulus when given multiple copper-containing samples in one session adaptation.
To control effects from aftertastes, 1-min wait periods were mandated between samples. A weak sucrose solution instead of distilled water was used as the control and rinse water due to the unpleasant taste of distilled water.
The copper range was 0. The low pH conditions would reduce likelihood of particulate formation until high copper levels several milligrams per liter were reached. This study concluded that only soluble copper provided a taste sensation.
Zacarias et al. This study, performed in Chile, also used the one-of-five protocol, and only one concentration was given per session to address adaptation. One-minute wait periods were mandated between samples to minimize aftertaste effects.
Copper chloride and sulfate salts were used, and no significant difference in threshold values was found for the two salts. However, neither confidence intervals nor guessing correction techniques were used. The effect of pH on soluble and particulate copper species was not addressed. Nose clamping was used to determine the retronasal effect on copper tasting; no significant effect was shown by clamping the nose.
Lawless et al. These previous studies show a wide deviation in threshold values for copper with conflicting results in distilled and other water sources. In addition, these studies did not provide detailed water quality data and therefore could not thoroughly evaluate the distinct effects of copper speciation in taste threshold determination.
The goal of this research was to specifically evaluate the role of free, soluble, and particulate copper in taste and do so at concentrations below and near health-based standards. The pH and presence of anions were used to control copper speciation. The specific objectives were 1 to determine the taste threshold of free and complexed soluble copper and 2 to evaluate the role of particulate copper in taste sensation.
Thirty-six healthy adults, with no previous copper taste threshold experience, participated in four studies. The panel consisted of 15 males and 21 females ranging from 22 to 54 years of age and reporting no chronic health problems. The sensory protocol was approved by the Institutional Review Board at Virginia Tech; all panelists signed informed consent forms.
All panel members underwent an initial training session to familiarize them with the taste of copper and the sensory test methods. Panelists were instructed to swallow the samples as many panelists reported tasting low concentrations on the back of the tongue and throat. Preliminary testing with five panelists indicated that an aftertaste was prevalent with copper.
Therefore, only one tasting session was administered per day, and only one copper concentration was tasted per session. All samples were prepared fresh daily to avoid increased precipitation with time. Fourteen concentrations 0. Concentration intervals used for this research were not uniform but were selected to emphasize health and aesthetic-based standards. Soluble and particulate copper concentrations in the test water samples were manipulated by controlling the pH. Filtration through a 0.
Free copper ion concentrations were measured using a cupric electrode accumet cupric combination electrode, catalog number , Fisher Scientific. Distilled—deionized water was generated from a Barnstead Nanopure system that was fed distilled water and subsequently deionized and carbon filtered. A mineralized water at pH 7. Taste thresholds were measured at pH 5.
Copper was soluble at all concentrations in pH 5. The nonsoluble, or particulate, copper resulted in formation of a fine precipitate. Soluble copper as a function of pH as measured by filtration through a 0. The amount of free copper ion also varied with pH. Figure 4 demonstrates that at pH 5. At pH 6. To test the effect of pH alone, panelists participated in a similarity test with distilled—deionized water adjusted to pH 7 or 9 with NaOH.
Panelists were told to choose the odd sample. Out of 53 respondents, only 15 correctly chose the odd sample. The results demonstrated that pH alone did not affect panelist's perceptions of water Meilgaard et al. Therefore, any differences in sensory perception for these experiments were not related to pH changes and could be linked to copper speciation and subsequent interactions.
The first experiment evaluated which copper concentrations consumers could taste in water. Preliminary research in our laboratory demonstrated that humans could readily taste copper at much lower concentrations than these published thresholds.
Copper chemistry and the effect of water quality were investigated in our research experiment by using pH adjustment and presence of anions to varying formations of free, soluble complexed, or particulate copper.
As described in Materials and Methods, the test waters were distilled—deionized water compared to a mineralized water at pH 7. The distilled—deionized water was generated from a Barnstead Nanopure system. For each test, the control, rinse water, and the copper solution were the same pH and mineral content. An ascending concentration forced choice test was used to determine human taste thresholds ASTM, , ; Lawless and Heymann, ; van Aardt et al.
To decrease the possibility of guessing correctly, the protocol was modified to five samples four controls and one copper sample. Beguin-Bruhin et al. Five 3-oz white plastic sample cups were coded with three-digit random codes, filled with 20 ml sample and presented to panelists in randomized order. One of the five samples contained an aqueous solution with copper added; the others contained the same aqueous solution with no copper. A session began by panelists rinsing with copper-free test water, then tasting the first sample, waiting at least 20 s, and then tasting the next sample.
Panelists were asked to use their own descriptors to describe the taste of copper; a list of descriptors was not provided. Only one set of five samples was evaluated per day; panelists were exposed to increasing concentration steps within the testing range on subsequent days. A positive report was defined when a panelist correctly identified three correct samples in a row. Panelists described the taste of copper mostly as metallic, but bitter and bloody were also used as descriptors.
Analyses of the individual threshold results in these two water samples provided insight on the effects of chemistry on copper tasting. A simplified equation follows:. Aqueous ammonia results in the same precipitate. Upon adding excess ammonia, the precipitate dissolves, forming tetraamminecopper II :. It does not react with water but reacts slowly with atmospheric oxygen, forming a layer of brown-black copper oxide. In contrast to the oxidation of iron by wet air, this oxide layer stops the further, bulk corrosion.
Oxygen-containing ammonia solutions yield water-soluble complexes with copper, as do oxygen and hydrochloric acid, which form copper chlorides, and acidified hydrogen peroxide, which form copper II salts. Copper II chloride and copper combine to form copper I chloride. The simplest compounds of copper are binary compounds i. Among the numerous copper sulfides, important examples include copper I sulfide and copper II sulfide. Attempts to prepare copper II iodide yield cuprous iodide and iodine.
Copper II sulfate forms a blue crystalline pentahydrate, which is the most familiar copper compound in the laboratory. It is used in a fungicide called the Bordeaux mixture. Polyols, compounds containing more than one alcohol functional group, generally interact with cupric salts. For example, copper salts are used to test for reducing sugars. This demonstration can be done with copper in the form of shot, pellets, thicker wire, or bars, but is a great deal slower than with copper wire.
Ira Remsen founded the chemistry department at Johns Hopkins University, and founded one of the first centers for chemical research in the United States; saccharin was discovered in his research lab in Like many chemists, he had a vivid "learning experience," which led to a heightened interest in laboratory work:. While reading a textbook of chemistry I came upon the statement, "nitric acid acts upon copper.
Copper was more or less familiar to me, for copper cents were then in use. I had seen a bottle marked nitric acid on a table in the doctor's office where I was then "doing time.
Having nitric acid and copper, I had only to learn what the words "act upon" meant. The statement "nitric acid acts upon copper" would be something more than mere words. All was still. In the interest of knowledge I was even willing to sacrifice one of the few copper cents then in my possession.
I put one of them on the table, opened the bottle marked nitric acid, poured some of the liquid on the copper and prepared to make an observation. But what was this wonderful thing which I beheld? The cent was already changed and it was no small change either.
A green-blue liquid foamed and fumed over the cent and over the table. The air in the neighborhood of the performance became colored dark red.
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