CA1256782A - Lead detection method and kit - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method and test kit are disclosed for quantitatively testing for the presence of lead in aqueous solution. The method comprises adjusting the pH of the aqueous solution to a value of from about 7.5 to 13, adding a predetermined amount of an aqueous test solution containing a soluble sulfide and a water-soluble glycol, and comparing the resultant colour of the solution with a standardized calibrated colour chart, so as to obtain a quantitative indication of the concentration of lead, if any, in the aqueous solution. The test kit for performing the method comprises a supply of aqueous alkali for adjusting the pH of the aqueous solution to be tested to the range of 7.5 to 13, a predetermined amount of a test solution containing a water-soluble sulfide and a water-soluble glycol in aqueous solution, a lead-free reaction vessel for mixing the aqueous solution with the alkali and test solution, and a standardized calibrated colour chart for comparison with the colour of the aqueous solution after addition of alkali and test solution.

Description

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This invention relateQ to an improved method of tcsting for the presence of lead in aqueous solution. It i8 known that amounts of lead in the range of from 4 to ~
ppm can be determined using dithizone as well as by various instrumental methods. ~owever, such tests require special equipment and chemicals and often involve complicated experimental procedures. They can therefore not be performed by the average layman at home.
U.S. Patent No. 3,809,53~ (Horine) discloses a crude testing method for the presence of lead in aqueous solution, which comprises addin~ a solution of a soluble sulfide and inspecting the resulting solution visually.
However, this prior teaching does not permit a quantitative determination of lead concentration allowing asse~sment of the seriousness of lead contamination in any ~iven instance. Furthermore, the prior method suffers from the di~advantage of evolution of unpleasant and potentially hazardous hydrogen sulfide odour, thereby inhibiting the practicality of the method in the home and the ease of use thereof, since inadequate ventilation ~hould be assumed with respect to testing in the home.
There is currently a need for a simple and efficient manner of quantita~ively detecting the presence of lead which is capable of being leached Prom the glaze of various pottery and ceramic containers. The lead-containing glaze is capable of being leached by food acids and this has given rise to many cases of lead poisoning in Canada. However, the lead content of pottery glaze, if ~5 ~ ~ 5 ~r~

any, i8 also readily leachable by the use of aqueous acetic acid. When lead is leached from pottery using 4%
acetic acid ~olution, the concentration limit presently ~onsidered unsafe by the United States Food and Drug Admini~tration i~ ~ ppm for small holloware. Leglslation specifying lower tolerable limits (less than 1 ppm) is under consideration.
Slmilarly, the Hazardous Products ~Glazed Ceramics) Regulations ~CRCc 925, paragraph ~a)) state that products releasing lead in excess of ~ parts per million may not be imported into Canada or sold to the public ~if produced locally). The evaluation i5 determined on the basis of a test of a product comprising steeping the product with 4%
acetic acld solution for 18 hours at 20C and determinlng the quantity lead present in the solution by an atomic absorption technique.
Accordingly, it is an object of the present invention to provide a simple test method for the detection quantification Qf lead, partlcularly in the range of 2 to 50 ppm, enabling members of the public at home or industrial users to test qulckly and in simple manner pottery, ceramics and other articles, to ascertain the saety thereof and to identify and separate potentially hazardous items o concern.
Accordingly, one aspect of the inventlon provides a method of quantitatively testing for the presence of lead in a concentration of from about ~ to 50 ppm in aqueous solution, which comprises ~a) adjustin~ the pH of the aqueous solution to a value of from about ~.5 to 13, (b) adding to the solution a predetermined amount of an aqueous test solution containing a soluble sulfide and a water-soluble glycol, and (c) comparing the resultant colour of the solution w~th a standardized calibrated colour chart so as to obtain a quantitative indication of the concentration of lead, 1f any, in the aqueous solution.
Another aspect of the invention provicles a test kit for testing an aqueous solution for the presence of lead, which comprises (a) an alkali solution for ad~usting the pH of the aqueous solution to the range of aoout 7.5 to 13, (b) a test solution containing a water soluble sulfide and a water soluble glycol in aqueous solution, and ~c) a standardized calibrated colour chart for comparison with the colour of the a~ueous solution after addition of alkali (a) and test solution tb) so as to provide a quantitative indication of the concentration of lead, if any, in the aqueous solution. The test kit may also include a lead-free reaction vessel, such as a lead-free ~lass test tube, for mixing the aqueous solution, the alkali and the test solution.
The soluble sulfide employed in the method is preferably sodium sulfide nonahydrate, but can also be potassium, lithium, ammonium, calcium, strontium or barium sulfide or hydrosulfide.
The glycol i5 advantageously glycerol, ~thylene glycol, diethylene glycol or propylene glycol and is l~S678~

conveniently employed in an amount of from about 1 to about 10% by volume, based on the test so~ution. The aqueous solution to be tested will normally have been obtained by leaching a suspect article, such as a pottery contalner, cutlery item, or paint chips from a suspect painted product. Such aqueous solution is advanta~eously acetic acid, such as distilled white vinegar (5~ acetic acid), but may also comprise citric, oxalic, tartaric, maleic, formic, lactic, benzoic, adipic, succinic or propionic acid in water. Thus, the aqueous solution is ~enerally obtained by steeping an article suspected ~o contain leachable lead in the acid solution for an extended period of time, for example for about 24 hours.
A particular feature of the invention is the ad~ustment of the pH value of the aqueous solution to the alkaline range of about ~.5 to 13 prior to contact with the test solution. This procedure substantially eliminates the evolution of hydrogen sulfide yas with accompanyin~ unpleasant odour during the duration of the test procedure. Adjustment of the pH value can conveniently be achieved by the addition of an alkali, for example selected from alkali metal and alkaline earth metal hydroxides and carbonates, such as sodium, calcium, potassium, lithium and magnesium hydroxides and carbonates, sodium metasilicate, trisodium phosphate, sodium bicarbonate, ammonium hydroxide, borax, ferrous hydroxide and magnesium oxide, which alkalis can be added ~X~678~

in solid or in solution form. A preferred alkali is about

2.5N aqueous sodium hydroxide solution.
The test solution is preferably an aqueous solution containlng about 0.64N sodium sulfide nonahydrate and from lX -to 10%, preferably about 5% by volume of ~lycerol. ~owever, any alkali or alkaline earth metal sulfide or hydrosulfide, such as lithium, sodium, potassium, ammonium, calcium, strontium and barium sul~ides and hydrosulfides, may be employed as the test solution, generally in concentrations of up to 50% by weight, based on the weight of the solution. Glycerol is preferably used as the stabilizer, but any water-soluble glycol can be employed, for example ethylene glycol, propylene glycol or diethylene glycol. An amount of glycol of from about 1 to 10% by volume is preferred.
The leaching solution is preferably acetic acid, for example in the form of white vineyar (5~ solution) which is readily avallable in most households.
However, other acids can also be employed to form the aqueous solution to be tested, such as oxalic, tartaric, maleic, citric, formic, lactic, benzoic t adipic, succinic and propionic acids in solutions generally up to 10% by volume.
The test solution is conveniently packaged in the kit in 25 ml samples, while the alkali neutralizin~
solution may advanta~eously be employed in amounts of 25 ml. The test kit may include a dropper, e.g. an eye dropper, for transferring measured amounts of the aqueous ~5~7~3~

solution into the reaction vessel. As mentioned above, the kit advantageously further includes a lead-free, non-reactive vessel for use as a standard reaction vessel, as well as a colour comparison chart for determining the approximate lead release.
The method and kit of the invention are also useful in determining the presence of other toxic metals, ~uch as copper and blsmuth which produce a similar brown sulfide insoluble salt~ They are generally detectable at a level of approximately 10 ppm. The presence of such other metals can conceivable result in an interfering reaction in lead detection. However, this interference is not generally a real problem when detecting lead since the above metals are seldom used i~ pottery making or found in cutlery or paint.
The method of the invention enjoys various advantages over the prior art procedures. In particular, the process is completely odour-free since the test ~olution is added to an alkaline environment thereby avQiding evolution of hydrogen sulfide and the characteristic rotten eg~ smell. As a result, the testing operation is considerably more pleasurable for the user.

In an acidic environment (prior method), the reaction proceeds according to the following equation:
Na~ + 2S= ~ 2H+ + Pb++ = 2Na+ -~ H2S~ + PbS~ (1) In the procedure of the present invention, the reaction proceeds as follows:
Na+ t S + OH- + Pb++ = Na+ + OH- ~ PbS~ (2) ~25678~
5ince hydrogen sulfide is not evolved durin~ the test, the procedure i5 much safer, since H2S ~as is a v~ry toxic gas, even in low concentrations.
In addition, the resulting test solution is clear, rather than a cloudy preGipitate as produced by the prior method of U.S. ~atent No. 3,809,537. In consequence the observation of a colour change, particularly at the lower concentrations tless than 4 ppm), is very difficult to detect utilizing the prior method.
The kit accordin~ to the invention includes a colour comparison chart which can be employed to determine the approximate lead (and other toxic metals) release, by comparing the colour of the resulting solution ~preferably in a standard glass container supplied with the test kit against a white background) to the colour of the test strip.
Futhermore, the test solution contains a stabilizer which increases the shelf-life of the product.
Thls is an important factor in the present field, since the use of a stale product may well ~ive rise to a misleading false negative or "safe" result.
The method and kit of the invention can furthermore be utilized to detect low levels of lead in potable water. Thus, lead contamination of a household water supply can be verified, this being an occasional occurrence either due to contamination from source or due to the use of lead solder (especially ~n new homes containin~ fresh solder on joints). In addition, lead 7~32 stearate is used in the production of plastic pipin~ and can give rise to the pre~ence of lead in drinking water emanating from plastic pipes.
Furthermore, the method and kit of the invention can also be employed to determine leachable lead content in soil collected around the home. Hazardous levels of lead in soil can be established and quickly acted upon.
Moreover, the method and kit of the invention can also be employed to determine the presence of leachable lead in cutlery, older silverware and painted surfaces or furniture.
The following Examples illustrate the invention.
Exam~les 1 to 10 A variety of different media were tested for the presence of lead utilizin~ a 0.64N sodium sulfide nonahydrate aqueous solution containing 5% by volume of glycerol. Distilled white vine~ar (5~ acetic acid) was employed as a leachin~ solution and the pH thereof was ad~usted to within the range of ~.5 to 13 utilizing 2.5N
sodium hydroxide solution. The pottery article was filled with the vinegar and allowed to stand for 24 hours. 1 ml of the vine~ar was then placed into a lead-free ~las~

reaction vessel and ~ drops (0.35 ml) of the neutralizer (sodium hydroxide solution) was added, followed by 7 drops (0.35 ml) of the test solution. The colour change, if any, was noted and compared with the calibrated standardized colors of a colour comparison chart.

g 12S6~82 For comparlson purposes the experiments were repeated utilizing the procedure described in U.S. Patent No . 3, 309, 53~, employing a similar vinegar and a similar amount of sodium sulf ide nonahydrate. The results obtained are ~ummarized in the following Table 1.

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~1 r-l ~ ~1 r- ,1 0 O O ~: O 0 h O10 J ~J p 'J r-l ~ O O O ~ ~r~ P ID rl r~ D 4 4 U) ~ rl , J .a IJ r1 0 ~rl ~a r-l r-l 3 0 .Q ~ ~ O hC~) ~ O O ~rl ~ al 11) h h ~ ~a ~ r 4 ~ X
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Examples 11 to 18 The above described test solution and neutralizer were also employed for analyzing the lead content of soil samples, potable water, cutlery and metal samples employing a similar procedure to that described above. In the soil analysis test, a factor of 100 is employed to compensate for the dilution of the vine~ar.
The results are given in the following Table 2.

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~ O ~, O P- p' U h ~ ~ O u) ~rl 4 C3 U) A .1 ~D

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cl 4 4 -- 4 a~ O h h 4 h h h ~: Id O
1~ 4 U) h .C g ` 0~ U~ h 0 1) L> I~s ~ o o o a) o h ` ~ ~ ~ h U
S~ J h h O ~ h O t~
o ~:` O O ~ O^
r-l O~ ~rt ~~ O ~ O O ~ ~1 4 ~-I O O ~r1 Il~ r1 .a .~ 3 4 0 ~ U) O O O ~ 1 4 l~
-~ ~o ~ ~u) 4 0 td ~ E3 0 0 ~
O ~ ~: h ~1 ~ U ` P P~ ~ U ` 0 ~ ~1 ~1 O O P O h ~I p~ h 1~ 4 td rl ~ ~ ` O a) ~ ` o ~1'a E!
4 ~a ~ 4 O Q-'a h -1 4 0 ~ O ~ h r~ tl) --I Q.
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h g~ 0 0 0 tlhl:: O O P.
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In summary, it will be appreciated that the method and kit provide a convenient means of detecting lead in the range of 2 to 60 parts per million. The method can be employed for the detection of lead in a variety of domestic areas, including pottery and china, cutlery, and painted articles, as well as soil, water and unidentified metallic items.

Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of quantitatively testing for the presence of lead in a concentration of from about 2 to 50 ppm in aqueous solution, which comprises (a) adjusting the pH of the aqueous solution to a value of from about 7.5 to 13;
(b) adding to the solution a predetermined amount of an aqueous test solution containing a soluble sulfide and a water-soluble glycol; and (c) comparing the resultant colour of the solution with a standardized calibrated colour chart so as to obtain a quantitative indication of the concentration of lead, if any, in the aqueous solution.

2. A method according to claim 1, wherein the sulfide is sodium, potassium, lithium, ammonium, calcium strontium or barium sulfide or hydrosulfide.

3. A method according to claim 1, wherein the glycol is glycerol, ethylene glycol, diethylene glycol or propylene glycol in an amount of from about 1 to 10% by volume based on the test solution.

4. A method according to claim 1, wherein the aqueous solution comprises acetic, citric, oxalic, tartaric, maleic, formic, lactic, benzoic, adipic, succinic or propionic acid in water.

5. A method according to claim 4, wherein the acetic acid comprises white household vinegar.

6. A method according to claim 4, wherein the aqueous solution is obtained by steeping an article suspected to contain leachable lead in the aqueous acid solution for an extended period of time.

7. A method according to claim 1, wherein the pH of the aqueous solution is adjusted to the range of about 7.5 to 13 by the addition of alkali selected from alkali metal and alkaline earth metal hydroxides and carbonates, sodium phosphate, sodium silicate, sodium bicarbonate, ferrous hydroxide, magnesium oxide, ammonium hydroxide or borax.

8. A method according to claim 7, wherein the alkali is sodium hydroxide.

9. A method according to claim 8, wherein the test solution comprises about 0.64N aqueous sodium sulfide nonahydrate containing about 5% by volume of glycerol and the alkali is an about 2.5N aqueous sodium hydroxide solution.

10. A test kit for testing an aqueous solution for the presence of lead, which comprises:

(a) an alkali solution for adjusting the pH of the aqueous solution to the range of about 7.5 to 13;
(b) a test solution containing a water soluble sulfide and a water soluble glycol in aqueous solution;
(c) a lead-free container for mixing the solutions; and (d) a standardized calibrated colour chart for comparison with the colour of the aqueous solution after addition of alkali (a) and test solution (b) so as to provide a quantitative indication of the concentration of lead, if any, in the aqueous solution.

11. A test kit according to claim 10, and additionally comprising a supply of aqueous acid for leaching a suspect article to form the aqueous solution to be tested.