Lead Sensor Development

Our Lady of the Lake University

411 S.W. 24th Street

San Antonio, Texas 78207

Elizabeth Walther

Directed by Dr. Charles A. Smith

Lead in our environment… Of the overexposures to harmful materials in industry, lead is one of

the most common and is a leading cause of workplace illness. There are many ways in which humans are exposed to lead: air, drinking water, food contaminated soil, deteriorating paint, and dust. Airborne lead enters the body when an individual breathes or swallows lead particles or dust once it has settled. Before it was known how harmful lead could be, it was used in paint, gasoline, water, pipes, and many other products. Old lead-based paint is the most significant source of lead exposure in the U.S. today. Harmful exposures to lead can be created when lead-based paint is improperly removed from surfaces.

The paint on the painted sample in the photo was made during the 1950s. Paint made before 1978 contains unhealthy levels of lead. The pad in the photo was coated with a solution of sodium sulfide and rubbed on the paint. A resulting dark patch can be seen indicating the presence of lead in the paint.

Reason for developing the lead sensing pad: the Alazan-Apache Apartments

In August 2004, the San Antonio Express News published an article that stated a five-year-old living in the Alazan-Apache apartments had high amounts of lead in his blood. The lead was traced to the bath tub he was using. The ceramic of the tub had been worn through and the underlying lead was exposed. The San Antonio Housing Authority responded by offering free blood tests for lead and not replacement of all bathtubs. It was up to the resident to determine the degree of wear on their bathtub.

The objective of our project was to develop a lead sensing pad that would change color in the presence of lead. It was planned that his pad would allow simple and direct tests for the current residents of Alazan-Apache apartments.

Using this information we came up with a flier that stated who, what, when, and why we wanted to test deteriorating bath tubs. However, little interest was generated from the residents. This may be due to the personal nature of the location of the analysis or fear of interacting with a governmental agency.

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sPbSSPb

HSHSH2

In order to develop a lead sensing pad a chemical reaction is needed. The chemical reaction chosen is:

The lead sulfide precipitate has a black color and should be easily seen on a white pad. The sulfide is in several forms in solution related by the below equilibria:

It is assumed from these equilibria that the reaction may proceed most efficiently at pH above 12.9 since sulfur will mostly be in the form of S-2.

2 SHHS

05.71 pKa

9.122 pKa

Chemistry of lead sensing pad

Method Development: pad material

To get the best pad to use on the wipe test we tested different cotton pads and cosmetic sponges. The cosmetic sponges provided the easiest to see lead sulfide precipitate.

Cotton swab Cotton square cosmetic round

Method Development: sensitivity To see what the absorbance was when we mixed a certain ppm of

lead with a different percent sodium sulfide, we used the UV visible spectrometer to obtain the absorbance. We then checked to see what the absorbance was on the different percent sodium sulfide alone. We over-laid the corresponding percent sodium sulfides.

30%Na2S

30%Na2S with 2 ppm lead

Method Development: pH

After using the UV/Vis spectrometer it was determined that a 30% sodium sulfide solution would work the best. Tests to verify that optimum sensitivity would be found at basic pH values illustrated on odd trend. It is well known that sodium sulfide solutions turn yellow in the presence of oxygen. However, it appears from the colors of the different pH solutions and the provided pKa values that HS- is interacting with oxygen and not H2S or S-2.

Concentration of Lead (Pb)

Resulting Color on Wipe

1000 ppm

100 ppm

50 ppm

10 ppm

Equal volumes of different concentrations of lead were placed on a surface and allowed to dry over night. The following day 30% sodium sulfide solution was placed on the pad and rubbed over each dried lead solution for thirty seconds.

Method Development: color of wipe at different concentrations

Objects Tested The importance of cleaning the surface was determined by

testing several different materials. Below illustrates the cleaning tests performed using both lead and non lead containing solder. It was found that we had to clean most surfaces with a slightly acidic solution (we used vinegar) in order to prevent false positives from being detected.

Digestion of Paint Samples

Digesting paint proved more difficult than initially thought. After discussions with technical support at Parr and with trial and error the best digestion method was found to be a 3:1 mixture of nitric and hydrochloric acids. Digestion times were ~ 35 seconds with two hours minimum of cooling then repeat without opening the bomb. The oven used was 1kW operated at high power setting.

x

sx mV

bCC

sample of massanalyte of mass

% wt

Standard Addition Technique and Relations

The concentration of analyte in the sample, Cx, is determined by the below equation:

where b is the y-intercept, m is the slope, Cs is the concentration of

stock solution added to each addition flask and Vx is the volume of original solution placed in each addition flask.

The weight percent is then calculated by dividing the mass of analyte determined through standard addition divided by the orignal mass of the entire sample digested.

m=0.79b=0.149COR=0.9

Volume of stock added

Sig

nal

m =1.01

b=0.04

COR=0.99

Volume of stock added

Sig

nal

The presence of lead in samples was verified using the atomic absorption spectrometer. Standard addition plots were prepared and calculations performed.

Weight percent of lead in digested old green paint=0.19%

Weight percent of lead in digested old black paint =0.79%

The Standard Addition Technique of Lead-based Paint

Conclusion and further work…

• A lead sensing pad was prepared that can detect concentrations of lead arguably below 10ppm (probably depends on your eyesight!)

• Get direct contact with a person who lives in Alazan-Apache apartments to introduce the analysis to friends living in the apartments

• Investigate further the cleaning procedures used before testing with the pad

Acknowledgments

• Dr. Charles A. Smith

• Welch Program

• McNair Program

• Summer Research Group

The paint on the painted sample in the photo was made during the 1950s. Paint made before 1978 contains unhealthy levels of lead. The pad in the photo was coated with a solution of sodium sulfide and rubbed on the paint. A resulting dark patch can be seen indicating the presence of lead in the paint.